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I am building a 2800 sqft, two-story brick home in Austin, TX. I have heard many good things about the benefits of insulating with closed-cell Spray Polyurethane Foam (ccSPF)1, and have requested that the builder use 2" of ccSPF in exterior walls instead of fiberglass batts. I am seriously considering asking them to completely seal the house by putting ccSPF above the sheetrock in the 2nd floor ceiling (and then blow fiberglass insulation on top of that).

At this point, I am in unfamiliar territory; I understand that houses with strong vapor barriers must have some form of air-exchange system to remove carbon-monoxide and other harmful by-products of household living. I have read that the best way to do this is with an ERV system.

With such substantial changes to the design of the house, I want to make sure there isn't anything else I'm missing. A few questions come to mind...

  • Q1. The spec house model (engineered for fiberglass batts) comes with a 14-seer A/C system. With whole-house ccSPF foam and an efficient ERV system, it seems that I should reduce the size of the A/C; if so, how should I go about estimating the impact of these changes on the size of the A/C myself? I would rather not depend on the builder to re-size the system; so far, the only people they will let me talk to are in sales and their purchasing department.
  • Q2. Does anyone have experience with ERV systems? If so, can you share what kind of maintenance issues you had to deal with? How do I reliably test that it is functioning properly five years from now? What model are you using?
  • Q3. ccSPF is very sticky; I'm concerned about what I will have to do if I need to replace some sheetrock down the road. Is there something I can do proactively to make it easier?

  1. In addition to being a fantastic thermal insulator, 2" of ccSPF effectively acts as an insect / vapor barrier, it is an amazing sound insulator, and also insulates against radiant heat transfer.
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2 Answers 2

up vote 6 down vote accepted

At this point, it's not a DIY project. You need to consult an engineer or an architect/engineer that is used to these kind of building envelopes. A sealed envelope with an ERV needs to be designed and PROPERLY sealed, and the ERV capacity needs to be determined and designed by an engineer in order to meet building code and pass inspections.

Note: I'm in TX and familiar with building code for high efficiency projects here, and my girlfriend is a PE but not in HVAC.

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It's a half-way DIY project. I probably will not ask the builder to have anything to do with the ERV system. It's not on their price sheet, and everything you get from them is marked up. I will fit the ERV into the house before we occupy it. BTW, the intent is not to build a closed envelope... it's just that ccSPF will probably bring me rather close to that point from what I have heard. The snag will be if an inspector refuses to certify the house for occupancy if I have whole-house ccSPF. In that case, I would ask the builder to leave fiberglass in the attic with no foam insulation. –  Mike Pennington Sep 2 '11 at 11:05

Karl is right on with his advise to consult an HVAC expert to help you with this situation. I agree that the foam is far superior to batt fiberglass and worth the extra cost. There will be several considerations such as types of fuels used in the home, ie; gas range, fireplace etc. Other items worth a look are possible Radon, window and door schedule. Your situation in Texas is greatly different than here in the Northeast where fossil fuels are still the norm and heating/AC appliances are normally installed indoors rather than outdoors. Congrats on your new home and great energy efficient planning.

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We will have a gas range, so I'm pretty sure I can't escape the air-exchange issues... besides, smoke from cooking rarely gets sucked through range-hoods well enough, and I also have to deal with by-product vapors from household cleaning agents. Any way I slice things, it sure looks like I need an ERV system. Looks like the consensus so far it talk with an HVAC specialist... I will do so... –  Mike Pennington Sep 2 '11 at 11:31
    
Yeah, the gas range makes it really critical for you to have an engineer look at it... and make sure that you have a hardwired smoke / carbon monoxide system as well, which is also code. –  Karl Katzke Sep 2 '11 at 21:26
    
Be sure your gas appliances are fitted with oxygen % sensors and auto shut offs. Your engineer will detail your needs. –  shirlock homes Sep 4 '11 at 0:06
    
Is your range dual fuel or gas only? In any case, please consider providing makeup air for your hood instead of depending on some other way of accidently getting makeup air. It will be safer and probably work much better too. –  Philip Ngai Jul 4 '12 at 16:47

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