Insulation question

Hoenes

Member
We're remodeling my sons kitchen. Older home, plaster lath, and no insulation. Complete demo, down to bare studs, now re-wired and ready to go back with sheetrock. We have a small area that is vaulted.( What was originally a porch area.) It's about a 3 foot wide vault area the length of the kitchen. Trouble is, ceiling stud cavity in this area through here is only 3 1/2". Is it OK to let fiberglass insulation touch the bottom of the roof sheathing? I don"t want any overheating of shingles, and/or condensation issues below. I know on new home vaults, it shows air flow travel above fiberglass. That is not possible in our situation. Should we leave this area un-insulated, or is this ok to insulate?
 
A little insulation is almost always better than no insulation at all.

Will insulation in this area prevent ventilation to the rest of the roof? If so, you could install channels like those intended for blown-in insulation to ensure ventilation.
Attic ventilation channel
 

I have been researching a far more difficult problem in my old post and beam house. What I have found is 2X what Dick 2 said. Closed cell foam will not let the moisture through, and the higher R Value per inch will reduce the heat the likelihood of snow melting and forming ice dams. The drawback to this is the cost. Another way to do it is using two layers of foam board insulation. One layer of two inch and one layer of one inch. They are fastened to 1/2 inch battens which are first fastened to the underside of the roof sheathing, so that you have a ventilation space. You use aerosol foam to seal seams and between rafters and the insulation board, to do a really tight job.
 
Make the rafters wider by using plywood and add a 2x6. That will give you 9 inches to work with. Always need ventilation and a minimum of R19.

Make sure you have eve and roof vents.
 
those channels look interesting, but how do you install them when your roof deck is full of nails from the shingles on top?
 
> those channels look interesting, but how do you install them when your roof deck is full of nails from the shingles on top?

I was thinking the same thing. I think the answer is "very carefully".
 
Thanks to all. I had forgotten about the attic ventilation panels. Around here they can be found, made out of styrofoam. A light push would solve the protruding nail problem. Also, no bigger an area that we have, we could clip the nails with side cutters.
 
Its not that hard to put in and the nails sticking through will stick into this material.I used this between every rafter in my house. I have about 20" of insulation in my attic.
a243031.jpg
 
After you get the insulation all up and are ready to install the sheetrock or paneling put up a layer of 6 mil poly as a continuous vapor barrier. This will keep moisture from cooking, bathing, and breathing from getting into the insulation. It will also stop any drafts. "Fattening out" studs and rafters is a great way to increase the wall thickness for added insulation. Do a good job of fitting the insulation for no gaps and no packed in places, both reduce it's efficiency. You only get to insulate once and it is worth it to take the time and do it right, adding when you can as you are doing. It will pay you back for the rest of the time you live in the house with money savings and a warmer more comfortable house. Insulation is a good investment.
 
An air gap between the insulation and the roof sheathing, with adequate ventilation, should help cool the roof, cool the attic, remove moisture from the attic and reduce your air conditioning costs.
 
I agree there should always be an air space between the insulation and the bottom of the roof sheathing
 
you really need to check the perm rating for the materials you are using---the greatest perm on the inside and then progressively less as you work to the outside wall---that way the moisture does not get trapped and condense as it hits the colder surface
 

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