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entropyracer 09-19-2008 01:18 AM

Spray Foam/ Foam Board Hybrid?

I have an 1865 log house in chilly Wisconsin. Before winter arrives, I need to decide on an insulation for the upstairs ceiling. Long story short- I am remodeling the upstairs and removed the old ceiling and cellulose insulation. The room is approximately 20'x25' and the ceiling sits atop a 4' log wall. Currently the underside of the roof decking is exposed. To hold a straight ceiling line in the future, 2x6's have been firred alongside the full size 2x5's leaving a cavity about 8" deep. The ridge is vented however the vented attic space is only about 4' tall by 8' wide. On the sides of the room, the ceiling is directly under the roof and there is no way to install a soffit vent. Because the whole roofing system is unvented, cellulose and fiberglass insulation arent my best choices. I am very interested in spray foam however I'm not sold on the price. I talked to a local insulation contractor who quoted spraying 4 inches of foam into the ceiling. My question is; Can I save money by installing a 1" R7 foam vapor barrier and then suppliment the R value to 30-50 by fitting relatively inexpensive foam panels between the rafters? The drawback I can see is there will be voids between the spray on foam and the panels. Will this cause moisture issues? If anybody has any ideas on this, please let me know! It just seems like a great idea to me.


Termite 09-19-2008 06:51 PM

What you're thinking about probably won't work as well as sprayed in Icynene (or similar) foam, but it won't hurt either. If you opt to use the foam board, you can seal up the gaps with spray foam. Great stuff is an option, but I'd suggest making a small investment in a spray foam "pistol" type gun that will spray in any position. The guns just take cartridges of foam, and work awesome.

If you opt for foam board, please please please plan on covering it with something like thin plywood or sheetrock. It is incredibly combustable, and is required by code and product listing (and common sense) to have an ignition barrier applied over it. If it is sealed off without much available air, it won't burn nearly as effectively, and there's no chance an outside source will ignite it. Ever see the video of the Rhode Island nightclub fire a couple years back? Yup, foamboard. I hate the stuff.

entropyracer 09-19-2008 07:12 PM

I'll definately be drywalling it. I had intended on putting both a layer of spray foam in and then a thick stack of foam sheets. Is the process you explained common practice to just use foam board and great stuff in a northern climate? Thanks!

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