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johnL
foam board wall insulation and air space

It used to be, if somebody was going to finish a concrete or cinder block basement, they would put up 1x furring strips, cut some kind of foam board so it fit between the furring strips, and put paneling or sheetrock on top.
Now it seems that they put a solid layer of foam board on the wall, put the furring strips on top of the foam and then put the wall board on the furring strips, so there's a 1x gap between the wall board and foam board. At first I thought it was so that there would be no breaks in the insulation, but now I hear that it's to leave an air gap between the foam and wall board to prevent mold.

So I have two questions:
1) Have I got all this correct?
2) There's still no air gap between the foam board and the concrete wall, so what about mold there?

If somebody could clarify this for me, I'd appreciate it.

jkirk
Re: foam board wall insulation and air space

when it comes to the interior of a wall there should be no open space. you either want framing and then all voids completely filled with insulation.. having any air pockets creates spaces where warm moist air can linger and can condensate.

there should be a vapor barrier directly under the drywall aswell this ensures no moisture laden air escapes out of the building. this is part of how mold occurs.

if its on the outside of the house, and foam is applied. then yes there should be 1/2" to 1" air space between the foam and wood or fibre cement siding.. this is to allow water that gets behind the siding the ability to freely drain down the wall and not be held against the building via capilary action

GeorgeM
Re: foam board wall insulation and air space

I am insulating my garage, I have already put the R28 pink insulation in the attic above the garage.The walls already had drywall on them but no insulation in side the walls I decided to leave the drywall in place and then put furring strips at 24" centers on top of the existing drywall and fill in the spaces between the furring strips with polystyrene foam boards my question is do I now put my vapor barrier on the wall covering the polystyrene insulation now before I instal new drywall.

George.

jkirk
Re: foam board wall insulation and air space

yes george.it goes directly under the drywall.

GeorgeM
Re: foam board wall insulation and air space

Thanks jkirk - I edited my message to make it easier for people to understand.

canuk
Re: foam board wall insulation and air space
johnL wrote:

It used to be, if somebody was going to finish a concrete or cinder block basement, they would put up 1x furring strips, cut some kind of foam board so it fit between the furring strips, and put paneling or sheetrock on top.
Now it seems that they put a solid layer of foam board on the wall, put the furring strips on top of the foam and then put the wall board on the furring strips, so there's a 1x gap between the wall board and foam board. At first I thought it was so that there would be no breaks in the insulation, but now I hear that it's to leave an air gap between the foam and wall board to prevent mold.

So I have two questions:
1) Have I got all this correct?
2) There's still no air gap between the foam board and the concrete wall, so what about mold there?

If somebody could clarify this for me, I'd appreciate it.

The method of applying the rigid foam directly and continously to the foundation wall is a superior method of insulating.
This gives a continous thermal break instead of the interuppted method between studs or furring strips. The interuppted method actually has a lower overall R value.
The air gap between the foam and wall board is a result of the thickness of the furring strips --- not for mold prevention.
Mold will also be an issue if there is moisture infiltration from the foundation walls. Any organic material -- paper on the drywall -- wood framing , etc. -- will encourage mold --- the rigid foam itself doesn't encourage mold.

You don't want an air gap between the foam and the foundation -- the insulation needs to have intimate contact with the foundation wall and depends how thick the insulation and if it warms the wall above the dew point. . Any air gap between the insulation and the cold wall surface will create a stack effect ( air moving ) which has a negative affect on the insulation.
If warm moisture laden air contacts a cold enough surface --- this will create condesation then mold will form -- air gap or not.

canuk
Re: foam board wall insulation and air space
GeorgeM wrote:

I am insulating my garage, I have already put the R28 pink insulation in the attic above the garage.The walls already had drywall on them but no insulation in side the walls I decided to leave the drywall in place and then put furring strips at 24" centers on top of the existing drywall and fill in the spaces between the furring strips with polystyrene foam boards my question is do I now put my vapor barrier on the wall covering the polystyrene insulation now before I instal new drywall.

George.

Two things come to mind ........

1 ) if you are not conditioning ( heating/cooling ) this garage space then a vapour barrier is not needed.

2 ) Now , I'm a large advocate of foam insulation , however , in your case I doubt you would be using thick enough foam insulation to be really effective the way you proposed.
You would be better off with removing the existing drywall --- putting in batt insulation -- vapour barrier ( if you want ) -- then re-drywall. This way you have at least an R12 --- won't loose any more space --- if there are electrical boxes on the walls they won't have to be extended --- besides it isn't much more work than you're orginal way.

2 cents worth.

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