# Changing to rigid foam insulation for basement rim joists?



## wrbrb (Aug 18, 2020)

Our 1952 house currently has a basement where half is finished and half is unfinished. The unfinished side where the hot water heater, furnace, and a workspace are located has fiberglass insulation on the rim joists in fairly poor condition, a lot of it missing, some daylight from outside visible, etc. We have seen mice outside, so I was wondering if it would be a good upgrade in general and to prevent them from nesting to do rigid foam with expanding foam in a can on the rim joists on the unfinished side for now, then also on the finished side later when the wall paneling is down and redone?

If so, would it make sense to use a couple thicknesses of rigid to reach a higher R-value? And any tips in general for the project?


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## rogerwh (Mar 1, 2021)

Sounds like a good plan. Cutting the rigid foam slightly short both width and height and filling the gap with expanding foam, is a good idea.

Roger


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## Nealtw (Jun 22, 2017)

Do have an open space to the wall cavity above?


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## SPS-1 (Oct 21, 2008)

Foam is not going to stop critters. Anyplace you see daylight, fit a piece of plywood in the cavity first.


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## Domo (Nov 9, 2018)

Rigid insulation will not stop mice and is more expensive and PIA to install than fiberglas - IMO.

If you see daylight use Great Stuff to fill the hole - then put in new fiberglass.


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## wrbrb (Aug 18, 2020)

Nealtw said:


> Do have an open space to the wall cavity above?


I haven’t checked this yet, but I think I likely do. I should pull out the fiberglass in a few of the bays and investigate, but what would need to be done if there is (or isn’t) an open space above?

I could pull out the insulation or cut it out if there’s a gap to the first floor above, seal any cracks to outside with a can of spray foam, cut rigid foam to fit, use adhesive to stick it in the bays, then more spray foam around that?

And would there be any vapor/moisture problem with this if I plan on leaving the basement walls unfinished (maybe just painting them) and there is one hvac vent heating and cooling the basement area?


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## Nealtw (Jun 22, 2017)

wrbrb said:


> I haven’t checked this yet, but I think I likely do. I should pull out the fiberglass in a few of the bays and investigate, but what would need to be done if there is (or isn’t) an open space above?
> 
> I could pull out the insulation or cut it out if there’s a gap to the first floor above, seal any cracks to outside with a can of spray foam, cut rigid foam to fit, use adhesive to stick it in the bays, then more spray foam around that?
> 
> And would there be any vapor/moisture problem with this if I plan on leaving the basement walls unfinished (maybe just painting them) and there is one hvac vent heating and cooling the basement area?


If it is open up the wall it should be fire stopped and foam board should not be left exposed in living area so you could do both at the same time with enough foam board that block the hole and then back that up with 1/2" plywood OSB, drywall or solid wood and fire caulk around that. 
If you have a fire in the floor area you do not want an open chimney that will take the fire to the attic. Check the interior walls too. 
If you have these holes, they should be blocked in the attic too.


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## wrbrb (Aug 18, 2020)

Nealtw said:


> If it is open up the wall it should be fire stopped and foam board should not be left exposed in living area so you could do both at the same time with enough foam board that block the hole and then back that up with 1/2" plywood OSB, drywall or solid wood and fire caulk around that.
> If you have a fire in the floor area you do not want an open chimney that will take the fire to the attic. Check the interior walls too.
> If you have these holes, they should be blocked in the attic too.


I pulled out a bunch of loose insulation, ripping it off at the top of the bay. It is open up the wall at least 10 inches above the subfloor, so I’m assuming it’s open all the way through the first floor, maybe all the way through the second floor and to the attic.

I think we have the first floor floor joists at the left and right of my photo, the first floor subfloor at the top, and the basement/foundation wall at the bottom.

Could you explain what the colored areas are?

I’m guessing red is a sill plate that sits on top of the foundation (bolted down? Maybe not in 1952?), yellow is exterior sheathing/siding, and have no idea why the short green board is there.

Looking up into the cavity at the back of the bay toward the left, the small green board just stops and past it (even with the joist? not visible in the first photo) is I believe a vertical 2x4 on edge - first floor wall stud?


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## Nealtw (Jun 22, 2017)

It looks like you have balloon framing where the stud goes all the way to the roof. the stud is the green in your picture


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## wrbrb (Aug 18, 2020)

Thanks for your reply - I was hoping we didn’t have that kind of framing, but at least it’s identified and I can try to make it a little safer from a fire prevention perspective. 

To clarify the process, I would do the following?

Caulk or spray foam or board up any holes to outside to prevent rodents or insects from entering
Cut rigid foam insulation into squares that fit into the joist bays, using a thickness of layers of foam that will be enough to block the gap to the first floor stud bay, to help with fire blocking while also insulating (maybe use two 1 1/2” pieces for 3” total, leaving half an inch still needed to close up using plywood?)
Push the foam back into the bays, hold in place by hand while fire caulking or spray foaming around the edges? Or use an adhesive caulk lightly on the rear face of the foam to hold it tight against the exterior sheathing, and fire caulk or spray foam around that?
Use 1/2” plywood (is any cheap plywood ok?) to cover the 3” of foam, bringing the total depth equal to the 3 1/2” stud bay cavity depth
Toenail the wood to the joists?
Fire caulk around the edges of the wood?


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## Nealtw (Jun 22, 2017)

wrbrb said:


> Thanks for your reply - I was hoping we didn’t have that kind of framing, but at least it’s identified and I can try to make it a little safer from a fire prevention perspective.
> 
> To clarify the process, I would do the following?
> 
> ...


Better to use 2 inch foam or 3 of the 1 1/2", the studs will likely be a full 2x4 as in 2"x4" and not 1 1/2x3 1/2 that we are used to seeing today. 
Holes big enough for critters, the better cover is a piece of sheet metal.


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## wrbrb (Aug 18, 2020)

Nealtw said:


> Better to use 2 inch foam or 3 of the 1 1/2", the studs will likely be a full 2x4 as in 2"x4" and not 1 1/2x3 1/2 that we are used to seeing today.
> Holes big enough for critters, the better cover is a piece of sheet metal.


Thanks. Do the rest of the steps seem ok?


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## Nealtw (Jun 22, 2017)

wrbrb said:


> Thanks. Do the rest of the steps seem ok?


Yes.
Many houses had an attic like this and you can see how it would be open to the attic.


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## rogerwh (Mar 1, 2021)

The first house I purchased had balloon construction. It also had 12” asbestos siding. This was before asbestos was bad. Before we hung the vinyl siding, we had cellulose insulation blown in. I believe they just plugged the lower part of the wall cavity, before blowing in the insulation, with cardboard. Than blow in insulation in the attic, laying fiberglass on top of the cellulose.


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## wrbrb (Aug 18, 2020)

Interesting. I would have thought a house finished in 1952 would have had more modern framing, but I guess not. Colonial style if that makes any difference.

I guess I can start with foam insulation blocking in the basement, then do the attic, then worry about any stud bays between the first and second floor if I happen to have the walls open for other projects. There does seem to already be fiberglass or rock wool in the exterior walls.


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## wrbrb (Aug 18, 2020)

After looking at more joist bays it looks like this might be a little difficult with everything in the way… any suggestions on how to tackle spots like this with EMT runs in the way?


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## Nealtw (Jun 22, 2017)

You just do the best you can.


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## squared80 (Feb 18, 2021)

Spray foam the hell out of it.


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## Scaredrabbit (Jun 10, 2021)

Got through about the first 3-4 on my house and said the heck with it. Got a spray foam kit and did the rest. Way too many little things in the way to make it worth it to do all the cutting.


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## Steve in MD (Jun 27, 2019)

If it was my home, I'd go with Thermax, fire rated caulk around the perimeter of the Thermax, and then finish off with Roxul. I don't know if this meets code, but all are fire rated materials and easy to work with.


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