More on insulation , probs...

Started by jb, February 11, 2005, 08:45:47 AM

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After reading so many inputs on insulation/moisture, I am really, totally confused as to what I need to do. Can someone please explain so that your average building novice can understand?...
I do have moisture, condensation concerns inside my cabin because half of it is bermed.So, in basic terms, when I insulate between the ceiling rafters, does the insulation have to touch the osb at top, or leave an air gap? Yes or no. When I insulate the front and back gable walls, do I staple on a barrier, like felt paper, on the inside, or would this cause more moisture to be trapped? Yes, or no. Thanks guys and gals, sorry to sound stupid...just not sure.

glenn kangiser

Check out this brief.  It shows most situations and explains below the picture about a third of the way down the page.
I read somewhere else that 30 lb roofing felt wrinkles and makes spaces for drying when wet allowing  moisture to move through.  It was superior to Tyvek types in this application.
"Always work from the general to the specific." J. Raabe

Glenn's Underground Cabin

Please put your area in your sig line so we can assist with location specific answers.


Thanks for the link...loads of help there but doesn't answer my questions...yes or no on an airspace between insul. and osb, and is the felt on the upper inside gables a good idea or a bad idea...sorry, but I keep asking and get long, drawn out explanations, but no one says do it, don't do it...

John Raabe

OK... Roofs need to be vented above the insulation. This means a 1" airspace open to screened vents top and bottom. On the inside (warm in winter) staple up an air/vapor barrier over the insulation. This could be poly or perhaps Tyvek. This goes under the ceiling finish which does not have to be airtight.

On the walls or gable ends, The felt paper goes on the outside and protects the structural sheathing from wet siding allowing it to dry. Insulation in the wall cavities on the inside should also have an air vapor barrier. This need not be poly and could be the facing of the insulation with drywall and a two coat PVA primer.

As I said on another post:

Almost all moisture problems come with airflow issues. Thus blocking airflow (from the damp warm inside to the cold dry outside) through an insulated assembly is the first priority. Do that and it really doesn't matter where the vapor barrier is. Forget it and there will be condensation and potential moisture problems.
None of us are as smart as all of us.