When insulating with closed-cell Spray Foam, what ERV and HVAC resizing considerations should I bear in mind

energy efficiencyervhvacinsulationvapor-barrier

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.

Best Answer

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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