High humidity issues in a spray foam attic
Have an interesting one here for you building science detectives.
Existing two story home built in the 1950s – Charlotte, NC (mixed humid) – approx. 3000SF. Existing mechanicals were a furnace in the crawl space with duct work distributed throughout the house and an air conditioner in the attic serving the second floor. Old batt insulation in the ceiling, wall insulation undetermined (we think none) and batts in the floor.
Owner received an energy audit and implemented some of the recommendations. Removed all batt insulation in the attic, removed both HVAC systems and all duct work, performed air sealing in the crawl space and the attic (at the floor and ceiling) and installed 5 ½” of Demilec open cell spray foam on the roof deck. Existing roof did not have a ridge vent – shingle color is a dark gray. Now that everything is installed, there is very high humidity in the attic and condensation on the unit and condensate drain.
There are already several threads on open cell vs. closed cell, vented roof vs. unvented and blown-in insulation vs. spray foam insulation so please let’s stay with our fixed variables we have here in our discussion.
Here are few readings we took. Interior temp. – 75 degrees and 45% RH. Exterior temp. – 89 degrees and 50% RH (low for here this time of year). Attic temp – 84 degrees and 75% RH.
Now the troubleshooting:
HVAC – we are asking for the manual J and checking the refrigerant charge to be sure the unit is sized correctly and performing properly. Typically in these situations the units are oversized and short cycling which is definitely not the case here – during the hottest part of the day the system is running non-stop and cannot keep up so do not think that the HVAC is the main issue although it could be contributing.
Spray foam – one of the problems with spray foam is there is not really any way to do quality control on it. Have read before that if the mix is not correct there will be odors, discoloring and the foam would be extremely hard. We do not see any signs of these issues. I was expecting to see large gaps where the old soffit vents were but it appears to have a tight air seal at the perimeter. We did inspect the foam insulation with a thermal imaging camera – temperature was 84 degrees for 90% of the area. There were several areas where the depth of the foam was not consistent and we saw about a 5 degree difference within the same rafter bay (Installer said this was typical). A handful of small ½” voids where the temperature got up into the 90s. None of which you would think would allow that much moisture to enter the attic. What else can we look for to determine the quality of a spray foam job?
Big question is how did the humidity get so high up there. I do not think it is coming from the house below – that air is conditioned and relatively low humidity – if that air was getting into the attic it would improve the situation. I do not think it is coming from the exterior either – if there was a hole in the foam/roof, the moisture would be going from the attic to the exterior. So how did it get there? I think two things may have happened – the day they sprayed the foam, the attic conditions were probably pretty similar to the exterior and probably even hotter. Let’s say hypothetically – exterior was 90 degrees and 70% humidity – inside the attic was probably 110 degrees with the same amount of moisture in the air as the exterior. After the foam was installed the attic temp. dropped from 110 to 85 degrees. The amount of moisture in the air remained the same thus the humidity would increase with a temperature drop. Secondly to compound the problem, the blowing agent for the spray foam is water – as the foam cures, offgasses, etc. it released all of the water used to install it so added more moisture into the air.
Since the attic is only passively conditioned through leaks in the duct work and the leaks in the ceiling, there is nothing working to remove the moisture from the attic. The only way for the moisture to leave the attic right now is through diffusion through the roof to the exterior and diffusion through the plaster to the conditioned area of the home. Diffusion is a slow and weak force so it could take weeks for the attic to equalize with the conditioned portion of the home.
Our plan of attack is to temporarily run a dehumidifier in the attic until we get the RH to match that of the conditioned area below. I think once we get the attic acclimated with the rest of the house the levels will somewhat mimic each other.
Any flaws in our logic? Any other things we should look into? Is there a way to calculate the size and number of dehumidifiers based on RH and volume? Any other suggestions on avenues we can explore to solve this problem? Thanks for the help.