# Proper cantilever insulation?



## Nealtw (Jun 22, 2017)

Almost on the far left you want a piece up against the rim up in the gap. 

And inside the house above the block wall


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## mac1990 (Mar 14, 2017)

Nealtw said:


> Almost on the far left you want a piece up against the rim up in the gap.
> 
> And inside the house above the block wall



So, put some foam board vertically on the header/rim joist, opposite of the siding?

and I already have foam horizontal above the cinder block wall inside the house?


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## Nealtw (Jun 22, 2017)

mac1990 said:


> So, put some foam board vertically on the header/rim joist, opposite of the siding?
> 
> and I already have foam horizontal above the cinder block wall inside the house?


Yes that would do it.


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## carpdad (Oct 11, 2010)

Just thought it up and is a wild guess, but hopefully, based on some past experience.:smile:
Air seal is most important. 1.5" xps board against the rim, AND same board on the floor side of the cantilever. Seal the joints with spray foam. Sides also. Then push in fiberglass insulation. This part does not have to be neat or pretty. Stuff the space and you won't be hurting any r value.


Better would be use the foam boards, 3" is available, to fill the space. Because it is cantilever and the floor is exposed to weather, more r value is better to keep that part of the floor warmer. If possible, I'd go for r50 but going past r60 gives you nothing for the money.


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## SPS-1 (Oct 21, 2008)

I would use XPS. EPS is not going to be a proper vapour barrier. XPS is, and will have higher R value.
Cut the XPS to a push-fit and then caulk (just cheap acrylic exterior caulk) the edges (if you really want to go the extra mile) rather than spray foam. Spray foam in the can will make a big mess, especially in an inverted application.


.


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## Bud9051 (Nov 11, 2015)

Hi Mac, you didn't say what your concerns are, heat loss to the outside or cold floors above the cantilever?

The advice so far is addressing mostly heat loss to the outside, however packing that space full of good insulation will do little to help warm the floors above.

If the basement space exposed to those joist cavities is heated then the other option is to insulate the bottom of the cantilever and leave a warm air gap above. I couldn't find the article that specifically discusses this approach but the article below does a little.
https://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces

Air sealing is always required but the floor above will have trouble staying warm without a source of heat. Warm air in the house is quickly pushed up by the cold air from that wall and or window,

Bud


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## Nealtw (Jun 22, 2017)

A solid sheet of plywood nailed down to the joists with glue is pretty much air sealed.


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## Bud9051 (Nov 11, 2015)

More reading:
https://www.greenbuildingadvisor.com/question/new-code-option-for-cold-floors

Bud


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## mac1990 (Mar 14, 2017)

SPS-1 said:


> I would use XPS.
> .


I choose EPS because it does not lose R value over time and will dry out if it manages to get wet. It is also a better value per R.

The EPS I purchased has a foil face, should be fine.






Bud9051 said:


> Hi Mac, you didn't say what your concerns are, heat loss to the outside or cold floors above the cantilever?


Something I did not even think about. The floors above the cantilever are cold in the winter.

All the areas above the cantilever are heated, but only half the area below the cantilever is heated. Half the cantilever is off an unheated garage, while the other half is in a heated finished basement.

So do the following when the cantilever is connected to a heated area:


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## Bud9051 (Nov 11, 2015)

If we use the example of the ceiling above a cold garage and below bedroom space above and there is no heated basement to warm that air gap, it is still beneficial to create the gap to allow it to share some of the heat from above. As long as there is sufficient space for a lot of insulation and the rim and other leak areas are well sealed I believe it a better approach. Cold floors are hard to warm up so any help they can get is good.

So, I would consider leaving the air gap for the entire cantilever, just blocking it where the joists are exposed to the unheated space.

Bud


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## Nealtw (Jun 22, 2017)

mac1990 said:


> I choose EPS because it does not lose R value over time and will dry out if it manages to get wet. It is also a better value per R.
> 
> The EPS I purchased has a foil face, should be fine.
> 
> ...


Solid blocking over the lower wall is code


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## mac1990 (Mar 14, 2017)

Nealtw said:


> Solid blocking over the lower wall is code



what?


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## Nealtw (Jun 22, 2017)

In yellow, no air thru that area.


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## mac1990 (Mar 14, 2017)

Nealtw said:


> In yellow, no air thru that area.



I removed that because @*Bud9051* stated the following: 
"If the basement space exposed to those joist cavities is heated then the other option is to insulate the bottom of the cantilever and leave a warm air gap above."

I thought the air gap wanted the warm air from the heated basement area.


I dont know if I am even going to mess with the air gaps. How much of an air gap is needed? It may mess up the thickness of insulation I was going to buy.


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## Bud9051 (Nov 11, 2015)

Neal, can you provide a reference for your statement "Solid blocking over the lower wall is code"? Just trying to clarify whether this is part of an energy star requirement or one of the many generations of building codes of which local authorities can pick and choose.

Bud


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## Nealtw (Jun 22, 2017)

Bud9051 said:


> Neal, can you provide a reference for your statement "Solid blocking over the lower wall is code"? Just trying to clarify whether this is part of an energy star requirement or one of the many generations of building codes of which local authorities can pick and choose.
> 
> Bud


 No but when we had to finish up some houses that other framers had started both the city and the engineers called for it when they had been left out.
MY book was so far out of date I tossed it.


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## Bud9051 (Nov 11, 2015)

Did more searching and was unable to find a specific reference to leaving the air gap when insulating a cantilever joist cavity. I try again later.

But the trade off whether the top of the wall is blocked or not is the air gap will peovide slightly warmer floors even without the contribution of heat from a conditioned basement. There is an obvious trade off in that less insulation will cost more but for that amount of space you might be talking a couple bucks per year.

On the other side the increase in temperature isn't going to be 10°.

With two of the best in the business, Joe and Martin, not able to agree doubtful we will be here either. That leaves it to you to flip a coin. If Neal can provide a relevant code requirement for the block above the interior wall that should be included but still leaves the air gap option.

Best
Bud

Just to note, Neal is in Canada and op is in PA


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## Nealtw (Jun 22, 2017)

https://basc.pnnl.gov/resource-guides/cantilevered-floor#quicktabs-guides=6


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## 3onthetree (Dec 7, 2018)

Nealtw said:


> https://basc.pnnl.gov/resource-guides/cantilevered-floor#quicktabs-guides=6


This is the detail I saw in that link


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## Nealtw (Jun 22, 2017)

3onthetree said:


> This is the detail I saw in that link


The picture I posted is what came up with the site in google??:biggrin2:


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## carpdad (Oct 11, 2010)

Although nealtw is in canada, I think warm part, assuming from what he said about the frost depth of the footing. PA is probably cold all over. Don't know how that affect the use of vapor barriers and such. For op in pa, cold but not necessarily arctic, vapor barrier inside (plastic behind the drywall) is not recommended (at least by me:smile. Am not a pro on insulation and if considering this kind of vb, should search for effects.

Also, about the last image. Foam board sandwiches the fiberglass insulation and that looks like a vapor trap. I would opt for foam board on the bottom, just because it is closer to weather, and leave the top so if any vapor, it has the top to escape. This is moot if there is air leak.


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## jpdx (Sep 17, 2020)

I have seen two different methods (two schools of thought?) for cantilevers:

1) Foam board installed just below the subfloor, plus foam board above the foundation sill plate to seal the cavity. This is what your image shows.

2) Foam board installed just above the soffit, plus foam board just inside the rim joist.

A variation/upgrade in option two is to open the soffit and install one large sheet of foam board under the joists, which will air seal really well and insulate the best since it prevents thermal bridging through the joists.

I would love to hear people chime in more on the pros and cons of each approach.


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## Bud9051 (Nov 11, 2015)

Unfortunately there is no one size fits all. The depth of the overhang, the conditioning of the adjacent basement, climate zone, and what ever is above all can affect the choice.

I like to ask if cold floors above are an issue and if so and the cavities are not too deep I like leaving a few inch gap above lots of well sealed insulation, assuming a warm heated basement. In this way the heat from the basement will keep those floors above nice and warm.

Here is a related article that uses the space above the insulation approach.
https://www.greenbuildingadvisor.com/question/new-code-option-for-cold-floors

Bud


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## mac1990 (Mar 14, 2017)

jpdx said:


> I have seen two different methods (two schools of thought?) for cantilevers:
> 
> 1) Foam board installed just below the subfloor, plus foam board above the foundation sill plate to seal the cavity. This is what your image shows.
> 
> ...



I changed my approach based on what people said in this thread.












I am having trouble in the red area, cant get a caulking gun in there. I am going to have to use some spray foam cans it seems.


..and yes putting foam below the joists would be great, if you have room.


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## Bud9051 (Nov 11, 2015)

Just want to add one scenario to help understand the problem of cold floors.
If you filled those cavities completely full of closed cell foam there would be no air leakage and plenty of insulation. Sounds good but where would the heat come from to warm the floors above? Any cold air from walls or windows will settle to floor level. The warm air in the room above will be pushed up away from the floors. The only heat source will be radiant exchange from the room above and even if one tries to manage that it is difficult to predict.

Heat from below is best.

Bud


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## mac1990 (Mar 14, 2017)

Bud9051 said:


> Heat from below is best.



I do like the idea of an air gap, but I just cannot trust ill completely air seal, in this renovation.


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## SPS-1 (Oct 21, 2008)

Bud9051 said:


> Sounds good but where would the heat come from to warm the floors above?


Mostly from convection from the room full of 77 degree air, and a certain amount of conduction along the sub-floor and perhaps flooring.


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## mac1990 (Mar 14, 2017)

I am thinking maybe it would be better to use zip sheathing attached to the bottom of the joists to create an air barrier. Might even be able to fit R3 insulated zip sheathing in there before the soffit.


I would then be able to get R30 Rockwool in the cavity with the foam board only up against the rim joist and blocking the inside.


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## jpdx (Sep 17, 2020)

What's your reasoning on using Zip sheathing instead of XPS in that area?


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## mac1990 (Mar 14, 2017)

jpdx said:


> What's your reasoning on using Zip sheathing instead of XPS in that area?



Well I am using EPS, not XPS, but..



In that pictured area, where the old J channel is, right above the bricks and right below the joists, ranges from ~3/4-1/2". It is the area I am having trouble air sealing. So..

I will be able to stick a 7/16" zip board all the way in there without damaging it. Tape the zip board seams, caulk the screw holes and have a nice air tight/secure place to nail my new J channels.

In the cavity, I can then have full R30 rockwool, with 1.5" pieces of foam at the header and blocking the entrance to the living space.

This seems like a more solid solution than trying to stick any type of foam in that tight brick/joist area.


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