AeroBarrier – How Tight Is Too Tight?
New construction in Phoenix, AZ (climate zone 2B). Single-level 2,200sf slab-on-grade with attached garage, but no combustion appliances. HVAC consists of a heat pump w/ outside air ducted into the return (central fan integrated supply ventilation strategy – CFIS). AHU will have an ECM blower. A motorized damper on the outside air duct will be controlled to supply the equivalent of 60 cfs continuous ventilation.
The flat roof has closed cell SPF roofing on top and open cell SPF insulation on the bottom (as well as on the inside face of walls above the top plate). Windows are casement w/ double gaskets (fairly air-tight). Walls will get damp-sprayed cellulose later. We’ve taken no special precautions sealing the house other than closing the “obvious big holes”.
Since our insulation levels & window U-values are well beyond code minimum, it makes sense to air seal beyond the local code minimum of 5 ACH50. To avoid needing to train/guide our builder to go beyond standard construction practice, we’ll be sealing with AeroBarrier late this coming week. Air sealing on auto-pilot. AeroBarrier CAN be taken to extreme levels (well below Passive House). But that’s not our intent.
My question is, how tight is too tight? At what point will further tightening of the building envelope interfere with ability of CFIS ventilation to move air through the structure? I have a number in my head, but now I’m second-guessing myself and I’m not finding any guidance online.
FWIW, the AeroBarrier will be performed before installation of 2 exterior doors (openings will be temporarily covered w/ OSB). They’ll be standard builder-grade doors, so I expect something of an increase in final blower door number compared to the AeroBarrier number.
Please, no discussion of how balanced ventilation would avoid this question. That ship has sailed and we’re well beyond the point of being able to put one in.
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