# Another Cantilever Insulation Question



## crank73 (Jan 29, 2014)

Hi guys. I was just looking for advice to see if I'm on the right track. I want to drywall the ceiling in my basement but before doing so I want to properly insulate and seal the cantilever cavity of the floor above. The cant. is approx 2 feet deep. I realize that there are many ways to skin this cat, some better than others. My limitations are that it's winter right now and I can't do anything from the outside (ie remove the plywood attached to the bottom of the joists outside) therefore I want to do this job all from the inside. I would however consider finishing this from the outside this spring if I have to. My goal is to do what I have to do before putting up my drywall ceiling.
I want to use 2 in. xps, caulking and expanding foam. Not sure if batting or Roxul is required to fill in space.

At the moment, looking into the cant. between the joists while standing on a ladder in my basement, my plan is to seal the rim joist with caulk. I would then adhere 2 in. xps to the top of the cavity against the subfloor above and seal it all around with expanding foam. I would then seal the mudplate and the joint where the outside plywood meets the foundation wall with caulk. I would then block the cant. between the joist at the top of the foundation wall with 2 in. xps and seal it with expanding foam.

Will this be an effective way of accomplishing insulation and sealing. Am I missing a fundamental principle of insulating and sealing cantilever floors.

Should I instead put the xps against the rim joist and bottom of the joists, laying on top of the outside plywood but on the warm side of the house.

Is placing FB in the empty cavity beneficial (whether the xps is on top or on the bottom)?

My floor upstairs has never been cold, but I imagine that's due to the warm air from the basement heating it. Once the ceiling is drywalled, that will be a different story.

I have attached a couple of pictures. At the moment, my plan is the second picture. Any advice would be greatly appreciated.

Yanick


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## Windows on Wash (Aug 30, 2011)

The best option, while still having a chance to prevent condensation, is probably being uninsulated or option 3.


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## crank73 (Jan 29, 2014)

Thank you for your reply. I understand that there are two issues here, sealing and insulation and doing either of them wrong will have negative consequences. I went to three difference home building companies today for advice and each of them gave me different answers. I have found different threads on forums and that mention each of my two options. 

I was just about to commit to doing it with the xps adhered to the top of the cavity (bottom of the subfloor above) and block it with xps at the entrance to the cantilever (top of the basement wall oposite to the rim joist) but am now doubting myself with your answer. lol.

I guess moisture is one thing, and a cold floor upstairs is another. I haven't had problems with either so far, but that is because it wasn't closed off with a ceiling.

I guess I'm assuming that if I block it off at the start of the cant., then the cold floor issue upstairs will be limited to the 2 feet closest to the wall, which I can live with. It's the moisture issue that I'm not super clear on.


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## Gary in WA (Mar 11, 2009)

http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces/

http://www.buildingscience.com/documents/insights/bsi-064-bobby-darin-thermal-performance

Foam board the rim also. Plywood over the foam on joist bottoms, air seal it also, but with caulking.

Gary


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## crank73 (Jan 29, 2014)

Gary, just to confirm then, you would foam 3 of the 4 sides? All but the top (under top floor subfloor)? Or does the rim joice foam foam replace the once at the inside wall end of the cant.?


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## Gary in WA (Mar 11, 2009)

If the basement is to be finished (heated), no need to fb the blocking over the bearing wall. If you want dead-air space under decking, fb the block, air-tight.

Gary


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## crank73 (Jan 29, 2014)

Thanks guys. Gary, the links you provided also answered questions I had reference my next project....dealing with my uninsulated crawlspace. I've got a lot of reading to do. Thanks again.


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## crank73 (Jan 29, 2014)

Alright, things are progressing. Last question....for now lol. I am progressing with option 3. 2 in. xps on inside of rim and 2 in. xps on bottom of cavity. 

The 3rd diagram above does not show the interior wall that is built in my basement. It was already built and drywalled by previous owners and is 2x4 framing with fiberglass batting and a poly vapour barrier between the frame and the drywall. A couple of inches of poly is sticking out at the top of the wall. The walls were built with a 1 in. (approx) gap between the framing and the concrete wall.

My question is, what length should I cut the xps which is lying in the bottom of my cantilever cavity (3rd picture). Should it end at the outside edge of my concrete wall? Should it end as drawn at the inside edge of my concrete wall? or can I / should I end it at the inside edge of my framed wall?

If it's not at the inside edge of the drywall, should I consider extending the poly that is sticking out of my wall and tacking it to the top of the xps to have a continuous vapour barrier?

Thanks. Back to work.


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## Gary Evans (Jan 27, 2014)

You've got the right idea crank.......insulate against the rim board and the soffit and maintain a small air gap under the rest of the floor.
Where I am minimum is R20 against the rim board and R28 under an cantilever.
I'd go R28 for both in this situation.....but I don't know if you get winter where you are.

The reason for R28 under a cantilever is your feet are far more sensitive to a cold floor than you would be to a cold wall.
If your using laminate flooring I'd go even more......that stuff makes it feel even colder.


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## crank73 (Jan 29, 2014)

Thanks Gary. I'm in the Edmonton area so winters are pretty cold. How long would you cut the soffit foam board? Flush with the concrete or flush with the interior drywall covering the gap between the studs and the concrete?


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## Gary in WA (Mar 11, 2009)

I'd extend it to the face of the drywall for a better air barrier. Leaving a 1" gap between insulation/concrete leads to convective loops, robbing you of R-value and promoting moisture for condensation there (esp. if the cavity is open to wood floor joists above); https://docs.google.com/viewer?a=v&q=cache:M_ByAk5oMT4J:www.certainteed.com/resources/Use%2520of%2520Vapor%2520Retarders.pdf+water+resistant+gypsum+board+permeance+rating&hl=en&gl=us&pid=bl&srcid=ADGEESgt9RDzAXFRomDcEbQmwtyBGjayITUg0VINVFrtfPJ92PgXgKurYS55o3WmUhfYQVJ8lLEstHevvLyLOw_jzEBcxg8oPtfb6V8oA5PdgcXk_DFpAflg99BVUppB7X5bE6Foimib&sig=AHIEtbQYAZ0SWjnVJNXhfHFsk1RrK9G2HQ
The poly may/may not be fine for the location, compare; ftp://ftp.cmhc-schl.gc.ca/chic-ccdh...ngual/Vapour_Permeance_Volume_1_Web_sept5.pdf

You can lose a lot of R-value with wet insulation; http://archive.nrc-cnrc.gc.ca/eng/ibp/irc/bsi/90-controlling-heat.html

Gary


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## Gary Evans (Jan 27, 2014)

I agree.......bring the insulation to the drywall.


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## Gary in WA (Mar 11, 2009)

Ah, found the one I was thinking about, middle of the article; http://www.google.com/url?sa=t&rct=...ONFE3Pdx0U39ZVg&bvm=bv.56988011,d.cGE&cad=rja

Gary


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