# Would "Great Stuff" or equivalent be ok for insulating rim joists?



## Red Squirrel (Jun 29, 2009)

I've been wanting to use rigid foam but don't have a truck to carry it to my house nor the cash at this point to buy some. Then I started thinking, could I just buy a few of the "mega" cans of Great Stuff and do it that way? Or is the R value very poor, and is there other issues with this? Considering that stuff expands quite a lot I'm sure it would not take all that many cans and probably be a very easy and fast job. 

I know they also have DIY kits for blown insulation, but if I'm going to buy that, may as well just buy the rigid foam boards and pay for the delivery. I want to do this right, so if the Great Stuff is bad idea, then I won't do it.


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## concretemasonry (Oct 10, 2006)

For your project, you not need a truck to carry the extruded foam.

Buy the thickness your need and score/cut it into manageable pieces for you job in the store or after you have bought it. Certainly, you have an approximate size of the pieces needed for you application.

When you get home, trim/cut for an approximate loose fit. Foam around the exterior cracks with foam. Quickly put in the extruded polystyrene and use the expanding foam as an adhesive to hold the foam in place while sealing against infiltration.

Later, stuff the space between joists with the cheap and reasonably effective fiberglass.

It is not sophisticated, but it works well.


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## Red Squirrel (Jun 29, 2009)

I was thinking this as well, I suppose I could just cut it at the store. People might look at me funny but really if I get the 2x8 I only really need to cut them in half so it's not like it will take much time.

Thinking further, I don't think the great stuff sticks enough to expand in place, I think it would fall off the flat surfaces. It sticks in small quantities (and VERY hard to get off hands!) but in large quantities it would probably fall in chunks.


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## concretemasonry (Oct 10, 2006)

The key word in the reply is "quickly" since it is easy to do roughly in advance and stick it in quickly after the foam in the corners (4) and let the expansion work ( with extruded foam in place quickly, the expansion is confined enough to do the job). You can always foam again later if you want to be a perfectionist beyond normal needs. - Just do one space at a time. Then do the next space 30 seconds later.

No sense in getting too fancy since you always have the thermal short-circuiting from the joists even in the perfect world.


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## Ron6519 (Mar 28, 2007)

You'll get about 5 rim joist bays from a can. You can do the math vs. a 2" 4x8 foam board at $20.00+ a sheet.
Ron


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## Red Squirrel (Jun 29, 2009)

Ron6519 said:


> You'll get about 5 rim joist bays from a can. You can do the math vs. a 2" 4x8 foam board at $20.00+ a sheet.
> Ron


Only that? I figured I'd get more. Guess I'm best to just stick to the foam boards for sure then and just use the spray foam for the perimeter of the joist cavities and to fill the gaps. I'll use caulk once the foam board is applied in the cavity. May even double up. I'll have to calculate how many I need, which probably wont be all that much, and just cut them in the store as suggested. Come to think of it, I might even be able to fit them at 8' given they are only 2' wide.


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## Gary in WA (Mar 11, 2009)

The guy at the box store cutting the drywall into rips to fit his weekend car is me..........

Gary


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## Ron6519 (Mar 28, 2007)

Red Squirrel said:


> Only that? I figured I'd get more. Guess I'm best to just stick to the foam boards for sure then and just use the spray foam for the perimeter of the joist cavities and to fill the gaps. I'll use caulk once the foam board is applied in the cavity. May even double up. I'll have to calculate how many I need, which probably wont be all that much, and just cut them in the store as suggested. Come to think of it, I might even be able to fit them at 8' given they are only 2' wide.


 How cold is it now in Ontario? You want enough insulation to keep the heat in and the Canadian Winter out.
Ron


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## Red Squirrel (Jun 29, 2009)

-30C this morning. 

I will most likely get the 2 inch, or bigger if they have some. I see there is 2.5 on the HD site. That would probably be about R12 which is what standard fiberglass batts are, so that is pretty good considering it also seals.


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## WillK (Aug 29, 2010)

Yeah, a can of Great Stuff doesn't really get that far. I had 2" gaps between studs around a window on an exterior wall, and I decided to use great stuff to fill it and it took 3 cans to fill 3 cavities that were about 4' long each.

Not sure if you have the same stuff available, but HD has 1/4" thick fan fold foam from Owens Corning, and I was considering that as an option for closing off the back side of my knee walls, and it basicly is a 40' long piece folded down to a 2'x4' package or something like that.


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## atxjoe (Mar 25, 2009)

*Moisture issue?*

I can't tell from your description if this would be an issue, but here's my concern: If you create a sealed air pocket where moisture can't escape you may have a mold problem. I was considering insulating under my floor by sealing rigid foam panels between the joists, using great stuff as the adhesive as described here. But that would create a warm, moist, air pocket under the floor and where I live almost certainly breed mold.
I decided instead to use spray foam which will adhere to the subfloor and prevent any mold growth.


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## Gary in WA (Mar 11, 2009)

The fan fold has a perm of .75, about the same as asphalt paper facing on f.g. batts, and a R-value of 1. http://docs.google.com/viewer?a=v&q...t1NNKe&sig=AHIEtbTocffEhhkCmbxwjlzaWjmPEaMcKQ

The house wrap, Tyvek, 58 perms., would be more effective at letting any cavity moisture out to the attic, if air sealed to stop the wind-washing.


Gary


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## High Gear (Nov 30, 2009)

I used 2" xps doubled up with great stuff window/door as the sealer ( R20).

I still need to cover it with glass ( can't find roxul around here ).

Use the window/door type if you use it as a sealer/adhesive ( and even that will expand more than you think, hold it in place for a bit while it sets up ).

Messy /nasty *****stuff , I would probably just use pl300 in the large caulk tubes ( lots cheaper per oz.) if I did it again.

2" may not snap that easy/cleanly , if you have a cordless circular saw and a chalk line do it out in the far end of the parking lot.

Anything you do at all starts paying you back.


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## Earnie (Jan 24, 2008)

You can special order it.

http://www.lowes.com/SearchCatalogD...&catalogId=10051&N=0&newSearch=true&Ntt=roxul


I plan to when I start my rim joist sealing project.


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## WillK (Aug 29, 2010)

The fan-fold suggestion was for convenience with transporting it, at 1/4" thick for this application I'd be building up a stack of them to put in to whatever thickness was needed.

For the back side of my knee wall cavities, I did end up using Tyvek actually.


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## Red Squirrel (Jun 29, 2009)

atxjoe said:


> I can't tell from your description if this would be an issue, but here's my concern: If you create a sealed air pocket where moisture can't escape you may have a mold problem. I was considering insulating under my floor by sealing rigid foam panels between the joists, using great stuff as the adhesive as described here. But that would create a warm, moist, air pocket under the floor and where I live almost certainly breed mold.
> I decided instead to use spray foam which will adhere to the subfloor and prevent any mold growth.


I was thinking this but from my understanding the foam is not 100% air tight. It will let air through very slowly so it should not get moist behind there. Someone correct me if I'm wrong on this.


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## mmcgirr (Feb 11, 2011)

I used polyisocyanurate, with caulk around the edges, then fiberglass batts. Works well. easy to cut in the parking lot also.


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## WillK (Aug 29, 2010)

Red Squirrel said:


> I was thinking this but from my understanding the foam is not 100% air tight. It will let air through very slowly so it should not get moist behind there. Someone correct me if I'm wrong on this.


My recollection from reading the buildingscience.com articles was that rigid foam works as an air barrier but not a vapor barrier, so it's good for putting on the outside. I think it explains in the "perfect wall" article.

http://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall


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## Ron6519 (Mar 28, 2007)

WillK said:


> My recollection from reading the buildingscience.com articles was that rigid foam works as an air barrier but not a vapor barrier, so it's good for putting on the outside. I think it explains in the "perfect wall" article.
> 
> http://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall


I used the 2" foil faced polyisocyanurate boards at the rim joists with no trapped moisture issues. I checked 4 of them, 2 in the unheated crawl space.
Ron


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