Should I insulate my rim joists?

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Old 10-18-18, 07:47 PM
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Should I insulate my rim joists?

I went out an bought some 2 inch xps 250 foam board. Now I'm having some second thoughts about it after reading that the foam board can trap outside moisture in between and can develop rot and stuff.

They've never been insulated (1971 house) and I can't see any mold or anything right now.

Will sealing up the rim joist with foam board cause mold, and water behind the foam board?
 
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Old 10-18-18, 09:03 PM
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Will sealing up the rim joist with foam board cause mold, and water behind the foam board?
Only you do a really crappy job of fitting and air sealing it. Cut it tight enough that it fits with no fasteners... friction fit. Then caulk the edges with a foam board adhesive like pl300. Add a 2nd layer if you want. Then caulk it in. The more insulation the better.
 
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Old 10-22-18, 02:09 PM
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Should I be worried about using that canned spray foam around the edges of the foam board?

Is it toxic to breath in, etc?

I heard about concerns of using "Spray foam" and the health effects, but think maybe they're talking more about the guys that come and blow it in?
 
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Old 10-22-18, 04:33 PM
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I don't think I said to use any spray foam.
 
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Old 10-22-18, 08:07 PM
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Not sure of your climate region Brian, but I assume "cold". Although the rigid foam and caulking will seal most you still want to seal between the sill plate and the foundation, and any other leaks you see. As for 2" pink or blue (no plastic covering) it still allows some drying to the inside, but primarily the rim drys to the outside. Note, they use housewrap on the outside under the siding because it allows moisture vapor to pass.

There are also fire codes that in some locations will require a thermal barrier over the rigid foam. That can be drywall of an expensive paint.

As an alternative, the cavity can be filled with Roxul and then the piece of drywall. The drywall serves two purposes, keep nesting rodents from establishing a home and to air seal. But Roxul and drywall are less expensive and the process is easier than the rigid foam.

Bud
 
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Old 10-23-18, 03:04 AM
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Originally Posted by XSleeper View Post
I don't think I said to use any spray foam.
No, but that's what all the tutorial vids and articles more or less say to use.
 
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Old 10-24-18, 03:06 AM
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Do you think it would be ok to seal around the foam board with that clay like "rope caulk"?

It looks like it would work and I would be able to take them apart to inspect for mold and stuff periodically.
 
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Old 10-24-18, 05:47 AM
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I've never considered rope caulk for that application, not very sticky, by design. But as long as you are neat and it stays in place you should be fine.

What do you have on the outside where any moisture would come from?

Bud

Just to note, the idea of removing the rigid for inspection is a requirement in some southern locations for termite inspection. Heard it on the forum so no citation.
 

Last edited by Bud9051; 10-24-18 at 06:17 AM.
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Old 10-24-18, 07:17 PM
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I feel like I'm wasting my time insulating the rim joists.

I put my hand up there and really don't feel any drafts I mean. It's 40F tonight, which is not really cold , but is this going to make a difference?

Also, someone said to caulk at the top and bottom of the rim joist where it meets the floor and sill. Is that correct?
 
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Old 10-24-18, 09:35 PM
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Since we cannot see what is behind, above, or to the sides where there may be a seam or butt joint, we advise to picture frame with the caulking. Then if a double sill plate the seam between them and between the rim and the foundation.

Natural air pressures in the winter force air into the lower areas of our home which in turn forces inside air out near the top so all air sealing is good.

As for not feeling the cold, heat loss is happening. If you were to view the exterior of the house on a cold night (night eliminates the sun) you would see the warm band corresponding to your rim joist. From an r-value point of view the rim with siding on the outside is about r-3, not much better than a window.

The place where you actually see the improvements is where they all come together, the heating bill. In my work I calculate the time frame required for any improvement to repay for the cost and effort required and insulating and air sealing the cavities does pretty good.

Bud
 
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Old 10-24-18, 10:01 PM
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Originally Posted by Bud9051 View Post
Since we cannot see what is behind, above, or to the sides where there may be a seam or butt joint, we advise to picture frame with the caulking. Then if a double sill plate the seam between them and between the rim and the foundation.

Natural air pressures in the winter force air into the lower areas of our home which in turn forces inside air out near the top so all air sealing is good.

As for not feeling the cold, heat loss is happening. If you were to view the exterior of the house on a cold night (night eliminates the sun) you would see the warm band corresponding to your rim joist. From an r-value point of view the rim with siding on the outside is about r-3, not much better than a window.

The place where you actually see the improvements is where they all come together, the heating bill. In my work I calculate the time frame required for any improvement to repay for the cost and effort required and insulating and air sealing the cavities does pretty good.

Bud
Thanks Bud, you've convinced me to keep going!

I'm using Alex Flex caulking to seal around the foam board. I'm using the pink XPS foam board that has a 250 on it, I think and a picture of a pink panther.

I believe the Alex Flex is safe for foam board.
 
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Old 10-25-18, 08:13 PM
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Have another idea.

My house is built in 1971 and so far I've never seen any mold or moisture on the rim joists, but of course I want to insulate them.

A few of them DO have batts stuffed in and I've taken them out and didn't see any mold or anything.

I'm kind of worried that maybe mold or something could form after I caulk in the foam board pieces. I've finished about 6 joist spaces so far.

What if I just friction fit a piece of foam board and then stuff some batting up there?

That way I could periodically check the joists for problems without having to cut all the caulking.

Or is it necessary to caulk all the edges of the foam board?


Some more questions

1. I should caulk the top and bottom of the rim joist where it meets the sill and floor above?

2, I have two 2x4's as a sill . Do I caulk the gap between them?

There appears to be some type of concrete mixture used to seal the front of the first sill plate to the foundation also.
 

Last edited by Brian1900; 10-25-18 at 08:43 PM.
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Old 10-25-18, 08:48 PM
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Ironically that approach might make things worse. The objective of using the caulk or other means of air sealing is to ensure no warm moisture carrying air reaches the now cold space behind that rigid. With nothing in those cavities the heat from the basement easily keeps the rim warm enough to not be a risk for condensation, but at the expanse of heating a high heat loss surface.

If your friction fit were tight enough to seal out any ait then it would work.

But we need to put "best practices advice" into perspective. I try to follow guidance from the experts far smarter than myself, but they have a problem, many different houses in many different climates. So how do the render their advice down to its simplest form and still ensure some high percentage of homes will be good. Their approach ends p being a bit of an overkill, good advice for 99% of those reading it. The 99% is my guess but I bet they have a number they target and 100% would be too expensive.

The bottom line is, you have more flexibility than I do, wink, wink. I can't advise other than best practices and code minimums, but you can do whatever you want. I think you have listened and read sufficiently to be a better judge of what you need/want than I can advise from long distance. Especially since I feel you will be checking occasionally and these areas are not buried forever.

I'm happy on this end.

Bud
 
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Old 10-26-18, 04:27 AM
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I'm kind of worried that maybe mold or something could form after I caulk in the foam board pieces.
You are definitely over thinking this.

If you extrapolate that thought then you would assume that insulation or sealing anywhere in a house could turn into a bad situation.

In reality you are placing an air thigh barrier between the heated/cooled conditioned interior air from the outside so that condensation does not occur which is what mold needs to grow.

Sort of like a plain glass of cold water vs a Styrofoam cup, the plain glass sweats but the Styrofoam stays dry!
 
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Old 10-26-18, 05:58 PM
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Thanks guys I think I get it now.

Just one last one.

Should I do a "picture frame" caulking around the rim joist?

I saw a video where a guy says to do it. I mean instead of caulking just the top and bottom, he also caulks the sides where the joists meet the rim joist.

Thanks again
 
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Old 10-26-18, 08:42 PM
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I would picture frame it as behind the end of those joists can be a joint or air path to the sheathing and outside.

Often times the opportunity makes even small steps easy. You are there and it only takes a bit of caulking and a touch of time.

Bud
 
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Old 10-27-18, 08:32 AM
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Sorry I guess I have another one.

I have an oil furnace and read something about carbon monoxide and the rim joists could make the house too "tight".

I do have a carbon monoxide detector down there and upstairs.

Do you think that would be a problem?
 
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Old 10-27-18, 09:08 AM
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Combustion air is always a concern and virtually no trades pay any attention to it other than energy auditors, which I am. We use a manometer sensitive enough to measure the required pressures under worst case conditions. However, unless your house was built using the extreme methods required to get it to the too tight condition (yours wasn't) you have nothing to worry about in regards to the whole house.

If on the other hand a furnace is located inside a sealed utility room then ventilation to the outside or to the house itself is necessary.

A house that is tight will typically have condensation issues on the windows during the winter and comfortable to high relative humidity.

Bud
 
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Old 10-27-18, 09:32 PM
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Originally Posted by Bud9051 View Post
Combustion air is always a concern and virtually no trades pay any attention to it other than energy auditors, which I am. We use a manometer sensitive enough to measure the required pressures under worst case conditions. However, unless your house was built using the extreme methods required to get it to the too tight condition (yours wasn't) you have nothing to worry about in regards to the whole house.

If on the other hand a furnace is located inside a sealed utility room then ventilation to the outside or to the house itself is necessary.

A house that is tight will typically have condensation issues on the windows during the winter and comfortable to high relative humidity.

Bud
Thanks Bud.

I DO have condensation on my windows and storm windows, especially now that it's getting in the 30's outside, but that may be because my windows are old wooden ones, etc. I do have carbon monoxide detectors though in the basement and upstairs.

My house is a "cheap" (builder grade?) 900sf ranch built in 1971.
 
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Old 10-27-18, 10:07 PM
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In 1971, same basic age as my current house, they were not worried about sir sealing, and real air sealing involves steps that can only be accomplished during construction.

How do I politely say, stop worrying? If you want a definite answer, have a blower door test done to measure the house leakage under pressure. While doing that they can also put the house into a worst case situation and measure the pressures near the combustion appliances.

Bud
 
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