Closed-cell foam on part of the ceiling – Fiberglass insulation on the other part
I live in climate zone 4, and I have a new build with scissor trusses that do not have a raised heel. R-38 fiberglass insulation cannot be used where the scissor trusses meet the top plate; I had to spray 6 inches of closed cell spray foam about 2 feet along the bottom cord of the scissor trusses, from the top plate, with the intentions of terminating the foam there and continuing on to the peak of the ceiling with R-38. The ceiling has recessed lighting, and I’ve built poly-iso boxes to go around the lights, and I have applied 5.5 inches of closed cell spray foam around the boxes. The foam on the boxes extends over into the adjacent truss bays, which will make it difficult to use R-38 fiberglass insulation in those adjacent truss bays near the recessed lighting. As so, I’ve also sprayed 6 inches of closed cell spray foam on the ceiling in those areas. I now have about 60% of the ceiling insulated with 6 inches of closed cell spray foam. Is there any good reason why I shouldn’t insulate the remaining 40% with R-38 fiberglass insulation? Should I just go ahead and insulate the rest of the ceiling with closed cell spray foam? Cost is not a factor.
My other concern is, because the closed cell foam essentially acts as a vapor barrier, a ceiling completely insulated with closed-cell foam will (I think) cause more water vapor to build up on the interior during the warm humid months of the year. Will this cause excessive moisture to infiltrate the walls that are insulated with R-13 fiberglass insulation (kraft faced, stapled to the front of the studs)? In theory, wouldn’t insulating the ceiling with closed cell spray foam have essentially the same effect on the conditioned space as insulating the underside of the roof deck with closed cell spray foam, and I’m worried for nothing about excessive moisture from the vapor barrier at the ceiling?
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Replies
Jason,
Your worry that it will be hard to install fiberglass batts adjacent to your small hills of spray foam (the hills covering the recessed lights) is justified. Blown-in insulation would be a better choice. More information here: "Insulating Scissors Trusses."
Q. "My other concern is, because the closed cell foam essentially acts as a vapor barrier, a ceiling completely insulated with closed-cell foam will (I think) cause more water vapor to build up on the interior during the warm humid months of the year."
A. You're wrong. Homeowners shouldn't depend on air leakage or vapor diffusion through ceiling cracks or ceiling materials to control indoor humidity levels. That approach (a) is nuts, and (b) wouldn't work.
Your indoor humidity levels will vary depending on a number of factors, including occupant behavior, ventilation system operation, and the existence and operation of a dehumidifier (if any), a humidifier (if any), or an air conditioner (if any). The leakiness of your home's thermal envelope is also a factor.
If you have problems with high indoor humidity, describe your problems.
Martin,
Thank you for answering my concerns. Because the structure is still being built, I am not sure if there will be an indoor humidity problem, which is the why I addressed these concerns here before I went too far one way or the other with the closed cell spray foam. This is my first build where I had to use spray foam (oversight on the scissor trusses for not having a raised heel). It is my own property, so if there are problems that arise with indoor humidity I can work to correct them however I need.
You mentioned a ventilation system, and I hadn't originally planned on installing a ventilation system, but I have researched it and I am now considering it for the health benefits, and also if it would help mitigate any threats/effects of excessive humidity. This is a workshop environment where large power tools will generate excess heat. I have a cyclone style dust collector that will catch most of the dust, but I'm concerned about the affect of lingering micron sized dust on an HRV or ERV. Can you speak to that concern?