how to insulate finished 45 degree angle


Old 10-20-10, 11:12 AM
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Join Date: Jun 2009
Location: Rochester, MN
Posts: 8
how to insulate finished 45 degree angle

I've got a room on the second floor of my house (built in 1940) that has knee walls for about 4 feet, followed by a 45 degree angle to the ceiling. I've blown in insulation on all the "flat" horizontal areas behind the knee walls and on top of the room where the 45 move up to, but I have yet to do the 45 degree areas. Currently, the only thing behind the sheet rock is newspaper which isn't helpful at all. I've heard the easiest way is to take rigid foam 4x8 sheets, tac them over the existing sheet rock and then just sheet rocking over that again. I'm guessing the only other way is to take down the existing sheet rock and paying someone to spray foam OR do the DYI spray foam. Any other options? What the easiest/most economical way of doing this?

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Old 10-20-10, 07:16 PM
Join Date: Jul 2009
Location: canada
Posts: 54
You could also slide sheets of foam down the space behind the drywall from the top or side attic, leaving an inch or more of vent space between the insulation and roof decking. Another option is to slide a vent spacer down and fill underneath (against the drywall) with blown cellulose or fibreglass. If you have a 5.5 inch space (2X6 rafters) you could even slide 3.5 inch batts of rockwool or fibreglass down and then cover them with something like tyvek or even cadboard to reduce windwash. Your idea of just attaching the sheets on the inside is definately the easiest. If you are in a cold location though, I would be a little worried about condensation on the exisitng drywal, since being behind the insulation, it will now be very cold and particularly if painted, it will serve as something of a vapur barrier. If you go that route, you will want to be very sure to use a vapour barrier (apply plastic or use foil faced polyiso) on the room side of the insulation and tape the seams well. Even then, I wouldn't be too confident of not getting moisture. Around here (Canada) they reccomend at least 2/3 of the insulation be on the cold side of any vapour barrier such as existing drywall in an application like this to keep the existing drywall warm so condensation does not form.
Old 10-21-10, 05:24 AM
Join Date: Oct 2008
Location: New England
Posts: 9,990
This is just my opinion, but despite a bit of mess, simply remove the old sheetrock and proceed from there. Applying new drywall over the old is no different than applying it over the new framing and insulation. Once it is all opened up, you can decide on the method that that meets your needs and it will be insulated, air sealed, and have the correct vapor barrier in the correct place.

Old 10-21-10, 05:30 AM
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Join Date: Sep 2010
Location: USA
Posts: 53
If possible, could you get a pic form the attic area showing the spot you need to insulate. If there is applicable room you should be able to get a R-30 bat style insulation and slide it in between the joists. Also what style blown insulation have you used. If you have gone with a celloulose it will allow you to blow on an angle from the attic.

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