# Consequences of sealing rim joists in basement?



## mklein49 (Dec 31, 2012)

Hi all,

I'm in the process of finishing my basement. I'm debating about how to insulate the rim joist area in particular right now. My Primary concerns are indoor air quality and moisture issues. I'm personally putting temperature control at the bottom of the list. As it stands now, there are visible air gaps in a couple of places near the sill plate. You can of course feel cold drafts in the winter and we have quite a few bugs in the basement. Rounding out relevant background information...the house is located in St. Louis MO and was built in 2008. The basement slab is roughly 40x40 in size. 

My concern with sealing that area is the effect on the indoor air quality. Any advice on the consequences of sealing things too tight? Or should I not be concerned about that occurring?

Thanks


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## cleveman (Dec 17, 2011)

Don't worry.


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## Gary in WA (Mar 11, 2009)

2008 house, you may have an ERV/HRV in your house.... http://www.greenbuildingadvisor.com/blogs/dept/musings/hrv-or-erv

Caulk first, above/below the rim joist, under the sill plate. Then add FF FB with canned foam- last paragraph; http://www.buildingfoundation.umn.edu/RimJoist/rimjoistphenom.htm

I doubt you would leave a small window cracked open, year-around- same as the rim joist expanding/contracting with the seasonal changes; http://www.buildingscience.com/documents/insights/bsi-023-wood-is-good-but-strange/

Gary


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## cleveman (Dec 17, 2011)

I just insulated the rim joist in the basement of a house built in about '56 and it was totally bare.

Funny story. I had a guy going out of business and he wanted me to go and clean out his warehouse and haul everything away. So I scored one sheet of 1 1/2" xps, two sheets of 1/2" foil faced, and a bunch of 1/4" fan fold, along with some unfaced R38 bats and some kraft paper R19, along with a bunch of other crap.

After looking at it in my garage for a year or so, I decided to insulate the rim joist in that house. I knew they were 2x10 joists, so I ripped everything down to 9 1/4.

What was useful were those piles of 1/4" fanfold insulation. I found that in hard to reach areas, those 1/4" pieces slip in much easier than a 1 1/2" piece. So I would put 6 1/4" pieces in a bay, or 3 1/2" pieces or 1 1 1/2" piece. Then I had some left over, so I went back and got rid of all my material.

This was about a 1200 sf house. Then I went through 3 cans of foam spraying around the perimeter of each bay and on the ends.

Next I cut the 24" wide r38 and r19 into 12" wide strips and put them in each bay, wrapping them in plastic to keep the fiberglass from moving around too much. This took some time. I had to buy one additional roll of r19 and I got the encapsulated stuff. 

Now the entire rim joist is insulated with r48 to r29 and the whole thing cost me about $28 and 3 short days of work.

I've heard more than once that this is the biggest bang for your buck regarding insulation and I believe it.

So if you guys ever find yourself with a lot of fanfold insulation left over, just set the blade high on your table saw and rip it all up.


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## mklein49 (Dec 31, 2012)

So it seems like the sealed xps approach is great for handling condensation along with providing a base for temperature control. I do not believe I have an HRV/ERV, so i still wonder about tightening up the building too much. Is my concern invalid?


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## Huskee (Feb 14, 2014)

Think of rim joists as little wall cavities. You wouldn't think of leaving a full 8' wall uninsulated in your basement (at least you shouldn't). Well, rim joists may only be from 8"-12" or so tall, but they're still important. Older homes rarely had them insulated because of lack of education at the time as well as costs of materials. But uninsulated rim joists (or "bond" as some call it) can and will lead to cold floors above, especially near those exterior wall areas. Air quality should be least of your worries, but to each his own. Think of the water pipes coming up in those areas, they need a barrier between them and the outside air temp, not to mention ductowork if you have any in those areas. The absolute best os to have them spray foamed at 1-2". You can even buy the do-it-yourself foam tanks (looks like a gas grill propane tank, with a hose and wand) and do it yourself....although it's messy. This would be the best both for insulation and air quality. Or resort to the good ol' fashioned UNfaced fiberglass insulation, unless it will be covered such as drywalling that ceiling, then you can use faced in most areas....but that leads to possible fiberglass dust.


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## Huskee (Feb 14, 2014)

mklein49 said:


> So it seems like the sealed xps approach is great for handling condensation along with providing a base for temperature control. I do not believe I have an HRV/ERV, so i still wonder about tightening up the building too much. Is my concern invalid?


No such thing as tightening up the building too much, unless you were to spray foam your entire house, walls and ceilings as one fluid coating throughout, then you'd need air handlers. In your case, no worries.


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## mklein49 (Dec 31, 2012)

Ok, that eases my concerns. Thanks for all the replies so far!

So for installation,I have the engineered joists. Would I need to notch each corner of the xps for a good fit. Or can I just fill that cavity with foam? What's the recommended installation method there? Also, what is the best method for cutting the xps?

Thanks


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## IslandGuy (Jan 30, 2014)

Spray foam the snot out of it. I did my entire 1st floor (it was a finished floor, 1 bed, 1 bath a walkin closet and a den) on slab when I gutted it, rim joist & walls. I also did the windows and doors with the best money could buy. Now the heat hardly ever comes on down there, it's like a thermos. And the upstairs floors near the exterior walls are as warm as in the middle of the room.


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