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Your HVAC System Is the Faucet; The Building Envelope Is the Cup

Hvac Building Envelope Insulation Air Barrier Faucet And Cup

hvac building envelope insulation air barrier faucet and cupLast Saturday, I was doing some stuff around the house, and my wife had the radio tuned into one of those home repair talk shows. I don’t listen to those shows much because they usually say something that drives me crazy. They do seem to be getting better, though, and what I heard last Saturday indicates that a little bit of building science and house-as-a-system thinking may be making its way out into the trenches.

Last Saturday, I was doing some stuff around the house, and my wife had the radio tuned into one of those home repair talk shows. I don’t listen to those shows much because they usually say something that drives me crazy. They do seem to be getting better, though, and what I heard last Saturday indicates that a little bit of building science and house-as-a-system thinking may be making its way out into the trenches.

Anyway, the host of the show was talking to his guest (an HVAC contractor, I believe) and threw out a metaphor that I’d never heard before. Here’s my paraphrased version of what he said:

“Your heating and cooilng system is the faucet. Insulation is the cup. If your insulation isn’t adequate, a lot of that heating and cooling you’re providing will leak out.”

I thought it was brilliant! The only problem is that he caught only the insulation part of the building envelope and didn’t mention the air barrier, which is a hugely important part of the cup. Still, it’s a great metaphor with the substitution of ‘building envelope’ for ‘insulation.’


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Photo by SMercury98 from, used under a Creative Commons license.

This Post Has 18 Comments

  1. Home talk shows are getting
    Home talk shows are getting better. It seems that air sealing is the part which confuses and eludes the old guard. Air sealing is an intangible thing, subtle and difficult to sell. Insulation on the other hand, is an obvious PRODUCT which does no harm, so they are motivated to sell the heck out of it. As a result where I live we have many houses where the fiberglass insulation simply filters the air infiltration.

  2. It is a brilliant analogy!
    It is a brilliant analogy! The power of words is always amazing. The cup runneth over without a well sealed lid. I got a hole in my cup where it all slips away! 

  3. I use a similar analogy,
    I use a similar analogy, comparing the envelop of a building to the hull of a boat, or ship.  
    All boats leak, but we all know that big leaks are bad. If the hull is leaky, I can install a larger bilge pump, or two, but that is attacking the symptom, not fixing the problem, and eventually the boat sinks. In the case of our house, the home gets too expensive to operate, and may even sink – start to deteriorate. 
    The alternative is – Maybe I can patch the hull up and then I can save a lot of money by installing a smaller pump, and maybe if I do a really great job, the pump will seldom run at all. The result is a safe, dry, comfortable and mildew-free boat. 
    So; why do we think it’s smart to build a house that leaks a lot, requires a big pump (A/C & furnace) that runs all the time and costs us lots of money? In some cases, we even see people routinely cleaning mildew stains, thinking a few water stains on the ceiling or walls are “normal” and buying dehumidifiers or room A/C to attack symptoms of a sick and dying building. 
    Maybe we should fix the leaks by properly insulating and sealing air leaks. 

  4. Meh. I don’t like it. The
    Meh. I don’t like it. The exclusion of air sealing from the analogy just perpetuates the lack of understanding. Those shows drive me crazy, too, so I don’t watch them that much anymore.

  5. That is a good analogy. Yes.
    That is a good analogy. Yes. Thank you.

  6. M. Johnson
    M. Johnson: Yep. Also, there are a lot more insulation contractors than air sealing contractors. 
    Don P.: Glad you like it.  
    Geoff H.: I wrote an article a while back comparing homes to submarines, too. 
    John P.: See the title of the article. The metaphor still works but is more accurate if you substitute ‘building envelope’ for insulation. 
    Christopher C.: I agree! 

  7. So to torture it a little,
    So to torture it a little, the cup is an insulated travel cup – it should be well insulated and non-leaky. 
    Not sure what the analogy is for an inefficient hvac system to a faucet – a faucet that drips when you’re not filling the cup? 🙂

  8. As you mentioned in a
    As you mentioned in a previous article, building codes need to be written so that a maximum tonnage is allowed for a certain amount of sq ft. Let the builder decide how to make it fit, I’ll bet the air sealing gets done 🙂 
    I’m in the process of air sealing my own home, there’s not a lot of information on the Internet going into specifics of how to do it. Yes, you can find general points of where leaks occur, but not how to FIX these leaks. HVAC register boots in ceilings and electrical boxes are my current challenge, there’s no “backing” behind the cracks therefore caulking and expanding foam won’t stay put. The electric gaskets work OK IF the electric box is perfectly flush with the drywall, but how often does that happen in the real world? 
    Knowing where the leaks are is another issue for homeowners, few are willing to shell out the money for a blower door test to find them all. The big leaks can be felt as drafts (I’ve fixed the big ones in my home) but smaller leaks can be difficult to locate w/o a blower door test.  
    And why only R6 as code for ductwork? IMHO it seems silly to have such a low R value for the surfaces with the greatest temperature difference. Consumers won’t think anything of going from R19 to R38 attic insulation while keeping the less than perfectly sealed R6 ductwork.  
    It would be nice if homeowners could have an unbiased “low hanging fruit” list of energy upgrades. Put things that would have the quickest payback at the top and work your way down to things that aren’t worth doing in most homes.

  9. “Doesn’t matter what the
    “Doesn’t matter what the r-value is if the windows are open!”

  10. Bob 

    I understand your frustration. Here in GA there are new rules in place for new construction as far as tightness of the building envelope and duct systems, but that does not help existing homes or home owners. 
    As far as the locations for leaks, yes there are common areas, but every house is different. “Shelling out money for a blower door test” isn’t the only way to find leaks, but for now its the best way to find leaks. The cost of the testing will be offset by the energy savings you will realize once the improvements are made. You can get a blower door test done for a reasonable amount and the person performing it can give you a better idea on how and what to use to fill those holes. 
    As far as the ducts go…how about not sticking them in the attic in the first place? 
    To your last comment, again, every house is different. Improvements that will work in one house may actually create problems in another. If it were that easy, it wouldn’t be called “Building Science”.

  11. Good luck getting builders to
    Good luck getting builders to “do it right” and not put ductwork in the attic. Since home buyers typically go for lowest cost per sq ft and “bling factor” the HVAC system is low on the list. With existing construction it’s difficult to move the ductwork out of the attic w/o substantial costs. Builders could at least try to reduce the surface area of ductwork in the attic by using fewer large ducts (think trunk and branch) instead of a spaghetti mess of 6″ if that’s what’s on the truck that day. No effort is typically made to compensate for solar orientation, exterior walls/windows or duct length. Duct size is determined by the rooms sq ft. 
    Register are typically the lowest cost models also, even though the commercial grade perform SO much better. Sure the commercial grade fully adjustable registers cost twice what the cheap 4way builder grade stuff does, but IMHO they are worth every penny. Comfort is better because air can be more precisely directed, pressure drop is lower reducing duct leakage. I highly recommend register upgrades to those who can afford to shell out the approximately $20 per register. 
    The HVAC contractor is already at their bottom dollar just to win the bid to do the work. They do everything possible to keep costs down such as paying low wage workers to do the ductwork and equipment installation, and have higher wage workers make the electrical/gas/freon connections charge/test the system. It’s the only way to win the bid and still make some profit. The best contractors don’t even mess with low margin new construction work. 
    This is why insulating/duct sealing rarely gets done right.

  12. Bob I have to agree and
    Bob I have to agree and disagree with you. Previously, you are correct, no one sealed anything as far as ducts went. Now, at least in GA, on new construction the ducts must test at <8% leakage to the exterior when tested post construction. 
    I currently do the envelope and duct testing for a custom home builder here in the Atlanta area. On the first house I did, he failed the building envelope test, but both duct systems came in at <4% leakage to the exterior. They used builder grade 13 SEER units and registers. After I showed him what to do to “tighten up” the building envelope, he never flunked another one. This shows that some builders are willing to “do it right” and actually are willing to learn. 
    Also, you can purchase R-8 duct wrap and add that to your existing R-6 ducts. Its not the perfect solution, but its better than nothing.

  13. Allison, I agree with *your*
    Allison, I agree with *your* analogy and use of the term “building envelope”. It was the quote (even though perhaps paraphrased) about insulation being the cup that I was expressing my disapproval of! 🙂 

  14. Jon, does the GA code address
    Jon, does the GA code address things like using larger ducts for longer runs? If a register is 25′ for the furnace and most of the others are 10-15′ shouldn’t the 25′ run be sized up? Around here builders typically don’t size up long runs, therefore the rooms furthest from the furnace are always too cold/hot. Is there anything in the GA code about a maximum exposed surface area of ductwork? Here in Oklahoma builders think nothing about using lots of small ducts (one for each register) straight off a box on top of the furnace. IMHO a trunk/branch system substantially reduces duct surface area, therefore reducing losses. Does GA code encourage getting the ducts out of the unconditioned space altogether? This is of course the best option, but rarely happens here in anything other than apartments.  
    The fact GA even has a “green code” is a step in the right direction, wish we had something like that here. In Oklahoma code is system must keep house at 68 inside during the winter design conditions, and a 20 degree spread relative to the outdoor temperature in summer. Of course builders meet this requirement buy oversizing equipment since the wholesale cost difference isn’t that much, maybe $100 between a 45k and 100K furnace, $500 to go from a 2ton to 3 ton unit. 
    I am encouraged to see some local builders advertise HERS ratings, so maybe things will turn around w/o government regulation, which IMHO is the best way to do things. I love it when consumers vote with their dollars to make things happen instead of the government mandating it.

  15. Bob, what are these $20
    Bob, what are these $20 registers you speak of? Are they only for ceilings or do they have better ones for floors too?

  16. If you have ceiling/sidewall
    If you have ceiling/sidewall registers replacing those with commercial models will make a huge improvement. They also list floor registers but I don’t see it making as much of a difference unless the originals block a lot of air. 
    Commercial registers, I use the 821 series for my house:;
    For floors I’d go with a 265 series unless you need the damper, then for with the 210’s:

  17. 4 things I’ve found when
    4 things I’ve found when using commercial registers for the ceiling: 
    1: Do not try to use the 821’s to control air volume, you will need to upgrade to a higher end register if you want volume control. IMHO registers shouldn’t be used for volume control anyways, it should be done by duct dampers 
    2: In areas with significant heating degree days point registers DOWN to get the warm air to the floor. This also helps with all the warm air not collecting at the ceiling like you have with builder grade registers. 
    3: Since these registers have so much more free area throw is affected if you try to make the air go multiple ways. I’ve found they perform best when throw is in ONE direction. 
    4: One you have installed these in your house you won’t ever be happy with builder grade registers again. Plan on buying more commercial registers for your next house. Yes, they are THAT good.

  18. Allison, the cup is a great
    Allison, the cup is a great analogy, but if we leave the cup of coffee on the counter uncovered, it will soon revert to room temperature. That’s one of the reasons we put lids on them (other than not getting coffee stains on our shirts). Perhaps that analogy will help.

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