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Rowden 09-02-2013 12:54 PM

Cantilever insulation
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I have a 1950's era rancher with two bedrooms that extend approximately 2.5 feet beyond the concrete foundation wall. Using the information from this site I plan properly insulate the basement prior to finishing the space (2-inch XPS, 2x4 framing, batting, drywall).

To that end, I ordered a DIY spray foam kit and plan to spray the cantilevered bays under the bedrooms, as well as the rim band around the entire foundation. I have already cut, glued 2-inch XPS into all the other bays & spray foamed the spaces around them. I have attached a picture of the bay that extends out from the corner of the foundation. For the cantilever bays I planned to spray all joints (red lines in picture), then cover the exterior joist surfaces (A & C) and overhead floor surface (B) with foam 2-inches thick (two 1-inch applications).

My question is should I spray the plywood surface that is nailed to the bottom of the joists (E)? Or, if I have enough material, hit all surfaces within the bays?

It is my understanding from reading other threads in this sub-forum that I should block off this open space above the rim joist with FB insulation. Can I cut 2-inch XPS and spray foam that block into place? Do I gain anything by doing the latter or am I just isolating any air from the basement from warming this bay in the winter?

burnt03 09-02-2013 09:16 PM

Bookmarked this thread from a while ago, lots of pics and seemed like the right way to do it?

Rowden 09-03-2013 05:01 PM

Thanks Burnt03, I did take a look at that thread on several occasions. While those projects were undertaken from the exterior and involved combinations of XPS & batting, I'm planning to hit the interior with a large volume of spray foam.

Maybe I'm being overly cautious but I'd like to get a few opinions on how best to distribute the foam before doing so because once it's on it's too late. I'm assuming spraying all the surfaces is the best option (twice on overlying floor and exterior joists) but I wouldn't be surprised if it was recommended to skip spraying the bottom and use that foam for a third layer on the top (R value of 12-14 on top and 6-7 on bottom vs R value of 18-21 on top only). Should I install some Roxul in the remaining void before sealing the box above the rim joist?

Thanks for bearing with me.

Windows on Wash 09-05-2013 05:59 AM

Good read in the other thread.

Can you post up a picture from outside of the construction and access?

Gary in WA 09-05-2013 04:01 PM

Airspace on top with f.b., Fig.7, airspace on bottom with SPF, Photo 6; Decouple it from exterior ground temps. Cover with soffit of choice.


Rowden 09-05-2013 04:23 PM

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Originally Posted by Windows on Wash (Post 1238309)
Can you post up a picture from outside of the construction and access?

It is standard construction with plywood sheathing covered by vented vinyl soffit. Approximately 3 feet of clearance from ground to soffit.

Rowden 09-05-2013 04:48 PM


Originally Posted by Gary in WA (Post 1238528)
Airspace on top with f.b., Fig.7, airspace on bottom with SPF, Photo 6; Decouple it from exterior ground temps. Cover with soffit of choice.


Thanks, Gary. If I understand this correctly since the floor joists are encased (as is the case in Photo 6), it would be best to place as much foam insulation on the bottom/sides and leave an airspace on the top (Figure 7). Which is counter to what I had proposed...not surprising though.

Since the bays are 7-inches in height and I'm shooting for at least 3-inches of foam on the bottom sheathing, that would leave enough space for me to insert 3.5-inch un-faced fiberglass batting. That's assuming it doesn't get torn up by all the nails penetrating the sheathing.

Unless I'm missing something that's the approach I'll plan to take.

Gary in WA 09-24-2013 08:29 PM

Little late, sorry...


Rowden 10-14-2013 08:01 AM

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Just wanted to follow up with a few pictures of the completed project. I used Foam It Green 202 spray insulation in all the bays and managed between 2 & 3-inches of coverage on exterior facing surfaces. The spray foam was very easy to use and I'd recommend it to anyone looking to do their own closed cell insulation job. Covered that with batting then sealed the space with 2-inch XPS. Finished with batting over the XPS.

Thanks for everyone's assistance.

Now onto the covering the walls with 2-inch XPS and framing.

Gary in WA 10-15-2013 07:31 PM

You will still get thermal bridging of the floor joists and bottoms are still radiation coupled per link. Will you remove faced insulation when finishing?


Rowden 10-16-2013 07:15 PM

I'm primarily focusing my attention & money on the interior of the space at the moment (insulating & framing walls, leveling floor, replacing windows, etc). To do the cantilevers correct, per the link, I'll have to revisit the exterior which will require some aluminum flashing, etc to accommodate the new layer of rigid insulation.

I was planning on leaving the faced insulation between the joists. I realize that now that the room will be heated the insulation does not serve any purpose beyond possible noise reduction. Is there any recommendation / documentation that says there is a benefit to not having the insulation in place?

joco101 06-28-2014 10:31 PM

Hi Rowden, I don't know if you are still visiting this thread but here's my question.
I'm planning to spray foam the overhang (cantilevered joists) in my basement using Foam It Green 202. I saw in your post that you applied couple of inches of the foam and then put fiberglass insulation on top of that. Do you see anything wrong if I fill whole cavity with foam it green? Also, the instructions say that after applying 1inch of foam I need to wait 10 min to cure but I will have to replace the spray tip after 30 seconds. Did you wait 10 min after spraying the first coat? Thank you

Rowden 06-29-2014 08:49 AM


I'm not an insulation expert but I would think that would be overkill. You're sealing off the air leaks with one layer, which is probably the biggest thermal issue. Each additional layer would have a benefit, but at some point I would assume it wouldn't be worth it anymore. Depending on the depth of the cantilever, you might need 2-3 spray tanks ($$$). Plus, as Gary mentioned earlier, you would still have thermal bridging from the joists.

I had 20+ bays to insulate so by the time I finished one pass, the first bay had sufficient time to cool (spray two bays, move ladder, spray bays, untangle hoses, etc). The spray tool is good and the first coat was fairly uniform, but subsequent coats were not as uniform because it became more difficult to get my arm into the bay without getting messy. I did have to change the tips a number of times, even though I kept working the entire time. If you sprayed the back face (labeled "A" in my 1st picture) and worked out you could probably do it without too much mess vs covering three sides at the same time.

I am pleased with the results of my insulation project. This past winter was colder than normal and the floors were noticeably warmed than before. The big issue was moisture and condensation on the inside walls of the cantilever corner, which we did not experience this year.

Again, just my opinion. Hope this helps.

joco101 06-29-2014 09:41 AM

Thank you for your fast response Rowden. I'm going to put one or two layers of spray foam to seal the air leaks and then stuff some fiber glass insulation.

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