# Re-thinking my basement wall insulation system



## OhioHomeDoctor (Sep 27, 2011)

Dont reinvent the wheel. Use full studded walls. You be glad you did.


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## Windows on Wash (Aug 30, 2011)

OhioHomeDoctor said:


> Dont reinvent the wheel. Use full studded walls. You be glad you did.


+1

I prefer a full studded wall as well.


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## 1jumper (Aug 30, 2011)

Thanks for the replies. Anyone else?


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## ccarlisle (Jul 2, 2008)

Well, that depends a bit on where you are because on the one hand you're looking at a wall assembly that has an R-value of about 7.5 (see later) and on the other you're looking at about R18. So, which one fits?You don't mention the need or not of a vapour retarder either, so I'll assume you're not dealing with cold winters.

But the better assembly for drywall-to-furring strips is the use of 1.5" of rigid foam boards, and 1.5" concrete screws - otherwise you'll be dealing with 2.5" screws over 2" rigid foam to get an R10...I don't like that configuration really. So, back to the 1.5" boards and screws. 

But whether or not you need an extra R13, only your situation will really tell. In some parts, it's code to have a certain amount and therefore not really worth it to implement your own 'hybrid' assembly.


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## AGWhitehouse (Jul 1, 2011)

I'd recommend 2" XPS foam glued to the conc. wall with taped seams and a 2x4 framed wall over the top. It'll give you the vapor control and the freedom for electrical and wall mounting.


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## 1jumper (Aug 30, 2011)

Thanks for the advice. I live in mid-Michigan so winters are cold. I guess I'm going with the original stud framing route. It'll keep things easier even if it isn't quite as simple as firring strips and 2"foam.


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## High Gear (Nov 30, 2009)

Get the large caulk gun as the pl is lots cheaper that way and don't be stingy with the glue.

Any air that can get around from the foam/concrete wall and you've lost 

the advantage of putting it there in the first place.

I put a heavy bead in the groove joint vs using tape.


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## concretemasonry (Oct 10, 2006)

Don't over-think the concept of insulating a basement. It really isn't worth it in many situations.

The exterior Soil temperature is extremely stable and does not reflect the exterior AIR temperature. The cool soil is actually a benefit on an annual basis. The frost depth is a figment of code writers imagination.

When you are dealing with theoretical R-values of lightweight insulation you are only looking at the theoretical short term laboratory values over a hour ot so for the insualation "material" in an ideal situation (dry) and not the effects of the thermal short circuiting of any studs. Granted, XPS is a far better value because it can be a complete thermal break.

A basement is actually a semi-conditioned space and if it has air returns, there are summer benefits from the temperature heat sink repersented by the exterior soil.

Dick


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## daphillenium (Oct 14, 2011)

ccarlisle said:


> Well, that depends a bit on where you are because on the one hand you're looking at a wall assembly that has an R-value of about 7.5 (see later) and on the other you're looking at about R18. So, which one fits?You don't mention the need or not of a vapour retarder either, so I'll assume you're not dealing with cold winters.
> 
> But the better assembly for drywall-to-furring strips is the use of 1.5" of rigid foam boards, and 1.5" concrete screws - otherwise you'll be dealing with 2.5" screws over 2" rigid foam to get an R10...I don't like that configuration really. So, back to the 1.5" boards and screws.
> 
> But whether or not you need an extra R13, only your situation will really tell. In some parts, it's code to have a certain amount and therefore not really worth it to implement your own 'hybrid' assembly.


I thought from reading In these forums that a vapor barrier is undesired in a basement. I've read a few places that it "needs to breathe."


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## BlueBSH (Oct 29, 2009)

daphillenium said:


> I thought from reading In these forums that a vapor barrier is undesired in a basement. I've read a few places that it "needs to breathe."


everything I've read contradicts itself it seems like.... 

seems like you need at least one direction for it to breath... either inward or outward... but then houses now days can be ICF which is insulated on both sides with 2" EPS but that isn't a complete vapor barrior like 2" of XPS is...


I just wouldn't put XPS on the inside if you have it on the outside and vice versa... anyone else think that is a good idea?


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## AGWhitehouse (Jul 1, 2011)

BlueBSH said:


> I just wouldn't put XPS on the inside if you have it on the outside and vice versa... anyone else think that is a good idea?


If you have 2" XPS or greater on the outside, then 1" max. XPS on the inside and vice versa. 2" of XPS is a class II vapor retarder, while 1" is a class III. EPS foam remains a class III retarder for thicknesses beyond most common applications.


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## AGWhitehouse (Jul 1, 2011)

daphillenium said:


> I thought from reading In these forums that a vapor barrier is undesired in a basement. I've read a few places that it "needs to breathe."


 Yes, wall systems need to breath. The problem for basement spaces is that the breathing only really happens to the interior because the exterior is underground and usually moisture laden. The reason it's undesireable to have a vapor barrier on the inteiror side of a basement wall system is that if moisture does drive in, a barrier will block and trap it, leaving it no-where to go. This trapped moisture can lead to structural decay of the wall systems over the course of it's functional lifetime. Having a vapor retarder (class II at most) directly against the wall will provide a membrane that slows the transfer of moisture during wet weather to prevent water-based damages, but will allow it to dry to the interior during dry weather conditions.


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## BlueBSH (Oct 29, 2009)

AGWhitehouse said:


> Yes, wall systems need to breath. The problem for basement spaces is that the breathing only really happens to the interior because the exterior is underground and usually moisture laden. The reason it's undesireable to have a vapor barrier on the inteiror side of a basement wall system is that if moisture does drive in, a barrier will block and trap it, leaving it no-where to go. This trapped moisture can lead to structural decay of the wall systems over the course of it's functional lifetime. Having a vapor retarder (class II at most) directly against the wall will provide a membrane that slows the transfer of moisture during wet weather to prevent water-based damages, but will allow it to dry to the interior during dry weather conditions.


what class would kraft faced insulation be in? when our house was built they did part of the basement with only R-13 kraft faced no XPS or foam inside or out


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## leungw (Apr 20, 2009)

BlueBSH said:


> what class would kraft faced insulation be in?


I believe it's Class II.


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## AGWhitehouse (Jul 1, 2011)

Kraft paper is considered a class II retarder. IF a vapor retarder is used in a basement assembly, it is highly recommend it be placed directly against the concrete surface. Faced batt. insulation against a concrete basement wall is not recommended by me.


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