Quote:
Originally Posted by scottorious
...It is a 1 and a half story house so the entire second story is is knee walls and slanted wall/ceilings. I believe the trusses are only 2x4 and this house is fairly old and I dont believe there was any insulation put under the roofing...
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Yep, sounds just like my house.
Mine was built in the late 1880s. 2x4 baloon walls, 2x4 in the roof, etc.
Mine had a ceiling that was about 8' or 9' wide. No insulation anywhere.
I started out putting in R-13 fiber glass, then R-15 when it came along. Better than nothing, but that's about it.
But fiber glass is not so good unless you absolutely prevent air from passing through it. And the R-value per inch is not so high... very important when you have so little space for insulation.
After several different shifts in method, I ended up tearing out the ceiling thing. I was told by an architect that I could remove every other stringer that tied the roof rafters together... which I did. I put in rigid foam that I cut to fit between the rafters. I cut the foam very carefully so that it just fit in the space. I eventually used a method where I cut the foam pieces 1/4" small all around and foamed the gaps. Much easier and much more air tight. My 2 x 4 were actually 2" x 4" (imagine that) so it worked out well for the foam which I bought in 2" thicknesses.
I started out using EPS foam but I have since gone to XPS foam, which has three advantages, it has higher R-value (5 instead of 4), it does not shrink over time, and it is quite easy to cut with a hand-held jig saw. All those advantages, and I am getting it at the same price.
It is to your advantage to lap the layers so that you don't have seams at the same spot. You might even want to use some kind of tape that is made for such a purpose. No, duct tape will not work.
I didn't know about thermal bridging when I did my roof... I nailed 2" strips to the rafters. Not so good.
You lose about 18% of your insulation value if you don't break the thermal bridges. Using perpendicular strips like the mooney wall would do it just fine.
I'm doing my kitchen right now, same method... when it was particularly cold, I read the temp of the foam wall and also read the temp of a 6" stud... the stud was about 5 degrees F colder. Thermal bridging is for real.
So my advice is to go for foam XPS or iso, foam the gaps, make sure each layer is AIR TIGHT before you proceed to the next layer.
You will curse me all the while you do the work, but after you're done, you'll thank me.
And when your comp roof goes to hell, you should plan on adding a couple more 2" layers and then a plywood nailing surface. and then the new roof.
Insulate, insulate, insulate...
-AC_Hacker
P.S. Don't forget to add modern wiring while you have the house torn apart.