# 2x6 Insulation in a spaced 2x4 wall



## alexlacelle (Feb 27, 2012)

Last winter, I had a basement leak which prompted me to have my entire foundation dug up and waterproofed with a membrane, covered by 1 inch styrofoam. 



I removed all of the previous insulation and framing in the basement and am starting from scratch. As I heat with electric, I'd like to insulate as much as possible while keeping the costs affordable. 



I plan on using a house wrap against the foundation walls to aid in preventing any potential moisture problems, but then I was hoping to frame a 2x4 wall, 2 inches away from the foundation wall, and then use 2x6 Mineral Wool insulation in the cavity seeing as how I'd have the room for it. I would then use a vapour barrier. This way I can get an R-22 rather than a standard R-14 mineral wool for 2x4 wall. Is this doable?



I'm in Ontario Canada if that makes a difference.


Thanks!


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## Bud9051 (Nov 11, 2015)

Hi Lemkie, top priority will be to air seal where the house rests on the foundation and all seams in that rim. Also use some rigid insulation cut to fit that rim and seal it in place.

The rim and exposed concrete are the worst for heat loss. At 2' below grade the benefit of the soil kicks in and it reduces the heat loss. Codes still insist on insulating the foundation all the way to the floor but it is the upper half where the heat escapes.

House wrap is designed to allow moisture through, high permeability. With a good moisture barrier on the outside and the 1" of rigid there is no need for a vapor barrier on the inside. Any moisture that gets into that wall will need a way to get out and it should be so little the conditioned basement will never notice. Check Canada codes as they may still want a VB but I have citations that suggest not.

As for the gap and the 6' insulation another layer of rigid and then the framing directly against it would be more than enough. Leaving a gap can transport moisture from below up to cold areas at the top.

That was a massive effort to install the membrane on the exterior you should be fine with 3.5" Roxul.

Bud


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## alexlacelle (Feb 27, 2012)

Bud9051 said:


> Hi Lemkie, top priority will be to air seal where the house rests on the foundation and all seams in that rim. Also use some rigid insulation cut to fit that rim and seal it in place.
> 
> The rim and exposed concrete are the worst for heat loss. At 2' below grade the benefit of the soil kicks in and it reduces the heat loss. Codes still insist on insulating the foundation all the way to the floor but it is the upper half where the heat escapes.
> 
> ...



Thanks for the quick reply Bud,


Vapour Barrier is still required here by code, so I will have to put that up. Rim joists have already been covered with 2 inch styrofoam, spray foamed around the edges, and covered with Mineral wool. Prior to that I did my best to caulk everything I could to prevent air leaks, so I'm all set there. 



Styrofoam on the foundation walls would be challenging because the wall is a little wonky, which is why I'm hoping for straight mineral wool option. That being said, do you still think I would be fine with the 3.5 R-14 or should I then opt for the 5.5 R-22?


Thanks!


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## Nealtw (Jun 22, 2017)

I don't think house wraps helps there. 

The problem that happens other that leaky walls, are when the cold from the outside gets to the warm surface or when the warm air can get to a cold surface. We do a 2x4 wall 1 inch from the concrete, insulate that with a vapour barrier. If you start with a 2" foam and tape and seal that you have likely solved that and the rest is comfort insulation.
Your first step should be the the top plate of the wall. It should reach all the way back to the sill sitting on the top of the foundation. That is called fire stop but it also stops air flow from the wall to the floor system above. We usually use 2x8 but it can be 1/2" plywood or OSB.


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## alexlacelle (Feb 27, 2012)

Nealtw said:


> I don't think house wraps helps there.
> 
> The problem that happens other that leaky walls, are when the cold from the outside gets to the warm surface or when the warm air can get to a cold surface. We do a 2x4 wall 1 inch from the concrete, insulate that with a vapour barrier. If you start with a 2" foam and tape and seal that you have likely solved that and the rest is comfort insulation.
> Your first step should be the the top plate of the wall. It should reach all the way back to the sill sitting on the top of the foundation. That is called fire stop but it also stops air flow from the wall to the floor system above. We usually use 2x8 but it can be 1/2" plywood or OSB.



Thanks for the tip on the top plate I hadn't considered that and will be sure to do that. 



Styrofoam would be tricky on the foundation walls I have, which is why I'm hoping for a straight mineral wool insulation plan. My walls are a little wonky which is why I was hoping to at least use an R-22 batt if possible. I want to avoid framing with 2x6 however again because of the way the walls are. That being said can I get away with my framing 2x4 2 inches away from the foundation and using a 2x6 batt or is that just a waste?


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## 3onthetree (Dec 7, 2018)

Lemkie said:


> my entire foundation dug up and waterproofed with a membrane, covered by 1 inch styrofoam.
> 
> I'd like to insulate as much as possible while keeping the costs affordable.
> 
> ...


I would not use a house wrap in the basement, it does you no good and could be detrimental. You do know that they have different moisture permeability properties on each side? 

In a basement, the moisture will be coming in to the space from the concrete. You want to be able to naturally dissipate that as much as possible. Since there isn't a huge difference in temperature and it is always stable, there isn't as much need for vapor barriers as condensation has less chance of forming. That is why the current thought is eliminate basement VBs in most climates to allow the walls to breathe. I'm not sure of your climate for a VB, but I wouldn't use a house wrap at all regardless if you use a VB or not.

Moving up the R value of batt insulation in a basement doesn't have a lot of return on value (especially having exterior insulation, which is the best thermal location). Your best bet is stick with 3.5", keep it away from the wall, and focus on sealing/insulating the rim joist area.

_Edit: I guess I type slow, so just repeating info from others here._


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## alexlacelle (Feb 27, 2012)

3onthetree said:


> I would not use a house wrap in the basement, it does you no good and could be detrimental. You do know that they have different moisture permeability properties on each side?
> 
> In a basement, the moisture will be coming in to the space from the concrete. You want to be able to naturally dissipate that as much as possible. Since there isn't a huge difference in temperature and it is always stable, there isn't as much need for vapor barriers as condensation has less chance of forming. That is why the current thought is eliminate basement VBs in most climates to allow the walls to breathe. I'm not sure of your climate for a VB, but I wouldn't use a house wrap at all regardless if you use a VB or not.
> 
> ...



I actually didn't know that there were different moisture permeabilities on both sides so thanks for that information. I had read that it was a good idea, but with your explanation I can see how it would cause a problem. I've use tar paper on foundation walls in the past, but now I'm wondering if even that wasn't the greatest idea. 



I certainly see how not using a vapor barrier would allow the walls to breath, but seeing as it's code I don't think I have much of an option there considering I'm staying away from styrofoam or spray foam. 



As for the insulation I'm feeling like I should just stop overthinking it and go with the 2x4 batts like everyone says.



Thanks for your time!


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## Bud9051 (Nov 11, 2015)

Here's an article by a Canadian author explaining how and why in regards to leaving a space behind those basement walls.
https://www.joneakes.com/jons-fixit...lls-myth-and-science---overview#a08Foundation

Bud


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## Nealtw (Jun 22, 2017)

Lemkie said:


> Thanks for the tip on the top plate I hadn't considered that and will be sure to do that.
> 
> 
> 
> Styrofoam would be tricky on the foundation walls I have, which is why I'm hoping for a straight mineral wool insulation plan. My walls are a little wonky which is why I was hoping to at least use an R-22 batt if possible. I want to avoid framing with 2x6 however again because of the way the walls are. That being said can I get away with my framing 2x4 2 inches away from the foundation and using a 2x6 batt or is that just a waste?



With a 1" gap that we have with good fitting insulation and vapour barrier 

sealed outlets.
That 1" gap also has a system the top of the wall can go to 0* and less but the bottom of the wall is a constant 55* F. So the outside of that 2x4 never gets as cold as a 2x4 exterior wall would get.


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## alexlacelle (Feb 27, 2012)

Bud9051 said:


> Here's an article by a Canadian author explaining how and why in regards to leaving a space behind those basement walls.
> https://www.joneakes.com/jons-fixit...lls-myth-and-science---overview#a08Foundation
> 
> Bud



Thanks I'll have a look!


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## alexlacelle (Feb 27, 2012)

Nealtw said:


> With a 1" gap that we have with good fitting insulation and vapour barrier
> 
> sealed outlets.
> That 1" gap also has a system the top of the wall can go to 0* and less but the bottom of the wall is a constant 55* F. So the outside of that 2x4 never gets as cold as a 2x4 exterior wall would get.



I'm sorry but I'm not sure I understand what you mean here with "that 1" gap"


I will be sealing outlets, etc to the vapor barrier.


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## Nealtw (Jun 22, 2017)

Lemkie said:


> I'm sorry but I'm not sure I understand what you mean here with "that 1" gap"
> 
> 
> I will be sealing outlets, etc to the vapor barrier.


 We have 3 1/2" insulation in a 2x4 wall 1" from the concrete so behind the wall there is a 1" gap. 



I live in a suite built like this with electric baseboard heating and have not had a problem with anything. Except one closet that should have had some heat included.


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## alexlacelle (Feb 27, 2012)

Nealtw said:


> We have 3 1/2" insulation in a 2x4 wall 1" from the concrete so behind the wall there is a 1" gap.
> 
> 
> 
> I live in a suite built like this with electric baseboard heating and have not had a problem with anything. Except one closet that should have had some heat included.



Ok I understand now. This will be what I will be going with too.


Thanks!


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## 3onthetree (Dec 7, 2018)

Lemkie said:


> I'm sorry but I'm not sure I understand what you mean here with "that 1" gap"


The gap can be a problem and I think that's what Bud could be referring to. Best current practice is block the chimney effect in the gap. 

For me I encounter so many leaky basements that I want _water_ to be unimpeded to the floor and not soaked up in the wall, which trumps any top _moisture_ and that hopefully using the best practice in rim sealing and outside insulation might reduce the chimney effect to some degree.


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## Nealtw (Jun 22, 2017)

3onthetree said:


> The gap can be a problem and I think that's what Bud could be referring to. Best current practice is block the chimney effect in the gap.
> 
> For me I encounter so many leaky basements that I want _water_ to be unimpeded to the floor and not soaked up in the wall, which trumps any top _moisture_ and that hopefully using the best practice in rim sealing and outside insulation might reduce the chimney effect to some degree.


 It is not a chimney effect, it is a weather system, cold from the upper foundation drops, heat from the natural ground temps rise. 

He has already made the upgrades to the outside of the foundation. 

So he is left with water that can wick up in the summer when the top of the foundation gets warm. Keeping interior heat away from the wall gives that water no reason to enter that space or to the wall and insulation.


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## carpdad (Oct 11, 2010)

This may be already noted. I didn't read all posts. Your idea is fine although air space behind the studs and along the foundation (because foundation wall is not as smooth) will decrease some of the insulation value. Air will be moving around in these pockets. This may be nitpicking.:smile: 

If you wanted as much insulating because of your energy cost, I would build the wall standing, put 1-1.5" xps foam boards behing the wall, slide the wall into position then rockwool the bays. Caulk the floor sill of the wall and fireblock and foam seal the top. For fire blocking the top, I'm in nj and 2" of rockwool was minimum required.


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