How Much Air Leakage in Your Home Is Too Much?
Whether you want to build a new home or fix an old one, the way to ensure that you get the best performance is to do the building envelope right. That means installing the right amount of insulation and installing it well, and it means having an air barrier with minimal leakage. But how do you know when you’ve done enough air sealing? How tight is tight enough?
I get asked this question a bit, and I love to talk about the measures for air leakage anyway, so let’s dive in. First, of course, you have to be able to measure how much air leakage the house has. That’s what a Blower Door is for. (If you’re completely new to Blower Door testing, make sure you read our section about testing for air-tightness.)
Then you have to choose how you want to specify the air leakage. The most common unit used by Blower Door operators is ACH50, which stands for Air Changes per Hour at 50 Pascals. I prefer cfm50 per square foot of building envelope, or better, cfm50 per hundred square feet of building envelope (sfbe). (A cfm50 is a cubic foot per minute at 50 Pascals.) The two reasons for that choice are that (i) air leakage happens at the surface, not in the volume, and (ii) it’s the best unit, in my opinion, to express what a Blower Door is really good at – measuring the amount of air moving across the building envelope at elevated pressure.
Please don’t talk to me about ACHnat (‘Natural’ Air Changes per Hour). I loathe that measure! If you’re using a Blower Door, you can’t measure it, and only researchers use tracer gas analysis.
Now we’re ready to discuss the actual question: How much air leakage should you aim for? OK, we’re not really there yet. I lied. First, we have to know about your house. Are you building new or fixing an existing home? If the latter, what’s your budget and how complex is the building envelope?
Let’s start with the easy one: new construction. The rule here is that a house can never be too tight. The Passive House program takes houses about as far as you can go with air tightness, and their threshold is 0.6 ACH50. I tested a net zero house a couple of weeks ago that was at about 0.5 ACH50. That’s really tight!
A target that’s more achievable for anyone – and which the 2012 International Energy Conservation Code (IECC) will require for most climate zones – is 3 ACH50. That’s also the level that Joe Lstiburek identified as a good target in his great article on Blower Door testing new homes, Just Right and Airtight.
That number — 3 ACH50 — translates to about 0.25 cfm50 per square foot of envelope, or 25 cfm50 per hundred square feet of envelope. Since roofers have already abbreviated 100 square feet as 1 square, I like the latter form the best. It gets it into a whole number form and is easy to remember. Get your Blower Door number down to 25 cfm50 per square (or below), and you’ve got a tight house. The house I built ten years ago came in at 14 cfm50 per square (1.7 ACH50).
The places to pay careful attention to in new homes are funky transitions in the building envelope, band joists, top plates, bottom plates, and myriad other details. The simpler the building envelope, the easier it is to find and seal the air leakage sites. We’ve published quite a few articles on this topic here, so click on the air sealing, air barrier, or building envelope tags to tags to the right to read about some of these details.
This can of worms is really too big for the little article I’m writing here, but let me try to shed at least a little light on it. (Hmmmm. Do worms even like light?) The amount of air sealing you’re going to be able to do in an existing home is limited – unless your budget isn’t. Generally, with a good attack on the holes, you can get about a 20% to 30% reduction in your Blower Door number (whether you specify it in cfm50, ACH50, or cfm50/sfbe). Sometimes you can get much more when you add surface area or volume by moving the building envelope.
The first thing you want to know here, though, is: How leaky is your home to start with? If you’re already at 25 cfm50 per square, it’s going to be really hard to get a 20% reduction. If you’re at 100 cfm50 per square, it should be a snap to reduce it to 75 cfm50 per square or even lower. The higher that number is, the more big holes you probably have in your building envelope.
If you’re starting at 100 cfm50 per square, however, don’t count on getting down to 25 cfm50 per square unless you’re doing a Deep Energy Retrofit. Sometimes Deep Energy Retrofits are called Deep Pockets Retrofits, for obvious reasons. The cost a fortune! Check out Martin Holladay’s recent post called The High Cost of Deep Energy Retrofits at Green Building Advisor.
I’ve done a number of air-sealing jobs when I was in the contracting business, and the results varied. Sometimes it was a frustrating 5-10% reduction, and other times it was an easy 20%. The most reduction I ever achieved was about 40% in a house that started with about 120 cfm50 per square. We encapsulated the crawl space and did a lot of work on the kneewalls and can lights on the upper floor.
If you’re looking to make your home tight and you really want some numbers to go after, here are my recommendations:
New Homes: 25 cfm50 per hundred square feet of building envelope (or 3 ACH50)
Existing Homes: Get a Blower Door test, and see if you can reduce the number by 20% or more.
Infiltration Occurs at the Surface, Not in the Volume
It’s the Hole – Understanding What a Blower Door Is for
This Post Has 28 Comments
For old houses:
Since we already know every production house ever built can benefit, why do we need to use time money to tell us something we already know? That does not sound like science, more like sales schtick or performance art.
Why not just show up and start.Why not make
make every action and cost count?
For folks looking to take time and cost out of the HP business model, is this not a good place to start?
pj: Good point. I wouldn’t go so far as to say that “every production house ever built can benefit,” but yes, a good crew can certainly go in and find a lot of the problems with a visual inspection and then test afterward. I know someone who used to say you could write your evaluations based on satellite photos. I think that takes it a bit far, but he makes a valid point.
Still, there are good reasons to test:
1. Some programs and types of financing (like Energy Efficient Mortgages) require it.
2. A homeowner probably wants to know how much improvement they got for their money.
3. Some leaks are difficult to find without a Blower Door. See the guest post on spray foam problems last week.
So, if you have a really,
So, if you have a really, really, leaky place, is it advisable to begin blower door testing with a more powerful blower door (i.e., commercial grade, higher HP fan, etc.) so you have a better chance of achieving your target pressure? I’ve heard tell of homes where the target depressurization can’t be reached, so a measure of ACH50 has to be extrapolated. Does this have any effect on the final results, or save time, or cost?
BTW, my houses are SO leaky, I currently measure them in units of SCH (squirrel changes per hour) or SPM (squirrel paws per minute) per square. Seriously. 😀
pj: One more reason to go ahead and do the Blower Door test upfront is that it’s extremely easy to do. I can set up and run the test in less than 10 minutes.
John P.: No, if the house is super leaky, accuracy isn’t so important. Using the extrapolation (finding the leakage at whatever pressure you can achieve and then multiplying by the Can’t-Reach-50 Factor) is all you need to do. (I assume you’ve derived the equations for SCH and SPM by solving partial differential equations, right?)
Well, of course I derive the
Well, of course I derive the SCH and SPM equations by solving PDEs. But it’s not strictly necessary. I just do it gratuitously. And for the arcane symbolism. 😀
And don’t forget that
And don’t forget that whenever we are air sealing we are preventing moisture drive. As moving air will increase your energy bills, moving moisture through our building envelopes will decrease the life of the home. We actually seal our PUR structural panels on both the exterior AND the interior seams rather than a single bead down the joint. Most people forget that you can have moisture drive from both the interior or the exterior of the building envelope and it is that “space between” that you want to keep pristine and untouched.
One two-story home that we monitor – that only used our panels for the walls and not the roof – measured 0.48 ACH50 and 0.039 ELR for you Southface/Earth Craft folks.
Charles: I agree. It’s best to seal walls from both sides. It sounds like they did a great job with the air sealing details in that home you describe.
I think we agree.
We built production houses to two specs:
1. Totally Sucks
2. Sucks worse
Pulte is fighting minimum wage violations even today.
We are wasting over a years worth of savings with an “Assessment” that tells us:
1. “we can save some energy”
2. The bureaucracy requires it
These don’t sound like they make any sense, why not show up with a good seasoned crew leader and go to work.
It takes hrs to make the appointment, drive to the house, play with the smoke bombs, laser beams, blower doors, x-ray vision classes, etc,then drive ace to your office, play with your comp-u-tator, then sell the job, then go back and do the job.
Why not just show up and go to work?
It just seams like a high tech waste of time, money, and appears to most closely match the old Kirby vacuum cleaner sales pitch.
And yes, I agree with your friend Michael Blasnick, we guarantee we can ourselves we can save money with out going to the house.
love you posts,
Pj: I certainly understand
Pj: I certainly understand your points, but Allison’s point is very simple. It’s about achieving MEASURABLE results.
There are a lot of contractors out there that walk in a door and say “Sure we can fix that and we can cut your energy bills by as much as 50%.” Oh Yeah? PROVE IT! and of course they can’t. Just ask the window industry which has recently been the target of the FTC for making overblown claims of energy savings of 50% when in reality replacing all the windows in your home will rarely ever achieve fraction of that kind of savings.
A blower door test is not that complicated (as long as you know what your doing), involves no bureaucracy, lasers, or thermography, and is not a “sales pitch”. It is merely a tool that can give you those measurable results to prove that the improvements you promised and the home owner is paying thousands of dollars for, are actually being achieved. Otherwise, what ever your telling them is just that, a “sales pitch”.
I don’t know you from Adam, but for every one of us that does business in an honest manner, there are 3 or 4 out there that don’t. If that weren’t true, then I wouldn’t have a lot of customers that have already had “improvements” performed on their homes, yet I’m out there trying to solve their original issue.
PJ said- “And yes, I
PJ said- “And yes, I agree with your friend Michael Blasnick, we guarantee we can ourselves we can save money with out going to the house. ”
I say — huh? (and Blasnik has no c)
I actually think people should use blower doors to air seal houses. I’m also a fan (pun intended) of blower doors and infra-red even during the audit visit — because they don’t take much time and provide useful information. Not every house needs air sealing and some homes need a lot more air sealing than others and it isn’t always obvious.
I don’t think there is any responsible way to air seal homes without using a blower door and once you need the blower door to check your work and assess ventilation needs than you might as well use it to test-in too.
I’m not a fan of duct blaster testing or elaborate energy modeling because they take more time than they are worth. We can use pressure pans instead of duct blasters (because we have the blower door already there) and we can use simpler models rather than complicated models for most homes.
Each diagnostic test has a cost and a potential benefit and it’s not always easy to figure out when you should use which tests. It’s especially hard if you are trying to design a program and need to come up with rules. I think there are many programs erring on the side of too much testing but there are also some with too little.
pj: Here’s another reason to do the testing: Even if we know a house was built by a builder who did a crappy job on it, we may not know what has been done on it in the meantime.
Jon: Yep. You can’t have measurable results if you don’t measure.
Michael: I’d love to see us be able to get away from Duct Blasters on new homes, too. Those tests take a lot of time, which we can save by putting all the ducts inside and getting the envelope tight enough. (See you in Baltimore!)
I can’t imagine a discussion
I can’t imagine a discussion about BD targets for a new home without addressing cost vs. benefit. The cost-benefit of achieving 3.0 ACH50 would look very different in Buffalo than in Tucson, on multiple levels.
Local energy costs and climate (including latent load) drive the benefit side, while local practice and crew skill levels drive the cost side. Not surprisingly, I often find it costs less to achieve a given target in cold climate zones.
As with much of what we do, there’s no one-size-fits-all. We should always ask ourselves which combination of efficiency measures will provide the lowest energy usage within a given budget, or in cases where there’s a specific energy goal, what’s the least expensive combination of measures that will achieve that goal.
David B.: Great point, and that’s why we need your input here. Besides, I can’t put everything in the article itself or we’d have no discussion in the comments. ;~)
Yes, of course, the benefit of tight houses is greater in the more extreme climates, especially those that are really cold or with lots of humidity in the cooling season. But the cost of getting to 25 cfm50 per square should be minimal. It’s mainly getting past the learning curve, and once builders figure it out, the extra cost for labor and materials should be minimal. Joe L. picked that number (well, its ACH50 cousin anyway) as being a reachable target even for production builders, so I don’t think that expense is a good reason not to use that as the threshold. We’ve had builders here in Georgia getting there just by trying to hit our energy code target of 7 ACH50.
Now, if I’d said 1 ACH50…
You also said:
We should always ask ourselves which combination of efficiency measures will provide the lowest energy usage within a given budget…
But air sealing is about more than just efficiency.
This is certainly a good debate to have, but let me ask you this: If the extra cost is minimal and you yourself would never build a house even that leaky (3 ACH50), would you really recommend leakier houses for builders who don’t want to do the work? And what would your threshold be? Or do you not agree that the extra cost is minimal? Or that there should be a threshold at all? I’m interested in hearing your thoughts on this.
For a “simple”
For a “simple” house what counts as square feet of building envelope? Clearly above ground walls/windows/doors and top-floor ceiling is in. What about basement walls & floor? I’d like to translate my last-measured 1400cfm50 into “per square.”
Eric: Great question. I didn’t really explain that in the article, but it would the whole area of the building envelope, including the below grade portions and a slab-on-grade. The Energy Conservatory uses a measure they call the Minneapolis Leakage Ratio that excludes the below grade portions, but I think all of the envelope should be included. That way, when you compare a house with a basement to another house with a vented crawl space, they get credit for the extra below grade airtightness.
Allison, thanks. I guess I’m
Allison, thanks. I guess I’m somewhere around 40 in this old house. Easy things are done. 😉
As previous commenters have
As previous commenters have noted, pre- and post-improvement blower door tests are a terrific analysis tool, and the measurable improvement gives the homeowner confidence that a quality job was done. I would like to see more duct blaster tests performed as well, for this very reason.
I do see some merit in pj’s comment about trying to make the energy retrofit process as streamlined as possible, particularly when stimulus dollars are in the mix. I would never give up blower door and duct blaster tests, but I do wonder about some energy programs requiring auditors to do extensive energy modeling on every house. Energy modeling is important, but I think this level of work is best left to researchers and energy geeks, who in turn can establish some key targets that energy retrofits should achieve (acceptable CFM50 per square, insulation R-value in the attic, etc.). I am concerned about proposed energy tax credit legislation that awards credits that are proportional to theoretical performance improvements. I see a great potential for user error, variation among software packages, or gaming the system. Allison, I would be curious to hear your thoughts on this.
I keep running into homes with beautiful looking insulation work.
Only problem, the homes are really leaky, and they have high energy bills (75c sf/yr)
The insulation guy thinks he does great work because it’s pretty. He has no clue. And since nobody tracks, the ho assumes they are saving the earth even though their consumption hardly budges.
If you don’t measure before the diet you have no idea what you’ve accomplished, can’t set meaningful goals, or measure progress toward the goals.
Without a blower door you can’t find the leaks your pretty insulation work missed. All you’ve got is a loose knit sweater on a 5f windy day.
Allison wrote: <
>…would you really recommend leakier houses for builders who don’t want to do the work?
No, that wasn’t my point. I just reject the idea of a national one-size-fits-all BD standard for new construction. Just as you recommend a more nuanced approach for existing homes, I try meet a builder where he’s at (in terms of air tightness) and move down from there by helping him identify things he might be missing. If the builder hasn’t had a blower door test before, I encourage him to have a recently completed home tested (often a non-starter). Each project is different.
I read Joe’s article, and certainly agree with his “it’s just a tool” philosophy, but I think we can (and should) do better than 3-ACH50 in the coldest climate zones, especially for homes without access to natural gas. But in hot climates, I’d rather fight to bring ducts inside (always challenge in non-basement homes) than to arbitrarily specify a 3-ACH50, especially in dry climates where latent loads are negligible.
In my area, for example, there are no air sealing crews per-se and the only crews who even talk about air sealing are the foam guys. Also, new homes around here are naturally tighter than back east. For example, my current home is a production built slab-on grade with EIFS and tight vinyl sliding windows. It tested out at 2.8-ACH50, and I know for a fact that no special effort was made to air seal, other than using AT ceiling cans. Back east, the ES provider I worked for ran an air sealing crew and we had to work hard to reach our 0.35 CFM50/ft2 target (4.5 to 5-ACH50). At the time, we charged 25 cents per ft2 for air sealing new homes for ES builders. Most were multi-story brick veneer built on crawls with double-hung windows. Add to that a bunch of knee-walls, dormers and architectural gobbledygook. I’m not sure what it would cost to get those home to 3-ACH50, but these were builders who had never had a blower door test before. Plus, electricity was cheap – less than 8 cents/kWh. Walk before run.
I’m afraid my previous comment made it sound like I come up with some precise number for each project based on all the factors. That’s not the case. I was just trying to make the point that cost-benefit must be part of the discussion.
> …air sealing is about more than just efficiency.
Pleae explain. Are you referring to IAQ? (note: this is a trap)
“when stimulus dollars are in the mix”
my son is 25 and I still support him, I give him money.
The Home performance biz is over 30 years old and I still give it money. (tax dollars, and utility tariffs).
My hope for both is they can get to the point where they can stand on their own two feet. I see no way for HP to work in the real world with all the cost going into an assessment.
Repeat: Every Single Production House ever built was built to the lowest possible cost and they all need work. So far, the only reasons I have heard for all the test in Performance Art is Feel Good, and We need it for bureaucrats, Hardly sound reasons.
You waste a years worth of savings going to the house, playing with your toys, going back, playing with your comp-u-tator, For What, to tell me something you knew before you left your office? Why?
As far as I’m concerned, you can play with your blower door till the cows come home, just don’t make me pay for it. Do you think the customer will pay for it?
I am the first to agree the models are Bogus, and I guarantee I can model savings as accurately from a sailboat off the coast of Borme as you can from their kitchen. The big problem here is the customer will change their lifestyle post retrofit, so all bets are off. When you think about it’s silly.
Once upon a time there was a concept called “Capitalism”. An idea where goods and services flowed to the natural best use and economic activity was fostered. My hope is Building Scientist someday learn about It.
I share your distaste of wasteful processes and red tape. I performed about fifty home energy audits last year which were subsidized by local nonprofit groups funded by the stimulus act. There is a frustrating amount of inefficiency in the workflow, and that will have to change if the process is going to be able to stand on its own, as you say.
But I wouldn’t lump the blower door test in with the stuff that needs to be thrown out. As Allison said, an experienced auditor can get the test set up in about ten minutes. I’ve worked on day-long air sealing jobs that yielded anywhere between 10% (frustrating) to 30% (satisfying) reduction in air leakage. It’s importanat to know what you’ve achieved. I wouldn’t want a carpenter building me a wall without testing it for squareness, or a plumber working on my pipes without testing for leaks. Finally, there are always some hidden air leaks that simply won’t be found without a blower door test, I don’t care how experienced the contractor is.
Maybe getting a little OT for
Maybe getting a little OT for the original post, but there are programs out there to skip past the blower door test and go straight to the most basic upgrades. http://thenec.org/programs/home-energy-squad for example. In terms of air sealing, they simply weatherstrip the outside doors. A small step, but a quick & efficient one.
While I agree that one can write a comprehensive work order from a remote location I do not agree with your sentiments that no audit is necessary. These provide info necessary to have inventory, materials, work orders. Sure I can walk up to a house and fix most of the problems, but is that a realistic approach for every crew everywhere?
“I see no way for HP to work in the real world with all the cost going into an assessment” What?! What assessments are you talking about? Rarely do assessments cost anywhere near 10% of the retrofit work being performed. A thorough comprehensive audit can be performed in a few hours that leaves the homeowner with an easy to understand audit report that allows the homeowner to pick their own HPC and fixes based on the recommendations I make. Also, zonal pressures performed with the blower door allow me to guide air sealing i.e. why air seal gaps cracks and holes that are already tight? Keep in mind, most air sealing crews aren’t made up of a team of building scientists(at least most of the ones that I know, my apologies if you are the exception)its John Doe laymen. As a laymen, would you rather see, “Air seal, install ventilation, insulate house” on a work order or ” Air seal chase in Back corner of LR, Air seal plumbing penetrations under master bath jacuzzi tub, insulate kneewalls to R13, and back with drywall, install 50cfm of continuous ventilation in half bath and pressure relieve all doors”
IR cameras and the BD are often used as a visual supplement to data that is collected via the blower door. I often find that IR images are more effective as a visual for the client as opposed to a strictly diagnostic tool. Hand the client the camera and walk around, and tell them about what they are seeing. This builds rapport with the client, which sells the job.
All in all, the audit is a critical part of the process(in my opinion)good luck with your approach…
PJ – your “just go in
PJ – your “just go in and do it” argument fits about like this: I’ve got a sore throat, but instead of the doctor taking the time initially to visit with me and make a proper diagnosis, he’s going to give me medication to treat the mumps, chicken pox, malaria, emphysema, athlete’s foot, pink eye, swimmer’s ear, and typhoid fever, hoping something will help. Where is the cost savings there? So maybe I’ll get better after spending more money than I needed to for fixes I didn’t need (and I have no idea what cured the ailment), or I still have a sore throat. Then what? Doc throws more stuff at me, costing me more money, or I live with a sore throat, because after all that cost, it’s obviously something that cannot be fixed.
There must be a diagnosis (initial blower door test) so the problem can be defined, targeted, treated properly; followed up (this is gonna hurt, PJ) by a second blower door test after the work is completed to show the homeowner results they can see/touch/understand/believe in.
Now read this twice through, get plenty of rest, and blog me in the morning.
I think that whether or not a
I think that whether or not a blower door test is done depends a lot on the scope of the proposed work. If you’re coming in for a quick fix on an old, obviously leaky and the doors, windows, and attic hatch have visible air gaps, it might be worth “just doing it” if that’s as far as the homeowner wants to go.
If you’re going for a serious whole-house air-sealing operation, you (and the homeowner) would probably want before & after blower doors to justify/verify the work…
Carrie, I posted this on
Carrie, I posted this on Allison’s blog today, but the same answer applies to your post. I am happy to prove the on site assessment is nothing more the a high tech version of the kirby Vacuum Cleaner sales pitch for any production house.
If you are telling me you can’t show up with a good crew chief and install team and go right to work remediating a production home, I would get a new crew your guys should not be there at all!
The savings from not doing the performance art can make your business model work without socialist policies and OPM.
I designed the HVAC systems for thousands and thousands of houses and if I designed the duct system in your home, guaranteed it sucks! We worked on a team for Carrier calling on the top 25 production builders and spent most of our time in Florida, Texas, and California. I recall meetings where we would be fighting over sixty cents a system, high volume RNC is all about lowest cost.
Since we had no idea what direction a house would face, all plans were assumed to face East-West to CYA on the loads. The Houses all got the same duct design regardless of orientation for the same reason. One large contractor even started a soccer club to entice enough migrant labor to install the ducts in the roughly 4-500 houses a week he was doing. So if I did my job right, the AC was oversized at least 75% of the time, the Ductwork system design was wrong 75% of the time, always installed in hot attics, we were lucky if everything was connected, we installed in the cheapest AC units produced and our competitors had to do the same.
This is why the whole idea of an “assessment” is a big waste of time and money, I know your house Sucks, I designed it.
Most of the information on your post is directed towards Residential.
Will most of this information also apply to commercial bldgs.?
Hi Allison, what do you think
Hi Allison, what do you think about the combination blower door IR scan? Or would you prefer a test with a smoke producing device?
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