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insulating outside basement walls new construction

Posted by gardengirl53 (My Page) on
Thu, Oct 15, 09 at 20:17

We are needing info on this real fast. We want to insulate the outside of our basement walls. We were going to put up 2" xps 4x8. We live in the Chicago area. Also cut an additional foot to put at the top that would be exposed above ground. I just talked to a contractor that took a LEED class to get certified and she said that you don't need to insulate below the frost level and that all that is necessary is the first 4' below ground level and then just stuff in between the inside joists like an R38 batted insulation. Do we need 2"? and is she right about that? Can we just turn the 4x8 horizontally and run it around the top portion of the exterior basement walls? Would that suffice? She said below frost area in the midwest never gets below 50. Any thoughts appreciated. Thanks much


Follow-Up Postings:

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RE: insulating outside basement walls new construction

you don't need to insulate below the frost level

I don't know your Code off-hand. That may be acceptable.

But from the view of long-term cost efficiency and comfort, it makes more sense to insulate to the floor. And maybe even under the floor. I'm assuming the basement will be finished.

Especially if you're planning to keep the basement above 50˚F. in the winter. Without insulation, you stand a good chance of getting condensation on the concrete from air currents in the finished wall.

Overkilling the wall with insulation for the first four feet (R 50+) then forgetting about the bottom two or three feet is rather odd.

Insulating only the band, as you suggest, makes no sense and is likely not Code compliant.

In any case, using exterior foam presents an insuperable problem on a masonry veneer home. Thermal bridging with masonry veneers negates the benefit of exterior insulation altogether. As well, unless the exterior insulation is meticulously protected before and after construction, it will be degraded. But, fatally to my mind, even if you get exterior insulation to work thermally, you've provided an interstate for termites. And, in Chicago, you've got them.

I've been a licenced builder in Toronto for 20 years. After fiddling with different systems, now I insulate the basement on the interior with: 1" XPS followed by fg batts; or with cc spray foam to Code minimum or followed by fg, I have the key joist band area sprayed with cc or oc foam. Other acceptable alternatives include EPS (it has to be much thicker than XPS), isocyanurate boards or fg with Membrain vapour barrier.

Here is a link that might be useful: Building Science Corp on basements


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RE: insulating outside basement walls new construction

So I'm confused. I'm not planning on finishing the basement at this point. Depending on dollars down the road perhaps.
So if I was going to insulate the outside how should I do it?
I was told the xps does not degrade except at the top that will be exposed to the elements where it is above grade. So if I don't need to insulate below frost level on the outside where if I even should would I start? It doesn't sound like I need to use full 4x8 sheets going down to the bottom at all.

Maybe even if I don't finish the basement now I should forget outside insulation and just insulate the basement on the interior and have blue or pink walls for now :)

Thanks Worthy for your expertise, I was hoping you would answer.


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RE: insulating outside basement walls new construction

bump


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RE: insulating outside basement walls new construction

the xps does not degrade except at the top that will be exposed to the elements where it is above grade.

That's the most important part! It's where the foundation meets the wood superstructure of the home and where there are a host of leaky joints.

Degradation of the insulation, thermal bridging with masonry veneers and the insect problem are why I wouldn't bother with external insulation at all. (Yes, I know there is at least one termite resistant foam insulation.)

It's simpler and more effective to insulate the inside with any of the systems I described. For all the details, see the Building Science Corp. link above. (Prefab systems with the insulation inside the concrete wall are very effective too.) However, you will have to provide a thermal barrier over any interior insulation system that uses foam. Drywall is the most commonly used barrier, which already puts you part way to a finished basement. To save money, you can just have those walls taped, not totally finished.

I hope this is of some help.


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RE: insulating outside basement walls new construction

Its immense help! Its freezing here already and we were going to do the exterior insulating. Thanks!! One last question though. I can't just put Owens Corning foam insulation around the inside basement walls without covering them?

If so, why not?


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RE: insulating outside basement walls new construction

IRC section R314.2.3, which has been adopted by most US jurisdictions, requires a fire barrier. The one most commonly used is 1/2" drywall. Some Codes allow foil covered foams.

If you've ever smelled foam burning, you would know why it should be covered. Plumbers in a recent build of mine accidentally set fire to a couple square inches of cc foam. The acrid smell sent everyone working in the home running outside.

As an alternative insulation, you could use fiberglass covered by MemBrain, if it is available in your area.


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RE: insulating outside basement walls new construction

Thanks Worthy, I'm not to happy about that we really don't have the finances to drywall the basement also for tax purposes we don't really need or want to finish it. I haven't heard of this MemBrain is it comparable to the foam insulation and price wise? Sorry to keep bothering you. This might be the reason people are insulating outside as they don't have to finish their basements but can still take advantage of the insulation from the foam.


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RE: insulating outside basement walls new construction

Okay, the cheapest effective way to safely insulate and not board is to use foil-covered polyisocyanurate boards extending four feet below grade on the interior wall, just past the frost level as your LEEDs lady suggested.

This way, you can finish the wall off at a later time if you want and you're getting the majority of the benefits of basement insulation without the insect, degradation and thermal problems of exterior insulation.

For further details, open link below.

Here is a link that might be useful: Building Science Corp. RR-0202


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RE: insulating outside basement walls new construction

Thank you Worthy xxx :)


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