Basement Insulation

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Old 03-23-16, 09:05 PM
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Basement Insulation

We are finishing off our basement in stages. Started with a bathroom that had one short exterior wall. We attached 2" of rigid foam directly to the concrete wall, built a 2" x 4" wall next to the foam and then filled the cavities with more rigid foam.

I submitted that plan to the city when we applied for the building permit and it was approved, but when the building inspector came out today he said he wasn't going to make me redo it but that what I did does not meet code. That code requires a 1" "air gap" between any insulation or framing and the concrete wall. He suggested in the rest of the basement that I build a 2" x 6" wall set 1" out from the concrete and insulate that with kraft backed fiberglass insulation being sure to still leave the air gap.

Am I crazy? I have searched high and low on line and can find no examples of insulating a basement with an air gap like that? I really want to insulate the rest of the basement the same way we did the bathroom. It was easy to do and seems to be really efficient. We used poly-iso and by my calcs were at about R-22 when done.

Is my inspector just wrong or has anyone else heard of an air gap requirement like that?
 
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Old 03-23-16, 09:23 PM
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I believe your inspector is wrong and I will search for some related articles specific to this issue. Are you on the warm or cold side ow WA?

Bud
 
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Old 03-23-16, 09:49 PM
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Eastern Washington.

Thanks
 
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Old 03-23-16, 10:19 PM
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Well, that was frustrating. What a pile of hear-say and old school advice. Unfortunately I could not find the article I was looking for so I'll just have to, for now, explain what the issue is. Now, recognize that the existence of 2" of air sealed rigid foam may eliminate this concern, but here is what the objections are.
1. A gap creates an opportunity for convective air flow which increases heat transfer.
2. The rigid foam is specifically used because it has a small amount of permeability to allow some drying to the inside. (I'll get to your issue of filling the cavities with more rigid.) Once that moisture enters a space between the rigid and the stud wall, it will be carried to the top which "may" be cool enough to result in condensation. This is where your 2" of rigid may protect you from this vertical moisture movement being a bigger problem.

Now, before I forget, continuing to fill those cavities with more low permeability rigid insulation may slow the moisture flow to the point where you effectively have a vapor barrier. Once you get past the inspector issue, I would use Roxul in the cavities directly against the rigid.

If you can figure out how to ask nicely, ask your inspector where in the codes the gap is required and what energy code you are currently under, 2009 or 2012 IECC (international energy conservation code) i say ask nicely because local code officials can set their own rules.

I'll keep looking.

Bud
 
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Old 03-24-16, 08:41 AM
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Thanks, good advice! Roxul is new to me, but I researched it on line and it looks like a great product for what we are doing. I'll tactfully talk to the inspector. He was a friendly guy, he wasn't a jerk but maybe just not up to date. After all it's our house and were just trying to do the best job we can and we are not trying to do the job on the cheap.
 
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Old 03-24-16, 09:24 AM
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Building science corp has many articles on basement wall insulation. Below is a short article that illustrates wall insulation. The second link is a longer version with more illustrations.

Been awhile since I read these top to bottom, but a quick review I can find no reference to the need for "no gap". But, I also find no reference for "must leave a gap". And in all the illustrations, there is no gap included.

The bad reputation for leaving a gap, IMO, came before the rigid insulation against the wall approach was popularized. The problem was the exposed very cold concrete at the top and the gap would allow air to circulate and transport moisture up to that cold condensing area, depositing moisture and even forming ice. But, with one or two inches of rigid insulation we no longer have those cold condensing surfaces to be worried about, as long as we don't add too much insulation to the inside of the rigid. There is a ratio where your area would want the rigid to be something like 30% or more of the total assembly r-value, reference available if needed.

As mentioned in the previous post, too much rigid can reduce the permeability to essentially a vapor barrier. Thus, if you switch to filling those cavities with Roxul (very dense but still very open) then you will restore the ability of the assembly to dry to the inside and that may be enough of a change for the inspector to approve this approach. I don't believe the one wall you completed will be a problem unless it was a known wet wall. All concrete walls will still dry to the outside at the top, but slowly.

Bud
Basement Insulation | Building Science Corporation
Understanding Basements | Building Science Corporation
 
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