# Rim joists question?



## Windows on Wash (Aug 30, 2011)

Spray Foam and rigid foam (sealed and installed) are effectively doing the same thing in that they are sealing the air barrier and insulating that rim joist location. 

Many folks will say that you need to use closed cell foam in this rim applications, but I have yet to see an issue with open cell. To be safe, not knowing your location, use closed cell foam to make sure you meet the minimum vapor control that you need. 

Can you insulate from the exterior, sure. Most do not because they do not want to engage in an exterior remodel and pushing out the rim joist area by 2" isn't feasible for most.


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## Bud9051 (Nov 11, 2015)

If by exterior you are referring to the (tongue twister here) inside of the exterior of the rim joist, yes, and do it well. 

Here is another approach. Insulate the rim joist as above and then almost fill the rest leaving a couple of inches at the top open all the way to the basement. Allowing warm air into that space (assuming your basement is warm) will help keep the floors above a bit warmer. Then cover with plywood to secure the bottom from critters and insects.

Now, I create that space above by adding strips of rigid on each side of the joist to allow me to add a layer of 2" rigid. Having rigid at the top and plywood at the bottom will encapsulate any batt insulation, like Roxul, to keep critters who find their way into your basement from making a home.

Bud


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## charles3526 (Oct 15, 2015)

Basically i want to fill tje cavity with wither apray foam or 2inch foam board. 










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## Windows on Wash (Aug 30, 2011)

No need to fill the whole thing up.

Just insulate all the surfaces that are adjacent to the outside wall. Bottom of that cantilever and the outside rim for the interior units and be sure to get the sides on the end joist bays of the cantilever.


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## charles3526 (Oct 15, 2015)

Well im asking cause under that is a piece of plywood thats old and bug get in from that area at least im thinking.

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## charles3526 (Oct 15, 2015)

This is what i want to do 










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## Windows on Wash (Aug 30, 2011)

By doing that, you will make the floor that is above that cantilever cold by cutting off the warm air circulation.


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## charles3526 (Oct 15, 2015)

What about if the basment is already cold 

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## Bud9051 (Nov 11, 2015)

If the basement is cold, that is a separate problem that needs to be addressed and as you reduce the heat loss down there ALL of the floor above will feel warmer.

The overhanging space above is being cooled by cold air flowing down from outside walls and windows. Any warm air above is being pushed up so how are those floors going to feel warmer? Yes, air sealing and insulating those cavities will help, but allowing the basement to be cold is not good. Where is your furnace or boiler? Are there a lot of water pipes down there? Air sealing and insulating the rim joist, just as you are doing with the overhang, will make a big difference.

Bud


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## Windows on Wash (Aug 30, 2011)

+1

Putting that insulation flush will the poured wall is going to move that whole floor section to the outside and therefore cut that section off from any roomside heat. That will cause those sections to get colder and therefore make them colder than they were before.


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## mikegp (Jul 17, 2011)

What if he puts the foam board on the inner edge of the cavity and fills the inside with insulation? Roxul would be best, fiberglass might work, but there could be condensation issues in the cavity once it's sealed off from the basement.


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## Windows on Wash (Aug 30, 2011)

No need to put fibrous insulation in there if the foam is of a sufficient thickness. 

If he is trying to keep the floors warm, you need to insulated the outer edges (i.e. the rim, bottom, and ends) of that cantilever and allow for circulation with the room side warmth.


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## Bud9051 (Nov 11, 2015)

Charles, here is a related article that discusses leaving an air space above the insulation to keep those floors above warmer. It has been awhile since I read this article so not sure what else it suggests.
http://buildingscience.com/documents/insights/bsi-064-bobby-darin-thermal-performance

Bud


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## Windows on Wash (Aug 30, 2011)

I am reporting this link usage accordingly.


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## Gary in WA (Mar 11, 2009)

charles3526 said:


> Hello i am preparing to insulate my rim joists. I wanted to use spray foam and fill the entire joist that hangs over the outside of the house. I hear and seen people just use 2" foam board and close it off when it meets the bsmt walls.------ correct, seal the basement air/moisture from the cantilevered cavities. Is that the same as spray foam? I know it would be cheaper then it but it is more efficient with insulation and also keeping bugs out from comming in via the outside ply being shot to sh*t. Thats my next questiom can i also install a 2inch foam board on the outside of the rimjoist and spray around it to seal up any gaps?---------- if you add foamboard on the floor sheathing, you don't need rim joist insulation, but it may help control air/moisture infiltration through the rim. My joist hang over about two ft over the foundation so thats what im trying to clear. --------- you NEED to air seal with plywood to stop the critters. Any help would bw great thanks in advance.
> 
> Sent from my SM-G900V using Tapatalk



See answers in red color.

Gary


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## Gary in WA (Mar 11, 2009)

Upon further reading, Post #8, 11, and 13 are wrong in that you don't want any basement air/moisture getting to the cantilever cavity. Basic air sealing/insulation, many links available- here are two; Fig.7; http://ccetompkins.org/resources/sealing-air-leaks

https://basc.pnnl.gov/resource-guides/cantilevered-floor

Post 15; "Growing old is mandatory, growing up is optional." I've added/changed my signature as recently a member private messaged (PM) me and said he was glad I would help him with his insulation planning because he was fearful of posting his question publicly in that he would be ridiculed. 

If we are to take the time/effort/money on a project, may as well try to do it right the first time.

Gary


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## Bud9051 (Nov 11, 2015)

Hi Gary,
Although I do agree that moisture concerns need to be considered, if the op does a good job of insulating and air sealing each bay, bottom, ends, and sides where exposed, then leaving the upper area of those cavities open to basement air should not be a problem. 

Now, the plywood should be repaired if it is bad, but once air sealed and with 4" of rigid those cavities will not be cold. As for the basement being cold, the op is already improving this area, not sure if he intends to do the rest, but once the entire rim joist is air sealed and insulated that basement will be much warmer. Also assuming there is a furnace or boiler down there. Charles hasn't answered that as yet.

Bottom line is, does the op want warmer floor above. If yes, then leave the gap between the floor above and the new rigid insulation and improve the temperatures down there. 

Best
Bud


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## Windows on Wash (Aug 30, 2011)

So which is it? You changed your answer in an attempt to make it look like I was wrong.. 

Insulating the floor of the cantilever will keep the floors warmer and more comfortable as well as consistent moisture levels with the interior. 

By closing them off, they will be heat and moisture synced with the ground. This is the same reason we try to cover cantilevers from the exterior. Putting foam across the bottom doesn't address the thermal bridging of the framing, but that will still be warmed by the roomside air. 

Short of cladding the exterior in foam, I would insulate the bottoms as stated previously. It will keep the floor warmer and if insulated sufficiently, both moisture and heat loss are mitigated. 

As far as the ridicule inference goes, I have never, nor would I ever ridicule a poster for any question. As a matter reference, I have vehemently defended posters before and chastised a regular poster that was less than friendly. 

Not sure what your reference to post 15 is for. The only reference I see was the "internet police". Are you the Internet police?


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## Gary in WA (Mar 11, 2009)

"then leaving the upper area of those cavities open to basement air should not be a problem." ----------- leaving *any* opening at the top is not "air sealed" as per Energy Codes; " Floors (including above-garage and cantilevered floors)
Insulation shall be installed to maintain permanent contact with underside 
of subfloor decking.
The air barrier shall be installed at any exposed edge of insulation." 
From; https://www.energycodes.gov/sites/default/files/becu/2012iecc_residential_BECU.pdf

The first link I gave said an air barrier:



Install a rigid air barrier or other supporting blocking to *separate the cantilever from the conditioned space.**-------- per code it is not part of the conditioned space due to exposed on the underside (to ambient temps) rather than in basement-- requires solid blocking or foam board over the separating wall space.
Seal *all seams, gaps,* and holes of the air barrier with caulk or foam. ----- all gaps would include any opening at top.
Install insulation without misalignments, compressions, gaps, or voids and align it with the sub-floor,* the rigid air barrier (A)*, and the exterior face of the cavity.----- so it is aligned on three (plus ends) sides which INCLUDES the over-wall blocking/FB.
Once insulated, enclose the cavity with a rigid air barrier material.From; https://basc.pnnl.gov/resource-guides/cantilevered-floor
These are Energy Codes, readers should at least know moving air robs insulation of R-value, especially fiberglass (or other fibrous and even SPF) insulation, eg.in a wall cavity- similar to floor cavity, just exposed on bottom rather than side; http://www.greenbuildingadvisor.com...ir-leakage-degrades-thermal-performance-walls 



All non-code literature I've read (of 25 recent ones) except one (Canadian contractor), required/suggested air sealing over the wall between conditioned/non-conditioned spaces. Gary


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## Windows on Wash (Aug 30, 2011)

Since when is "air sealing" what would be referred to as creating a barrier between conditioned spaces? The point of insulated the bottom, back, and sides of the cantilever is, in effect, moving it into the conditioned space. 

The top of the cantilever is, as I understand, the conditioned floor of the kitchen or main floor room...is it not. 

The open area of the cantilever is communicating with the conditioned space at that point and hence the reason that I am recommending it stay open to keep it warm. 

The outer edges and corners of the cantilever should be caulked and sealed prior to that application of rigid foam board to the bottom, back, and end bay-sides. 

You didn't answer the question about why you changed your answer I see.

When will have to agree to disagree here if the situation is as I understand it.


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## charles3526 (Oct 15, 2015)

Hello thank yous for all ur answers. Im looking cut off any out bugs and air from comming in idk where else they woulf be comming in from. But besides the hang overa of the house. I also want to cut off the air comming from them caise all there is thats blocking it is some old ass ply wood its not rotted but u can tell its old. Im basivally looking to keep iut the crickets. So i was thinking of filling the cavity of the rimjoists with either spray foam,2" foam board or just stuffing them with batt insulation
And then covering each up with a piece of wood or foam board to its even with the interior cement wall. 

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