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Polyiso foam in basement: a rim joist question

Hi all,
Love this site, great hints
Now Im in Rockland county a little north of NYC and have a 2007 2 story colonial
I have added to the R38 with another 8" of cellulose and now am looking at the basement.
It is unfinished but I has r19 batts in the ceiling (unfinished)
So I was looking at putting 2" Poly Iso board glued to the basement walls, the ceiling height is 8ft so perfect fit for the boards
My question is the rim joist I did a cheap job of putting blocks of 1" expanded polystyrene in and sealing with Great Stuff, now if I put up the Poly Iso do I need to add another 1" of say spray foam (no time for patience to cut all those board pieces again) or because the boards are flush with the ceiling and basement wall, there will be no more humid air touching the foam blocks I put in.
Im worried that I need to add more because before I put the blocks in the batt plugs the builder put in where wet.
So do I need to add anything else to the joist space or because the iso will be flush with the floor joists once I put up sheetrock on the framed walls and ceiling I should be ok
Id hate to make a mistake to find rotting sill plate in 10 years
Any advice would be appreciated

PS I will mention this again, I LOVE THIS SITE!!

Asked by Darren Finch
Posted Tue, 02/11/2014 - 13:23
Edited Tue, 02/11/2014 - 13:51


11 Answers

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In general, it's a good idea to insulate rim joists to the same minimum R-value as above-grade walls.

In your climate zone (Zone 5), the 2009 IRC requires walls to be insulated to at least R-20.

You have insulated your rim joist to about R-4. That's not much. If you add an additional 2 inches of polyiso to your rim joists -- and from your description, it's hard to tell whether or not you are planning to do so -- you would end up with a total of about R-16. That's better than R-4, but it's still not R-20.

If you did a good job of air sealing the perimeter of each piece of expanded polystyrene that you installed at your rim joist, then I don't think your rim joist is in danger of rotting. But it's certainly true that your rim joists are poorly insulated.

For more information, see Insulating Rim Joists.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 02/11/2014 - 14:02
Edited Tue, 02/11/2014 - 14:04.

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Dear Darren:

A couple of additional tips that may be helpful, if you haven't finished your project:

> Another option for your rim joist: We packed Roxul rock wool into the end of every floor joist bay, filling the area against the rim joist. Roxul is dense, unlike fiberglass, and inhibits airflow. It has a clay-like consistency; you can tear off chunks and stuff them into the end of a joist bay, and they will stay there. Roxul is also fireproof, so the 'packed' area extended to the top of the wall stud bays -- to seal and fireblock this area, as noted below. Check with your inspector first; some require solid fireblocking.

> Fireblocking: Did you install fireblocking at the top of your basement walls, and every 10' vertically along the walls -- with a gap between the foam panels: This may be required by your inspector, is relatively easy to do and a good idea. Here are a couple of good resources:

> We sealed the joints in the foam board panels with the 'Windows' version of Great Stuff, which remains flexible. This should eliminate cracks as the assembly expands and contracts.

> If you fill and seal holes drilled for electrical wiring and plumbing, be aware that the "Fireblock" version of Great Stuff is actually very flammable -- and will ignite at just 240 degreees F, a much lower temperature than the ignition point for wood. We switched back to non-flammable fireblock sealant made by 3M and DAP. (Also sold at Home Depot and Lowes.)

I hope this is helpful.


Answered by Mark Hays
Posted Tue, 06/17/2014 - 21:24

Helpful? 0

You wrote, "We packed Roxul rock wool into the end of every floor joist bay, filling the area against the rim joist. Roxul is dense, unlike fiberglass, and inhibits airflow."

I wish you luck; your rim joist may not turn spongy for a very long time. However, your suggested approach is not recommended in cold climates.

Roxul mineral wool is air-permeable; it does nothing to prevent warm, humid interior air from contacting a cold rim joists. If you use mineral wool or fiberglass in this location in a cold climate, the rim joist will accumulate moisture over the winter. The eventual result is mold and rot.

The only two products that work in this location are rigid foam (with the edges carefully sealed) and closed-cell spray foam.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 06/18/2014 - 04:39
Edited Wed, 06/18/2014 - 04:40.

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Ive put in 3" polyiso foil faced using the Great stuff Pro foam to seal , it makes up the R20 Martin told me to minimise the chance of rotting
I certainly felt the difference after I had installed in
Next stop 45 sheets of 3" poly iso for the walls
One question I did have, should I buy a 200bft spray foam system to seal the sill and top of the foundation?
I know it may have minimal effect or is it not worth the money and effort?

Answered by Darren Finch
Posted Wed, 06/18/2014 - 09:13

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Martin, air permeability is a scale, not a boolean. Roxul is pretty significantly air retardant -- air is not going to just randomly waft right through it. If you have a leaky rim joist, then sure, maybe the stack effect will drive air through it, but as long as you seal the rim joist leaks first, I don't see how much air is going to get through several inches of tightly packed Roxul. (Even if there are leaks, stack effect will be driving cold air in, not warm air out.) When I stuffed those plugs of Roxul into my rim joist, they expelled big dust-filled blasts of air. If they were completely air-permeable, that wouldn't happen. The words "it does nothing" just don't seem true at all.

Answered by Nick Welch
Posted Wed, 06/18/2014 - 13:02
Edited Wed, 06/18/2014 - 13:03.

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Seriously even though I had the 1" polystyrene in the rim joist and had sealed it with Great Stuff standard, at the beginning of last winter we had a heap of snow but there was a 6" gap around the house at the rim joist level which showed how much heat was going through the rim joist even air sealed (The basement is not a conditioned space but Id like to finish it)
It will be interesting to see next winter since Ive put the 3" polyiso in the rim joists and resealed, if I will get that same melted snow gap

Answered by Darren Finch
Posted Wed, 06/18/2014 - 13:43

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The spaces between the mineral wool fibers are filled with air. This air isn't dry outdoor air; it's indoor air, and therefore it is warm and humid. It isn't magic air. Just plain old indoor air -- in contact with the cold rim joist.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 06/18/2014 - 14:13

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Nick: Roxul (when installed perfectly) may be sufficiently air-retardent to slow air infiltration to manageable levels from a thermal point of view, but it's orders of magnitude short of being able manage moisture transfer issues. Roxul is quite a bit more air-permeable than 3lb density cellulose, which is nowhere near good enough from a moisture transfer point of view.

The water vapor permeability of any fiber insulation is through the roof- even if it were somehow magically solidly air-tight, the 100+ perms of vapor diffusion would still make the moisture transfer rates too high. The much higher density ComfortBoard IS product is even more air-tight than batts, and tests out at 30 perms according to the spec.. That 30 perms figure is...

...2-3x as vapor open as Typar housewrap (and nearly as vapor open as Tyvek)...

...10x as vapor open as one inch of Type-II EPS...

....on the order of 100x as open as a kraft facer on a batt...

...on the order of 1000x as vapor open as 6 mil poly....

... and rock wool batting is at LEAST 3x as vapor permeable as ComfortBoard IS- so vapor open that it isn't even tested- it's presumed to be a very high-permeance layer.

Bottom line, the insulation has to be truly air tight, and sufficiently low-permeance to be fully protective of the band joist.

Answered by Dana Dorsett
Posted Wed, 06/18/2014 - 17:54
Edited Wed, 06/18/2014 - 17:57.

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OK, OK, fair enough on the Roxul point... but why is the rim joist so special? Typical wood stud walls are similar creatures -- they have cold wood on the exterior and fibrous insulation on the warm inside portion -- why is this arrangement fine for them? A 2007 house should have sill gasket, so there should be no additional moisture in the band joist from the foundation.

Answered by Nick Welch
Posted Wed, 06/18/2014 - 18:31

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Nick even I know the law of heat transfer, a cold basement with a relative humidity of 20% with a even colder outside with zero or close to it humidity will draw the heat from the warm(er) basement through the wood which is colder due to being exposed to the colder air outside so the warm moist air comes in contact with the cold wood, leading to condensation forming
When I moved in the fibreglass stuffed into the rim joists were wet with moisture, thus my initial posting so anyway Im going to consider this question closed since the polyiso will stop that condensation on the cold rim joist because the warm air wont reach that cold rim joist
Thank you all for your input

Answered by Darren Finch
Posted Wed, 06/18/2014 - 19:08

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Q. "Why is the rim joist so special? Typical wood stud walls are similar creatures -- they have cold wood on the exterior and fibrous insulation on the warm inside portion -- why is this arrangement fine for them?"

A. I'll give you two answers: First, it's not entirely clear that everything is fine with the typical wall. Many walls have mold and rot. For more information, see How Risky Is Cold OSB Wall Sheathing?

Second, walls usually have painted drywall on the interior; this layer acts as an air barrier and vapor retarder. Most rim joists lack this interior layer, so interior air can access the rim joist more easily (especially when the rim joists are insulated with fiberglass or mineral wool).

Answered by Martin Holladay, GBA Advisor
Posted Thu, 06/19/2014 - 06:48

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