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-   -   Cantilevered floor insulation (http://www.diychatroom.com/f19/cantilevered-floor-insulation-14927/)

rickswa 12-30-2007 06:09 PM

Cantilevered floor insulation
 
Hello,

I have a question about a cantilevered floor. I saw a similar post but just want to get info more specific to my situation. I have a poured concrete unfinished basement and my family room is cantilevered out two feet on the back of the house. It seems cold in our family room so I went to the basement to check it out. They have paper faced R-30 batt insulation between the studs and the bottom of the cantilevered area. Outside is the OSB type plywood that they used for the subfloor throughout the house. When I put my hand by the 2x4 that is bolted to the foundation, there was cold air pouring in between the 2x4 and the plywood on the bottom of the floor joists that faces outside. I was able to squeeze my arm under the cantilever and put caulking along the seam and this seemed to help. However, I found one area on the far outside corner that actually had a hole (about 4" x 4") in the plywood! I used a scrap piece of plywood to fix the hole and sealed it with caulking.

Now I have 2 questions. They put the paper backing of the fiberglass insulation facing down (towards the plywood board that is on the "outside" of the house). Should this paper backing face UP towards the family room floor? There are heat vents in our basement but we keep them closed most of the time. Also, Should I install a foil backed bubble insulation or something similar outside to the plywood on the underside of the cantilever? I just don't want to install something outside and then find out it makes moisture retain in the house. Also, once I get this insulation thing straightened out, should I leave it just like it is with no boards sealing off the cavities. I mean now you can just look up between the joists in the basement and see the insulation. Should this area be "capped" somehow? Thank you for any help you can give me!!!

pavola 12-30-2007 09:47 PM

There should be blocking between the joists at the foundation wall. The paper side of the insulation should always be to the warm side, so flip it over before putting the blocks in. Also make sure the OSB is not exposed from the outside under the cantilever.

When redoing the insulation, make sure it completely covers the bond (the perimeter of the cantilever). because R-30 may not completely fill the joist space (height-wise), leaving a cool area between the insul. and the plywood subfloor in your family room.

rickswa 12-30-2007 09:53 PM

Thanks so much for the help. I'll flip the insulation over since (well no surprise) they did it wrong. Then I take the same width lumber, cut it 16" and block it off? Also you said "make sure the OSB is not exposed from the outside...". Well, basically I have the outside, the osb, then the inside. Again, given my builder, no surprise. BUT should I just use Tyvek or similar to block it from outside? Thanks again for your help!

pavola 12-30-2007 10:12 PM

To make it easy, use 1/2" treated plywood. If the cantilever is higher off the ground use the same material that creates your soffits under the eaves. House wraps like Tyvek shouldn't be left exposed to the elements either, but you could install that too.

rickswa 01-01-2008 11:39 PM

thanks again for your help. I just want to make sure I'm going to get this right. For the blocking, I have to block off between the basement wall and the cavity where the cantilever extends out two feet, so the cantilever ends up completely boxed off on all 4 sides, right? As far as not having the OSB exposed on the bottom oustide, can you tell me what I could put on the bottom of it. The OSB exposure worries me a bit because I have another cantilever in the kitchen as well as a bay window in the front that by looking at it from the inside was done the same way but on those two I cannot get between the ground and the bottom of it to cover the OSB on the bottom. Now that I went and looked at it, the R-30 is shorter in height than the joists. Should I just put some loose fiberglass insulation in there to make it fill? If I do, can I put it on the top of the paper side between that and the subfloor above? Thanks again for your replies!

pavola 01-02-2008 01:08 AM

Quote:

Originally Posted by rickswa (Post 84682)
thanks again for your help. I just want to make sure I'm going to get this right. For the blocking, I have to block off between the basement wall and the cavity where the cantilever extends out two feet, so the cantilever ends up completely boxed off on all 4 sides, right?
Yes

As far as not having the OSB exposed on the bottom oustide, can you tell me what I could put on the bottom of it. The OSB exposure worries me a bit because I have another cantilever in the kitchen as well as a bay window in the front that by looking at it from the inside was done the same way but on those two I cannot get between the ground and the bottom of it to cover the OSB on the bottom.
See the previous post for some options. You could check the others with your hand if you have at least that much room. OSB will have a different "texture" than plywood. Plywood, if not treated, can be painted where as OSB can't. If it's OSB, you should dig out enough to redo it because in a relatively short time OSB will become soft.

Now that I went and looked at it, the R-30 is shorter in height than the joists. Should I just put some loose fiberglass insulation in there to make it fill? If I do, can I put it on the top of the paper side between that and the subfloor above? Thanks again for your replies!

You can use some loose insulation to fill the gap, but just use a few inches to cover the bond (the perimeter joists). Don't cover the entire paper as this is your vapor barrier. Or it might be easier to precut some 2" foam to fit against the bond between each joist and then put the R-30 back in.

Good luck.


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