# Insulating Basement Walls



## SPS-1 (Oct 21, 2008)

I don't understand how the exterior insulation effects anything. Even if there is no exterior insulation (and possibly even with exterior insulation) there would be tar-like damp-proofing on the exterior of the foundation. 
Condensation happens from warm, humid interior air hitting a cool surface, not from moisture in the walls.
I suppose if the exterior insulation is thick enough, you could hope that the foundation wall does not get cold enough to cause condensation, but I wouldn't count on it. In a cold climate, I would want a vapor barrier. Be careful about what you read on the internet about vapor barriers --- the answer depends on what part of the country you live in. If they make blanket statements, then right off the bat, they are giving poor advice.


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## Nealtw (Jun 22, 2017)

The insulation on the outside is great for an un finished basement. It only works if you are heating the inside of the wall. 
Once you build a wall inside you are cutting off heat to the wall so in a cold spell it is just a matter of time for the concrete to be the same temp as the outside.
We build the wall with a space and insulate that and use a vapour barrier.
Condensation is caused when warm air from the house can get thru the wall and the insulation and get to a cold surface. 
The important stuff is a good fitting insulation so cold does not get to the back of the board and air can not get into the wall and get to cold. 
I think with or with out foam against concrete is good, sill gasket under floor plate, fire stop the top, vapour barrier the walls or seal the drywall, electical boxes can be a problem.


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## Bud9051 (Nov 11, 2015)

I feel your pain, too much conflicting information and it would take an encyclopedia to sort out your concerns.
You have probably already seen this link but just in case:





BSD-103: Understanding Basements


Now that basements are becoming livable spaces keeping them dry, comfortable and contaminant free is proving to be anything but simple.




www.buildingscience.com





I try to use advice from known reliable sources like building science above or GBA (greenbuildingadvisor) below:








How to Insulate a Basement Wall - GreenBuildingAdvisor


Here at GBA, we regularly receive questions from readers about the best way to insulate a basement wall. Since these questions pop up frequently, it’s time to pull together as much information as possible on this topic. In this article, I’ll try to explain everything you always wanted to know...




www.greenbuildingadvisor.com





From my reading the air gap is out. Vapor barriers maybe optional where you are but out in most cases. Rigid foam board without a foil or plastic covering is considered a vapor retarder not a perfect vapor barrier like a layer of plastic.

I like the rigid foam board tight to the wall and taped with studs tight to the foam board. Stud cavities then fill as desired with a plus to Roxul and no VB.

Good luck.

Bud
Note both links above have many articles. GBA may require you to register but they have a short term free option.


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## ballaby (Dec 26, 2017)

Bud9051 said:


> I feel your pain, too much conflicting information and it would take an encyclopedia to sort out your concerns.
> You have probably already seen this link but just in case:
> 
> 
> ...


I appreciate the response. I have read both sources you posted in the past couple days. As I am sure you are aware, it isn't always easy to tell what is a reliable source and what isn't on the internet .

I am leaning towards 1" XPS foam board against concrete wall and seal as best I can with tape at the seams and canned spray foam around the bottom. Then build the 2x4's against that and fill the stud cavities with unfaced fiberglass since I already have it. Theoretically there is no way the fiberglass insulation can touch the concrete and should never be able to get wet. 

So there is no concern with having rigid foam board both on the internal and external side of the foundation walls? Doesn't concrete need a place to dry? With the approach I am leaning towards I would worry about moisture building up between the concrete and foam board, but maybe I am overthinking it at this point.


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## Bud9051 (Nov 11, 2015)

concrete loves moisture. As for buildup read about vapor retarders. they allow a small amount of moisture to pass which avoids any unusual accumulation.

The 1" of rigid plus whatever is on the exterior will ensure the concrete remains above the dew point.

Bud


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