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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Replies
garet_denise,
Not advocating for balanced ventilation, but why not provide a path for most of the air movement through the envelope with passive vents, rather than relying on all sorts of leaks in spots you don't choose. That would eliminate the need to worry about whether the house was too airtight.
Yes, this. Relying on defective air sealing to ventilate your house is pretty poor design. Put some vents, preferably adjustable, in strategic locations. Probably a good idea to include easily accessible filters as well.
Answers will be subjective to some degree but I think 1.0 ACH50 is a good target, with diminishing returns for getting tighter, but no downsides to getting tighter other than cost.
60 CFM continuous intake from the air handler can probably be made into a balanced system by linking a bathroom exhaust fan to kick on at the same time as the air handler.
It now occurs to me that pressure from the outside air being blown into the structure through the AHU will push open the backdraft dampers on the bath fans, acting as a relief outlet.
OTOH, is it possible that the building envelope could be so tight that bath fans don't operate effectively?
Depends on the CFM of the bath fan and what your final air tightness number is.
Get your CFM50 number from your blower door score as a benchmark. 50 pascals is about 0.20” w.c. Depending on the pressure rating of your bath fan exhaust, it may be able to reach that amount of differential pressure. If it can, it should be able to exhaust out the same infiltration rate as your CFM50 number. A full blower door test should include a chart with infiltration rates at different differential pressures, so you can use that to estimate leakage at different pressures if your bath fan has a lower pressure rating.
The moral of the story, if your house is sufficiently tight, you may be running a “blower door test” every time you attempt to turn on your bath exhaust. You can try to make this a balanced exhaust system by linking your AHU to kick on whenever your bath exhaust kicks on, but it may not be worth it. The ideal scenario is of course designed balanced ventilation. Something like an ERV so you are not having to kick on your big AHU blower motor just to exhaust your bathroom.
Will you tell us about how AeroBarrier has bid this work?
It seemed to me that they got very expensive getting under 3ACH50 but the pricing model may have changed.
I think 1 ACH50 is a great target.
My guess is that you will be able to turnoff CFIS without any noticeable comfort affects, because I am guessing there will be 1000 cubic feet per person hour of occupation.
Walta
Pricing is a flat rate down to 1.5 ACH50 (around a buck-and-a-half per square foot), then it gets more expensive. There is also an upcharge if starting leakage rate is 6 ACH50 or higher.
Given that pricing I would set my goal at 1.5
Walta