Foam insulation on underside of roof - anyone have it?

[Kurt Veltman]

I am dealing with a mold issue in my attic, and have removed (for the most part) all the insulation and am ready to have the mold dealt with. (If anyone has experience with this, good or bad, I would like to hear about it). The next step is replacing the insulation. I talked to a salesperson today that recommended spraying the underside of the roof with their insulating foam, and sealing off all air flow in/out of the attic. I have done some construction, and have talked to several people, and the conventional method has been to seal off the attic from the outside of the exterior wall inward, and maintain an air gap between the roof and the insulation, and have either gable vents or ridge and soffitt vents, to keep the attic approximately the same temp as the outdoors, thus preventing ice build-up on the roof.

So here's the question: does anyone here have this sprayed on foam insulation in their attic? If so, for how long? And how has it affected your house and energy bills?

Thanks for any input on this.

[JC Whitney]

Have you determined the cause of the mold? It needs moisture and a food source to thrive - eliminate both as much as possible and you'll be in good shape. One common cause is bathroom exhaust fans vented into the attic instead of being ducted to exterior air. Next step is to get a good vapor barrier on the correct side of your insulation. Which side is the correct one is dictated by your local climate - it wants to be on the warm side of the insulation to prevent condensation when warm moist air hits cold and dumps its moisture load. The right spray foam system installed correctly can act as both an insulator and vapor/air barrier.

Whether to insulate the roof rafters above the attic space vs ceiling joists benath it has a lot to do with the specifics of your house... I typically see foam (Icynene seems to be the most prevalent around here) used on the underside of the roof when there's HVAC equipment located in the attic. The thought behind this approach is that rather than treating the attic as part of outdoors, you're bringing it into the interior conditioned space - thus no need to vent the attic. You might want to take a look at Icynene.com for additional info.

Ice damming often happens when water from snow melted by the heat in an attic hits the lower cold edge of the roof (no warm spaces below down at the gutterline). If you insulate the attic/roof very well, in theory the snowpack will stay on the roof, and only melt when ambient air temperatures are such that you won't get that drastic change of temp down at the roof edge. Look around after a fresh snowfall and you'll be able to see which of you neighbors have a lot of heat going up through their attics...

It's good stuff (and priced accordingly), we use it on high end houses whenever possible. It provides a nice, tight wall / roof assembly.

[Andy Birko]

I do not have an attic insulated this way but at one point we were investigation finishing our attic and getting the spray foam like you say. There is a product made from pre-formed styrofoam sheet type things that you're supposed to put between the rafters to create an air space between the roof sheathing and the insulation. I found a pic here:

I seem to remember them being wider but something to keep in mind.

[Kurt Veltman]

Thanks for the input guys. JC, the main issue I have with foam on the underside of the roof is that you are basically adding hundreds of cubic feet of space to keep heated. I understand that in this scenario the attic will be warmer than the house in the summer and cooler in the winter, but it seems like it would be more efficient to insulate the floor of the attic, maintaining the air gap that Andy mentioned.

The other issue is this: does the foam insulate so well that you don't get any snowmelt in the winter?