Basement rim joists

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Posted by: from Bourbonnais
4/23/2019 at 10:52:32 AM

Is it ok to insulate the basement rim joists if you are un sure if the exterior rim joists are insulated? I live in a 5b climate zone.

Thank you!

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Date/Time4/23/2019 at 12:36:56 PM

Hi Jeff,

Assuming I understand your question properly, in cold climates its common and a matter of building code requirements that the interior header space between the joists and next to the header/rim joist be insulated and has a vapor barrier. The r-value for new homes is stipulated but if grandfathered into your situation you may want to confirm whats best for your application.

Don Merson

Cobourg Construction and Reno

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Date/Time4/23/2019 at 2:41:44 PM

Hello Jeff;

I think Don is correct, your question is a little confusing. If you are asking if you can insulate your rim joists, yes. Additional insulation can be a benefit. Many builders get by with building to minimum code requirements.

Just make sure you don't insulate in a way that causes problems down the road. You can't over insulate but you can insulate improperly.

Consult your local building inspector or ask a trusted building professional for advice.

Paul Clement RSE

Home Handyman On Call

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Date/Time4/23/2019 at 4:43:11 PM

Hello Jeff,

Thanks for your question. It is definitely okay to insulate your rim joists regardless of whether there is exterior insulation over them or not.Even just building to code requires insulating the rim joists. Unless insulation has been added to the exterior of your home it is unlikely that your rim joists are insulated from the outside.

Paste the below URL into your web browser to view a cross section of an insulated joist.

As stated by a previous poster. More insulation is usually a good thing but if you do it improperly you might not be accomplishing much and at worst you could be introducing moisture problems to the area.

There are lots of videos on YouTube on how to insulate rim joists, many specific to situations where the ceiling will be unfinished or at least not insulated itself. I'd take a look at some of those if you are planning on doing this yourself.


Ethan Borle

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Date/Time4/24/2019 at 11:30:40 PM

Yes. Houses lose a majority of the Heat through the rim joists. You could also install 6 mm vapor barrier in all the individual openings with tuck tape stop.

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Date/Time4/25/2019 at 12:12:23 AM

Hi Jeff,

It's important to remember, dry climate or not, that insulating isn't ALL about keeping cold OR hot air out. It is also important for keeping the air you desire INSIDE your own. A/C for example. For this reason alone, yes...insulate the rim joists...just do it right.

Also, there is such thing as over insulating and negative results from it, but it typically applies to attics.

Aim for an R15-R20 in the rim joists. Check with your local building authority.

All the best....

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