Case Closed: Get Those Air Conditioning Ducts out of the Attic
Remember that article I wrote about ducts installed against the roof deck and how I said it was probably the absolute worst single location for installing ducts? Well, in the comments, Dave Roberts, a senior engineer at the National Renewable Energy Lab (NREL), wrote about a paper he co-authored last year and included a link to it. Up against the deck may be the worst place in the attic to install ducts, but Roberts shows that putting them in the attic at all is the worst place in the house you can install ducts.
The report, Ducts in the Attic? What Were They Thinking?, summarizes the research that’s been done about putting ductwork in unconditioned attics and basically says it’s about the stupidest thing we do in homes that do a lot of air conditioning. I encourage you to download and read this report. If you’re building or remodeling a home, make sure the general contractor (if it’s not you) and the HVAC contractor get copies.
I love the analogy they use to introduce one of the main problems with this location. “Heat exchangers,” they write, “are designed to transfer as much heat as possible from one fluid to another.” Comparing this configuration to a solar water heater, they make the case that putting air conditioning ducts in a hot attic is an effective way to heat up the conditioned air as it travels from the air handler to the conditioned space inside the home.
If you’ve studied heat transfer at all, you may recall that the rate at which heat moves from a warmer to a cooler body depends on the temperature difference, which we abbreviate as ΔT. An attic can get up to about 130° F in the summer, and the conditioned air entering the ducts is about 55° F or so. With hundreds of square feet of ductwork surface area in the attic and a ΔT of 75° F, the air coming out of the vents in your home will be significantly higher than 55° F. Throw duct leakage into the mix, and the problems are even worse.
What Roberts and his co-author Jon Winkler did, in addition to reviewing the literature about this topic, was to model the savings possible when you relocate the ducts from an unconditioned attic to the conditioned space inside the building envelope. They chose Houston, Phoenix, and Las Vegas as the locations for their modeled houses. The table below summarizes the main results.
In addition to comparing ducts in the attic to ducts inside the building envelope, Roberts and Winkler also looked at electricity savings of other measures, such as adding insulation, installing better windows, and using higher efficiency air conditioners. The table below shows that moving the ducts inside is the first thing you should do to save the amount of electricity you use.
In addition to saving on air conditioning operating costs, the upfront cost of cooling equipment is lower in efficient homes. Roberts and Winkler looked at moving the ducts inside compared to other building envelope improvements, and again, moving the ducts inside beats all the other methods for achieving this objective, as shown below.
This report, which the authors delivered at the ACEEE (American Council for an Energy Efficient Economy) Summer Study in August 2010, shows definitively that putting ducts in attics in cooling dominated climates is a practice that needs to end.
Download the paper here: Ducts in the Attic? What Were They Thinking?
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has a book on building science coming out in the fall of 2022. You can follow him on Twitter at @EnergyVanguard.
The Basic Principles of Duct Design, Part 1
Make It a Mechanical Room, Not Just a Closet
7 Ways to Improve Ducts in an Unconditioned Attic
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Very impressive research. One
Very impressive research. One thing would be nice to see in the future, would be case studies where people actually relocate ducts without building a new house.
It’s great that we have
It’s great that we have research to support this but isn’t it “intuitively obvious”?
If you’re trying to get cold air to a room, you don’t run the duct through the hottest place in the house.
In the winter, the exact opposite happens…
You’re trying to get warm air to a room, and the duct is running through the coldest part of the house.
It’s done this way because it’s easier and cheaper to run a flex duct through a wide open space than to fit it through and around floor and wall framing. It also makes maintenance and/or modification easier.
Those are valid reasons to do it, so the real answer is to make the attic (at least semi-)conditioned space.
Then the attic doesn’t get as hot in the summer or cold in the winter and you cut down on thermal conductivity through the ceiling of the upper floor as a “bonus”.
If you’re building a new house or replacing the roof, you can start with lighter colored shingles, and go from there.
Ira, that’s a great point
Ira, that’s a great point regarding the cost and maintenance. Allison, have you looked at the costs associated with installing duct inside conditioned area vs. sealing off the attic (spray foam?). The added advantage of sealing off the attic is you are lowering heat transfer from the attic to your home.
People like to talk about
People like to talk about 150F attic air and this is seldom questioned. I would like to question how they arrive at this number.
For my own house on a day which was officially 99F, I placed sensors in 3 places and measured hourly:
1. One sensor hanging about 2 feet below radiant barrier.
2. One sensor placed atop the radiant barrier.
3. One sensor placed about 1 foot above the attic insulation.
One would expect #1 to be most representative of the threat to a man working in the attic. One would expect #3 to be representative of the thermal load on ducts. #2 is measuring how much difference a radiant barrier makes.
On this typical summer day (nearly cloudless) close to Houston TX, the peak temperatures were found at 4pm:
#1 = 113F
#2 = 125F
#3 = 101F
These measurements have been taken sporadically throughout last summer and the highest temperature was measured at 132F atop the radiant barrier. So I would like to see more about the methodology which claims 150F as if that were typical.
I agree completely. A vented attic is nothing more than a shoe box with the top off. How can anything stay in the shoe box without a top? Even a pair of shoes will fall out without a top on the shoe box, so there is no question in my mind that air will too!! CLOSE the shoe box an air/vapor/thermal barrier and put the duct work any pace you like inside the box.
Mark J.: I
Mark J.: I’m sure some work has been done in that area. I’ll see what I can find.
Ira E.: You’re absolutely right that it’s intuitively obvious, to those who understand heat transfer anyway. What they added in this paper were quantitative results on savings and comparisons to other measures.
Chris K.: See pp. 7-8 of the paper for info about the extra costs associated with bringing the ducts inside the building envelope.
Mark J.: Yes, some people do throw the 150° F number around when talking about how hot attics get, and for some reason the authors of this paper put that in once near the beginning, too. The Florida Solar Energy Center (FSEC) has done a lot of research on attics, and I wrote an article about shingle temperatures that was based on one of their review papers. If you download that paper, you’ll see that their results for attic temperature are close to yours (pp. 18-19), and that’s why I use the number 130° F.
Gene: I can always count on you to come up with interesting analogies.
M Johnson, how old is your
M Johnson, how old is your house?
I have no idea what the temperature is in my house’s attic, which was built in 1957, but I bet it is a hot as any house’s attic in Florida will ever be…
I actually will have to replace my A/C sometime soon, my unit is over 16 years old, which is ancient in Florida.
I have to consider new duct work as well and will see if it is possible to place the ducts under the house.
Anyone want to pay for the new unit and duct work? I’ll be glad to give you all the data you need!!!
This is a difficult thing to
This is a difficult thing to get nearly any builder to do, even on new houses where the cost to do so is minimal compared to what it would cost in a retrofit situation- basically an upgraded truss or floor package, a little planning/design work, and some extra sheetrocking. There are some folks working really hard to change that, but it is difficult convincing contractors why they should do this, especially if they’ve been getting “good enough” results by running ductwork in attics and just oversizing the equipment. For retrofits, Ductless heat pumps are an obvious option. “cathedralizing” the attic to bring it inside conditioned space presents some challenging air sealing details and brings into question how much one should leave the attic coupled to the main body of the house. There is also at least one manufacturer pushing high velocity systems utilizing 2″ ductwork. I’m sure it is pricey…
If you have a crawl space, does this make installation and retrofitting easier?
I don’t like floor vents, but surely the space under my house is cooler than the attic, plus I only have to cool off 1400 square feet.
Lastly, if I were a builder, I’d take this and run with it. It makes perfect sense, it would certainly save you money quickly, and it’s good for the environment.
Graham, my house with the
Graham, my house with the temperature measurements was built in 1989.
This is quantitative proof
This is quantitative proof that in new buildings the duct work HAS to be placed in the conditioned space.
Retrofits are virtually impossible from a practical, logistical, and financial standpoint so existing homes are forced to work with best retrofit practices. Those include spray foam encapsulation (with combustible air venting for furnaces), increased insulation, radiant barrier, duct sealing, etc.
Great article and study, thanks for the link.
Maybe try not using duct work
Maybe try not using duct work at all as a retrofit…try ductless mini-splits instead…For new buildings, in the NW we are stating 18 inch between floors, which is more then enough room for larger duct work and part of the envelope.
There are alot of comments
There are alot of comments directed at builders. Please keep in mind that we are bound by our client’s budget contraints. To say it doesn’t cost much to insulate and sheetrock the attic is relative and the money to do so usually goes into stainless steel and granite in the kitchen. People pay for extras they can touch.
Graham: Yes, you can put your air handler and ducts in the crawl space and save energy. The problem you face there is moisture
Thomas A.: Yes, getting builders to do this can be difficult but the paper discusses 4 alternatives for getting the ducts out of the envelope and the associated costs. See pp. 5-6.
Steve: You’re right about existing homes. Unless they’re willing to incur the big expense of getting a whole new duct system, which is warranted occasionally, they basically have to spray foam on the roofline to do this.
Robert H.: I love mini-splits and want to go that route if and when I build another house. Did you see our two articles on mini-splits?
Duct-Free Zone – The Advantages of Mini-Split Heat Pumps
The Ductless Mini-Split Heat Pump’s Big Brother – CITY MULTI
Mike: Yes, the builder’s the one who has to manage all the work and the costs and make sure to keep the client happy, too. It’s a tall order, to be sure. If you haven’t already done so, I encourage you to read the paper, especially the section on pages 5 & 6 called What It Costs. They discuss 4 methods of getting ducts out of the attic, with expenses ranging from a savings of $800 to a cost of $4000. Plus, you should get to save some money on the cooling system by being able to downsize it.
Graham: I forgot to finish what I was saying to you above. Because vented crawl spaces have moisture problems in humid climates (and I think Florida qualifies for that), you’ll have to either encapsulate the crawl space or do a bang-up job on sealing and insulating the ducts if you put the system down there.
In warm climates, moving
In warm climates, moving ducts out of the attic is probably the lowest hanging fruit there is, at least in new construction. Unfortunately, it’s far from obvious to most home designers how to make this happen.
A ‘green’ architect hired me a few years ago to resolve a ‘slight’ problem — she designed her new home specifically to accommodate the air handlers and ductwork inside the thermal envelope. She was so proud of herself. However, when the mechanical contractor did his pre-rough-in walk-through, he discovered an impossible challenge. She had failed to allow space for turn radius and cross-overs. What a mess.
This is not something that’s taught in school. My advice to home designers and architects is to partner with a mechanical contractor who excels in engineered duct systems.
Perhaps the AIA should develop a continuing ed course: “Incorporating HVAC into conditioned space”
In my opinion, encapsulating the attic is a cop-out. It’s less expensive AND more efficient to bring the ducts inside. It just takes someone who knows how to make this happen.
I agree on some points, but..
I agree on some points, but..
good luck getting better cooling in the upstairs in an old home using a basement HVAC system than one in an attic. Sure, if you can find enough space and afford the large cost to have someone poorly carve up the inside of your house to get the proper sized ductwork through the house.
Quit writing papers and go try a seat of your pants experiment. I sure sleep well in my nice cool 2nd floor, thanks to the attic supplied AC. Rarely have I found someone in an old house satisfied with the cooling from the more traditional basement install.
Mark: The paper I wrote about by Dave Roberts of NREL focused on new construction. When someone’s designing a new house, they have a lot of options to get the ducts inside the building envelope. I agree that in existing homes, it can be difficult to condition the second floor without going into the attic. Not impossible, but definitely more difficult. And putting the system in the basement is NOT the only option.
“Quit writing papers and go try a seat of your pants experiment.” What’s your point? I’ve worked on plenty of retrofits to get the ducts inside the envelope as well as building a high performance home with ducts inside. All were more comfortable than a conventional setup. And they were much more than ‘seat of the pants experiments’ too.
The article is called "
The article is called “Cased Closed…:”
My point is simply the vast majority of homes in America are existing structures wherein the energy benefit of installing the ideal system is usually outweighed by the cost, structural modifications, etc.
Being green isn’t the number one priority for most Americans.
I don’t think the case is closed for attic ductwork.
There are varying options,
I didn”t intend to question your energy saving knowledge. Most old home enthusiast are generally distrustful of the modern energy industry. Same people that will tell you how inefficient your old wood windows are, and to install vinyl windows ASAP, when in reality….
Mark: Yes, actually the case is closed for ducts in attics when, as I described above, you’re building a new home or doing a major renovation in a cooling dominated climate. To do otherwise is just stupid.
One thing to keep in mind is
One thing to keep in mind is that basement/crawlspace retrofit isn’t possible for regions where high water tables and/or clay soils make below-grade excavations and construction problematic and expensive. As a result, most houses in this region (think Oklahoma, Texas, etc) are built slab-on-grade. In the 70’s and 80’s, in-slab HVAC ductwork was commonplace, and took advantage of the relatively stable temperature of the slab at or near the ground.
With newer foundation systems involving embedded rebar or tension cables making in-slab HVAC ductwork nearly impossible, there’s little other option but to move it to the attic…
All home in my region are
All home in my region are slab-on-grade. If edges are properly insulated, actually good from efficiency standpoint. If home is two floors above grade, ducts go in the second story floor system. In ranches on slabs, you have to create soffits (fur-downs), that are isolated from the attic. Hard to get builders on board with this, but in my experience, it doesn’t cost as much as closed attic, and has better performance. But if home is really cut up, it may not be an option.
David W: You statement that “there’s little other option but to move it to the attic” is a common misunderstanding. You can get the air handler and ducts into conditioned space in several ways without needing a basement or crawl space: (i) Lowered soffits around room perimeters; (ii) lowered ceilings in hallways; (iii) special trusses that create space below the unconditioned attic; (iv) conditioned attic, and more. See the article I linked to for more details.
If you’re talking about existing homes, yes, it’s very difficult to get ducts out of the attic. I was mainly discussing new homes in this article, though.
I live in a condo where we
I live in a condo where we cannot change the placement of our ducts. What is suggested if mold has grown around the vent boxes and ducts? I cannot get back far enough to clean them? If they are black, is that mold or is that wear from freon passing over them for 30 years?
Can someone advise me on the
Can someone advise me on the proper way to clean A/C ducts? How will I know if a professional is legitimate or a rip off? I have ads anywhere from 18.00 to 88.00 but none will tell me “how” they clean them. I don’t want someone coming in and spraying chemicals that claim to kill the mold. Is it worth it to replace the ducts if they are over 20 years old? Is the chemical spray only temporary? Please advise.
I believe that moving ducts
I believe that moving ducts out of the hostile temperature environment of the attic is obvious. However, unless you have new construction is highly doubtful that you can find an alternative. Instead, I covered my attic ducts with insulation. I chose unfaced batt type so that it would stay in place.
I haven’t collected empirical evidence, but my average power bill has halfed (due in part to other improvements as well). Does anyone have any similar experience or data on insulating over the ducts?
Diane: To answer your first comment, if your vents are black with mold, you could have several things going on. Do you know how high the relative humidity runs in your home? It could be related to the indoor air. That could be caused by an air flow problem in AC itself or too much infiltration (if you live in a humid climate). The best thing to do is get it checked out by someone who understands HVAC and building science. Here’s an article that might help you:
How to Choose a Company to Do a Home Energy Audit
Regarding duct cleaning, you may benefit from it, but you should be wary of companies who just want your money and don’t know what they’re doing. Here’s an article about the topic:
Should You Get Your Air Conditioner’s Ducts Cleaned?
Don R.: Keep a very close watch on the ceilings under your ducts. If you’re in a humid climate, burying ducts in the insulation could lead to condensation and ceiling damage.
Yes, you’re right that it can be difficult to get ducts out of conditioned space in existing homes. Spraying foam insulation on the roof deck is the most popular method to do this. It’s effective but not cheap.
Can you use pvc (plastic
Can you use pvc (plastic pipes) for duck work ?
my sister has a unit on her
my sister has a unit on her roof and it works when it feels like it she turns it on in the morning and it will cool her house but when it starts to get hot like at 12 noon it stops working can some one help please some ac guy said it is sucking air from the attik can that be my problum?
I have no choice but to keep
I have no choice but to keep my ducts in the attic and it gets 106 in eastern oklahoma. if i add a gable fan or turbine would that make any difference at all to keep the duct work from heating up? old small 1000sf house,no other vents but the gable.
We are homeowners researching
We are homeowners researching central air for our house. This blog is very interesting and eye-opening.
Our house is a 2 story, forced hot water system without ductwork. A basement is below and an attic is above the main part of the house. A two room addition on the 1st floor has limited attic space and a 4′ crawl space.
It’s easy to see your point about eliminating ductwork from the attic; however, I have two questions if I want to centrally air condition my entire house.
o Is air conditioning the 1st floor effective with floor venting if I run ductwork in the basement.
o Can you efficiently air condition the 2nd floor by running ductwork fom the basement to the 2nd floor in wall joists and running flex along the floor joists with floor registers?
o Would all this be cost prohibitive?
o What is an average cost be to install an effective system in a 2 story house, like ours?
@Stu: floor diffusers are
@Stu: floor diffusers are fine for cooling your main level. It’s just a matter of diffuser placement (eg, avoiding furniture). You should be able to extend ducts from the basement to the 2-room addition through the crawl space.
This article addresses new construction. In an existing home, trying to retrofit ducts into a closed floor system above your main level can get crazy expensive (and messy) unless you’re doing a gut rehab. At a minimum, you’d have to remove your ceilings (or floors above). Also keep in mind that ducts cannot across joists bays unless they’re open web trusses. Bottom line, you’re probably better off running the supply ducts for the second floor in the attic. Just make sure they’re R-8, verified tight, and suspended horizontally just above your insulation, thus avoiding the hotter parts of the attic. You’ll need to find a corner, such as in a closet, to build a duct chase to get ducts from basement to attic.
You’ll also need to provide a return path from a basement air handler or main level return to the second floor. As long as there’s an unobstructed path (e.g., no door that can be closed), the upstairs doesn’t need a dedicated return. If you have a door at the top or bottom of the stairs, you can replace with a louvered door. Finally, you will need to provide return paths for the bedrooms. This is typically done with jump ducts or transfer grilles (see http://bit.ly/oQtlYh for guidance). Tamrack makes a transfer grille with a honeycomb noise suppression filter.
One more thing, if your hot
One more thing, if your hot water is electric, propane or oil fired, you should pay a few hundred extra for a heat pump, which would save $$$ on your heating costs. You could use your hydronic heat to pick up the supplemental load during mid-winter.
You might also want to consider ductless mini-split heat pumps.
I have a 15 year old heart
I have a 15 year old heart pump system (actually 1 on each side of the house) with vents in the slab. I want to put in new heat pumps in the attic. The HVAC contractor is saying I need to put condensers on the outside with the pump in the attic. I don’t understand, can you please help explain this to me. I thought it was all one unit.
@Mariann: a heat pump can
@Mariann: a heat pump can either be a package system, which has one cabinet mounted against foundation or on rooftop, or it can be a split system, with two cabinets. In that case, the condenser is located outside and the air handler located inside the house, attic, garage or crawl space. The evaporator coil is in the air handler. Not sure what your contractor meant by putting ‘pump’ in the attic. The ‘pump’ in heat pump is actually the compressor, which is located in the outside (condenser) unit with a split system.
Hope this clears things up.
Thank you David, that was
Thank you David, that was clear and easy to understand unlike my contractor. I’m thinking I need to find a different contractor. This one had great references but is not clear when you need an explanation.
Mariann: David summed it up nicely. If you want a little more detail without much jargon, check out this article I wrote last year:
The Magic of Cold
I agree that central HVAC
I agree that central HVAC ducts in the attic are ridiculous. Most of the commentary is about cooling, but what about heating? I’m in Sacramento, and in spite of our hot weather reputation, it is primarily a heating climate.
Retrofitting my house would be expensive and disruptive. When I look in my attic, I wonder if a lot of the ductwork couldn’t be shortened or eliminated. Do I really need a separate duct to a small bathroom surrounded on three side by conditioned room space? Do I really need to duct air all the way to the side of the room farthest from the distribution plenum? I think I could get rid of at least 50% of the ductwork up there, for almost no cost.
Jerry, whether or not a given
Jerry, whether or not a given room needs supply air depends on wall, floor and ceiling exposures, and especially window exposure. I’ve designed a number of systems without a supply in small baths pantries and walk-in closets that have minimum exposure, but typically not if there’s an exterior wall.
The other point you made about moving diffusers back from the perimeter is valid, but again, it depends on how good your walls and windows are. The more efficient the shell, the less important diffuser placement and throw patterns become.
In any case, if you’re going to change your duct system, you need to start with an accurate room-by-room J load calc.
Interestingly, the DOE Challenge Home program conducted a webinar this morning on different methods for installing ducts in conditioned space, as required under their new guidelines. The presentation focused on burying ducts in the ceiling insulation.
In humid climates, this is a condensation risk. The Challenge Home program mitigates this by requiring R-8 ducts to be covered with 1.5″ of closed cell foam, with a minimum depth (can’t recall the specs) of blown in insulation above the ducts. The presenter made the point that the 1.5″ foam layer isn’t necessary in dry climates, although I don’t think the program makes that distinction.
Spraying the ducts with 1.5″ of foam would be expensive, and may not be worth the savings, depending on a number of factors.
Before I would consider that, I would first make sure the ducts are very tight and make sure the boots and plenums are well insulated. Also, it’s important to keep the duct runs horizontal and as low as possible, supported just above the insulation (unless of course you’re going to completely bury them).
due to heigh ceiling builder
due to heigh ceiling builder has mounted flex ducts between rafters and are placed against roof deck,im having foam insulation spray at roof deck,only an inch between duct and roof deck, sheet rock will be secured to rafters.location is south florida please advice.
@Erc, you may be boxed into a
@Erc, you may be boxed into a bad situation with no other alternatives. Those ducts not only displace a hell of a lot of insulation in the rafter cavities, but will be exposed to very high heat gain through the roof deck.
If your spray foam will be open cell, I would get them to spray a flash coat (1.5″) of closed cell (has roughly double the r-value) to mitigate the additional heat gains. Better would be to install a couple of inches of XPS on top of the roof, if shingles have not yet been installed. Or you could look for other pathways for ducts, for example, fur-downs and/or high wall diffusers on interior walls.
My hvac contractor wants to
My hvac contractor wants to spray my existing attic ductwork with foam to stop leakage of air in my old ductwork. Does this work and is it feasible to do this?
We have lived in Spring Texas
We have lived in Spring Texas since 1977 and due to two hail storms in that period have had two roofs installed. New house had shake shingles which breathed well but after several homes burned the HOA began allowing asbestos shingles – as long as they were one of the darker shades.
I have always wondered why white or at least lighter roofs are not permitted.
The Greeks have been using it millennium.
Wouldn’t that severely reduce the ambient heat even entering the attic in the first place?
Also if the shiny stuff is below the shingles, does it reduce the shingle life with the constant baking process?
I live in Montreal and had a
I live in Montreal and had a Lennox air conditioner and ducts installed in the attic. I was wondering is there any figures on the expected energy/temp lose. What I’ve observed
Attic Temp 50C -> Vent Temp 21C
Attic Temp 37C -> Vent Temp 18C
Attic Temp 25C -> Vent Temp 13C
Not sure what the R-Value off flexible duct or main metal duct is.
@Alberto, discharge air
@Alberto, discharge air temperature shouldn’t exceed about 15C (59F).
The supply air temperature at the vent isn’t just affected by heat gain in the duct system. It also depends on
a) return air temp
b) outside air temp
c) airflow across evaporator coil
d) return air leaks from attic
If you measure supply and return temperatures at the grille and at the air handler, you can get an good idea of where your biggest losses are occurring. At 21C, I’d guess you have a lot of leaks on the return side.
Flex duct has been around for a while, and the newer flex ducts usually are better R value.
Newer ones will tend to have an R6 or R8 rating.
The silvery colored ones add radiant barrier to that.
Older ones may be more like R2
Thank you for your comments.
Thank you for your comments.
I haven’t gone up in the attic, since it’s been hot up there…will it be easy to measure the temp.?
Also can it be due to low freon? I haven’t had a techincian check it since last year. I did a simple visual check of the pipe outside and there was water on it.
@Alberto, a low refrigerant
@Alberto, a low refrigerant charge cannot explain the wide variance in supply air temperature you observed.
To measure the inlet and outlet temperatures at the unit, you’ll need a suitable temperature probe. A good digital meat temperature probe will work. I suggest the Taylor 9842, available on eBay or Amazon for under $15. This requires that you drill small holes (4mm) in the metal plenums or ducts closest to each end of the air handler or furnace. Do not drill holes in flex duct, and never drill into the cabinet where the refrigerant coil is located!
You can set up in the early AM when its cool in the attic but you’ll need to conduct the test when the attic is hottest (it doesn’t take but a few minutes). Be sure to measure temperatures at the main return grille and a representative supply grille at nearly the same time. When you’re finished, cover the holes with electrical or duct tape.
My builder in St. Augustine,
My builder in St. Augustine, Fla. insists on putting my R-6 flexible ductwork in the attic. My plan is too ask him to have ductwork sprayed with 3-4 inches of closed cell foam and then insulate the whole attic with 12 inches of cellulose. Any opinions?
@den, you should check out
@den, you should check out this DOE webinar, which has a section on coating ducts with foam.
DOE Webinar – Ducts in Conditioned Space
If you go that route, I believe you need to use metal duct instead of flex. Unless you’re using closed cell on other parts of your home, spraying just the ducts may be cost prohibitive.
We just purchased a 1955
We just purchased a 1955 ranch-style home here in Dallas. The house is single-story and has a 3-5 ft crawlspace underneath. The prior owner just had a new AC system installed in which they switched from the equipment and metal ducts in the crawlspace to flex ductwork/equipment in the attic (where temps have regularly been in the 130-150 degrees this summer). The old ductwork and vents are all still in place, and in good shape (even insulated). The new system does not seem particularly efficient. My wife and I were considering having the ac system moved back to the crawlspace. We owned another home with old crawlspace ductwork in place that had been abandoned in favor of attic flexduct, so this isn’t our first time looking at this. Any info? Thank you for the great blog.
@ the person from St.
@ the person from St. Augustine, Fla. who wants to spray closed cell foam on the ducts…Why? Its a band-aid approach. Granted, its a cooling climate , I assume, although heat run through the ducts at any time might release isocyanates into the home, whats a better choice you ask; build a frame within the envelope where you could to hold the ductwork…thats the best approach, or get a new builder!
Spraying closed cell foam on
Spraying closed cell foam on ducts will not release anything when the ducts are carrying hot air.
Once the foam “cures”, it doesn’t release anything unless you take a blow torch to it.
@Brandon, if ducts are
@Brandon, if ducts are original, they’re unlikely to be sealed. Metal ducts are naturally very leaky. This isn’t just an efficiency hit. Leaks on the return side will draw air from the crawl which is often humid and worse.
There are a couple of ways to seal existing metal ducts… spray sealant (Aeroseal) on the entire interior surface, or brush on mastic along the seams from the outside (requires removal of insulation wrap).
Either way, sealing in-place metal ducts can be an expensive proposition. You might want to investigate the cost of encapsulating the attic as an alternative to moving the system.
But first, you should contact the previous owner or the installing contractor to find out why the crawl duct system was abandoned.
I am told there was a class
I am told there was a class action lawsuit involving 260 plaintiffs, involving a major builder who had many roof deck failures using foam at the roof line in a hot-humid climate. And that the settlement included a requirement that the plaintiffs not discuss the matter. I do know Pulte Homes built with this design at one time in Cinco Ranch (Houston TX), and will not build that way today. Wish I could cite some good references, but hearsay is just about all I have. But this hearsay comes from pretty good people.
For someone learning building science, it is maddening to hear of evidence being suppressed. If someone could inform us what must be done to prevent such roof failures, that would go a long way toward settling these uncertainties in my mind.
M. Johnson: I talk to a lot of people who know a whole lot about what’s going on in the industry, and that’s the first time I’ve heard about this lawsuit. Doesn’t mean it didn’t happen. I just think I’d have heard something if it did.
I addressed the roof rot issue in a previous article:
Will Open-Cell Spray Foam Insulation Really Rot Your Roof?
That would be a better place to post your comment.
I have been saying this very
I have been saying this very thing for years. The books that I learned from stated that in general terms that the systems size had to be increased 1/2 ton for every 100sqft of exposed duct in an unconditioned space, that is both supply and return.
I have my duct work in the
I have my duct work in the crawlspace of my home its about a 4ft crawl. Here is my problem. My floors have soaked the moister from the condensation of air vent that now it has rotten my joist where the vent touched them and subfloors are as well. My home is 20 yrs old. is this normal? should I have the ducts lowered away from joist a few inches?
You are saying that your floors are wet from condensation from the air vents, and the joists are rotted where the vents touch them. Do you mean the vents that actually blow air into the room, or the ducts that run through the crawlspace and are fastened to the joists?
If it is the actual vents, then you have a bigger problem, but the solution is the same regardless of whether it is the vents or the ducts that are making the condensation.
If it is the vents, you can’t lower those or make them not touch the floor or joists because they stick through the floor and blow air into the room.
Assuming it is the ducts, the short answer is that you can lower them so they don’t touch any wood. That will not stop the condensation though.
Doing that will allow you to put insulation on the ductwork which will result in less condensation. Its a good idea for energy efficiency too.
The real source of your problem is too much humidity in the crawlspace. That can be from several causes. The most common is that crawlspace vents are commonly left open during the summer, and warm humid air comes into the crawlspace. The cold air blowing through the vents then condenses the humidity into water which rots your wood.
Keeping your crawlspace vents closed is the simplest solution, and will most likely solve that problem.
You may have a high water table in your area and moisture is getting in through the walls or floor of the crawlspace.
If that is the case, you may need to put a dehumidifier in the crawlspace (that drains into a sump pump) to lower the humidity. You can also have one or two vents put into the ductwork to blow air conditioned air into the crawlspace which will lower the humidity, but remember to shut those in the winter so you don’t waste heat, and remember to open them again the next summer.
@Ira, it is not a good idea
@Ira, it is not a good idea to close a crawlspace (in a humid climate) that wasn’t designed that way. In particular, there must be a continuous vapor retarder on the exterior walls and ground. Trying to dehumidify a non sealed crawl would be an exercise in futility.
Since this is a vented crawlspace, condensation can easily be eliminated by properly insulating supply ducts with a product that has a continuous vapor barrier on the outside, with complete coverage of boots and every cold metal surface, and all joints taped. As long as moist air can’t reach cold metal, there’s no possibility of condensation. The air handler itself has internal insulation on the supply side to avoid this problem.
If you have a wet crawl floor, you should address the source (roof and site drainage away from foundation, and/or sump pump at low point if water is coming from ground within the crawl. But that has nothing to do with wet ducts. In a humid climate, uninsulated ducts in a vented will always sweat, and properly insulated ducts will not sweat.
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