Condensation new build
We are in the building process of a custom home. The electrician is installing lights in the ceiling and noticed condensation on the closed cell spray foam on the underside of the roof. We are in climate zone 5 with cathedral ceilings.
We did our research prior to insulating and used 3 inches of closed cell spray foam (air impermeable) with blown in fiberglass to fill the remaining space (permeable) around R 30 of fiberglass. Based upon articles on this topic we needed around R 20 of air impermeable insulation, if total R value was 50, to reduce the risk of condensation on the roof sheathing. We are currently in winter temps of 20-30 degrees F outside with some temporary electric heaters in the house but our HVAC system is not connected yet. Temps inside are in the 60’s consistently. House is air sealed with blower door score of 0.8 ach50. Any ideas on why this is happening? Should I be concerned for long term issues?
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This is a bit beyond my expertise but I want to follow it and also share a couple of thoughts. Hopefully others more knowledgeable will contribute.
1. The concrete and framing in your house are "young" and probably still quite wet. So you may have high humidity, which should drop as the materials dry out.
2. If water is condensing on the surface of the foam that means that surface must be rather cold, I'm guessing 45-50 degrees. Your thermostat may be measuring an air temperature in the 60s but given the current R20 insulation level being relatively low, your heaters may simply not be able to keep up with the level of heat loss.
I guess both points suggest to me this is a short term problem that should resolve itself as the final insulation goes in and the humidity levels drop. As a precaution you may want to use a variable perm membrane behind your drywall.
I just want to reiterate this is beyond my technical expertise. So take it with a grain of salt. Hopefully it will stimulate discussion from others more knowledgeable than myself.
To piggyback, a new build will have high moisture levels unless your builder has taken care to monitor moisture levels and delay the finish after the frame has dried (months).
Spray foam is not a great air sealer. Very possible that the spray foam has cracked or separated from the framing somewhere, or a penetration isn't sealed well. Also possible you have an inconsistent foam application or a combination of these problems. Spray foam install quality varies immensely based on the installer. Some are good, most aren't. Possible you have 2 inches in one area and 4 in another. If you've installed the minimum amount required to prevent condensation, there will be areas that do not meet your requirement. Thus, I do not allow R6 for spray foam as a design standard. Much less.
Given that you have a new build with high moisture levels and non operable ventilation, condensation will rear its ugly head somewhere. Your roof assembly seems like an obvious place. Do you see any moisture on the windows?
Get some moisture readings. RH should be less than 40%. More than likely, you won't have a problem once your ventilation system is running provided you keep the RH down. You may wish to place some meters in the roof assembly for long term monitoring.
Flash-and-batt, flash-and-fill and exterior impermeable insulation ratios all require a class 3 vapor retarder on the interior (i.e., latex paint over drywall) to limit the amount of air and moisture getting into the assembly. Without that vapor-retarding layer, you will get condensation.