Attic foam insulation on rafters
Can I add ceiling insulation?
I currently have foam insulation sprayed on the roof rafters, which purports to create a "sealed" attic. I am thinking of adding a layer of insulation directly above the ceiling, but I am concerned about moisture. Do you know if moisture would be a concern in this situation? If so, what is the best way to address it?
Answer from Green Energy Efficient Homes
I'm surprised anyone would have had the objective of having a sealed attic. While sealing an attic can keep moisture out from above and prevent drafts, it can also keep moisture in once it does get in. This can lead to rotting wood, mildew, and a lowering of the insulating value of any fiber insulation already in the attic.
Although insulation on the rafters will slow down heat transfer into the attic from the hot sun striking your roof, the usual solution to solar heat making the attic hot are to use as light a color of shingle or other roofing material as possible (so more of the sunlight is reflected away from the house before it can turn to heat); to add a reflective or radiant barrier such as Reflectix to the undersides of the rafters, so radiant heat gets reflected back outside; and to ensure adequate ventilation of the attic.
You can get good ventilation through passive vents in the roof and soffits (you need them both at the top and bottom to create a stack effect, where rising air inside goes out the top vents and pulls cooler air in through the bottom). If that's not enough, you can add ventilation such as a whole-house exhaust fan, which pumps warm air from the house into the hot attic on hot evenings, thereby drawing cool outdoor air into the house, so both the house and attic get cooled.
I would not recommend installing foam on the underside of a roof, unless the roof isn't made with a wood base. Given that that's what you have, I think you will still want to add attic ceiling insulation, but you'll need to change your attic from a 'sealed' attic to a well-ventilated one first.
You should never put a vapor barrier between your house living space and your attic. Given the vapor barrier between your attic and the outside (since it's a sealed attic), you will have problems with moisture from the house entering the attic and having no way to escape. With attic ceiling insulation such as fiberglass, mineral wool, or cellulose (whether blown in or batt), that moisture will quickly reduce the R value of the insulation. But adding attic ceiling insulation shouldn't make any current moisture problems any worse. You just won't be getting full value from the insulation.
The simple solution would be to add sufficient vents to get a proper air flow in your attic, and then add R-49 to R-60 insulation to your ceiling. Most roofing companies can recommend appropriate levels of venting, and can install it while avoiding costly roof leaks. If it was a roofing company that sprayed the rafters, I'd at least ask for another roofing company's opinion as well, even if you ask them back for advice.
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