# Insulation type for Basement ceiling side Cavities



## Windows on Wash (Aug 30, 2011)

Rim joist. 

Search the forums in the insulation sub section. Several very good threads on the different approaches to sealing and insulating those cavities.


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## DeeEyeWhy (Jan 19, 2016)

Windows on Wash said:


> Rim joist.
> 
> Search the forums in the insulation sub section. Several very good threads on the different approaches to sealing and insulating those cavities.


Thanks. I figure that with the wiring it may be easier to use the rock wool, also for the fire protection. 

It also seems that the rigid foam board is better for the basement walls themselves down to the frost line which in my area is about 36" below ground level.

But of course what do I know, which is why I ask.

Thanks


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## Bud9051 (Nov 11, 2015)

I agree with rigid down to and a bit below frost line however codes will often carry their requirement to the floor. But there is another advantage to covering the entire basement wall with rigid and that is to prevent humid interior air from contacting a cool surface. List a big city so we will know your climate.

As for the rim joist cavities I like a layer of rigid against the rim so the interior surface never drops below the dew point, varied with location but 2" is nice and air sealed with caulking or can foam. Where mineral wool is also nice I have see occasions where nice love it also. It can qualify for your thermal barrier over the rigid but it needs a solid cover on the inside. The mice will hate me.

As Windows mentioned, lots of threads here.

Bud


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## DeeEyeWhy (Jan 19, 2016)

Bud9051 said:


> I agree with rigid down to and a bit below frost line however codes will often carry their requirement to the floor. But there is another advantage to covering the entire basement wall with rigid and that is to prevent humid interior air from contacting a cool surface. List a big city so we will know your climate.
> 
> As for the rim joist cavities I like a layer of rigid against the rim so the interior surface never drops below the dew point, varied with location but 2" is nice and air sealed with caulking or can foam. Where mineral wool is also nice I have see occasions where nice love it also. It can qualify for your thermal barrier over the rigid but it needs a solid cover on the inside. The mice will hate me.
> 
> ...


I live in NNJ next to NY. Cold winters here and humid summers.


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## Nealtw (Jun 22, 2017)

Any holes from that cavity into the floor above should be sealed, then insulated with what ever and back that up with a piece of 1/2 drywall sealed in place with caulk.


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## DeeEyeWhy (Jan 19, 2016)

Nealtw said:


> Any holes from that cavity into the floor above should be sealed, then insulated with what ever and back that up with a piece of 1/2 drywall sealed in place with caulk.


Do you mean holes in the subflooring? 

I thought putting in insulation into these cavities would close up the holes from the floor above?


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## Nealtw (Jun 22, 2017)

DeeEyeWhy said:


> Do you mean holes in the subflooring?
> 
> I thought putting in insulation into these cavities would close up the holes from the floor above?


Plugging or sealing those holes is fire stopping. But if there are air leaks in that cavity you don't want it going up into the wall cavity. Once you start insulating you have to be care full, things done incomplete can cause mold problems. Anything you do you want to consider air flow possibilities and fire stopping.


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## DeeEyeWhy (Jan 19, 2016)

Nealtw said:


> Plugging or sealing those holes is fire stopping. But if there are air leaks in that cavity you don't want it going up into the wall cavity. Once you start insulating you have to be care full, things done incomplete can cause mold problems. Anything you do you want to consider air flow possibilities and fire stopping.


Would caulking or using expanding foam around the insulation close any remaining holes to accomplish this?


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## Nealtw (Jun 22, 2017)

Foam in the house should be covered with drywall or some other fire resistant material for a certain amount of time. 
Drywall is cheap and easy to work with.


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## DeeEyeWhy (Jan 19, 2016)

Nealtw said:


> Foam in the house should be covered with drywall or some other fire resistant material for a certain amount of time.
> Drywall is cheap and easy to work with.


Speaking specifically about the rim joists and not the walls, If I chose the mineral rock wool instead of the rigid board would I still need drywall?

Sorry if I sound dense, this is my first time in this specific area with insulation.

Thanks


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## user_12345a (Nov 23, 2014)

Do not use batts alone in that space, it's nearly impossible to put in a properly sealed vapor barrier up there, it always fails.

The batts make the surface cold yet let air through, resulting in condensation. Batts are also less effective when there's air movement.

Use rigid roxul board or foam board cut to size -> foam around the edges. Produces a thermal break. For foam board, it needs to be covered by something approved to stop fire -> cut drywall or roxul batt insulation. (the rigid insulation stops any condensation from forming so no vapor barrier required)


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## DeeEyeWhy (Jan 19, 2016)

user_12345a said:


> Do not use batts alone in that space, it's nearly impossible to put in a properly sealed vapor barrier up there, it always fails.
> 
> The batts make the surface cold yet let air through, resulting in condensation. Batts are also less effective when there's air movement.
> 
> Use rigid roxul board or foam board cut to size -> foam around the edges. Produces a thermal break. For foam board, it needs to be covered by something approved to stop fire -> cut drywall or roxul batt insulation. (the rigid insulation stops any condensation from forming so no vapor barrier required)


Sorry if I wasnt explaining myself correctly, but yes Roxul was what I was referring to:









ROCKWOOL Safe 'n' Sound 3 in. x 15-1/4 in. x 47 in. Soundproofing and Fire Resistant Stone Wool Insulation Batt (59.7 sqft) RXSS31525 - The Home Depot


ROCKWOOL Safe'n'Sound is a stone wool insulation for use in interior partitions of residential wood and steel stud construction where superior fire resistance and acoustical performance are required. Withstanding temperatures up to 2150°F (1177°C), Safe'n'Sound is non-combustible and will not...



www.homedepot.com


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## user_12345a (Nov 23, 2014)

yes, - know.

I was talking about the rigid roxul board...
I am against using batts only along the rim joist.


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## Nealtw (Jun 22, 2017)

DeeEyeWhy said:


> Sorry if I wasnt explaining myself correctly, but yes Roxul was what I was referring to:
> 
> 
> 
> ...


Foam board has to be covered, any other insulation needs to be sealed against air so air can not get past the insulation to the cold surface.


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## DeeEyeWhy (Jan 19, 2016)

user_12345a said:


> yes, - know.
> 
> I was talking about the rigid roxul board...
> I am against using batts only along the rim joist.


Ahh ok, so the one I posted isnt smart to use then...


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## DeeEyeWhy (Jan 19, 2016)

Nealtw said:


> Foam board has to be covered, any other insulation needs to be sealed against air so air can not get past the insulation to the cold surface.


Thanks, so its either rock wool by itself or rigid foam and drywall on top?

I cant close my ceilings because they are too low (6.5 ft from the floor), so my plan was to leave the ceiling open and just seal these rim joists on the perimeter.


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## Nealtw (Jun 22, 2017)

DeeEyeWhy said:


> Thanks, so its either rock wool by itself or rigid foam and drywall on top?
> 
> I cant close my ceilings because they are too low (6.5 ft from the floor), so my plan was to leave the ceiling open and just seal these rim joists on the perimeter.


If you were going to drywall the ceiling you could leave foam un covered but as it is un finished cover what ever insulation you use.


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## user_12345a (Nov 23, 2014)

Any rigid insulation needs to be sealed to prevent room air from hitting cold surface.

When batts are used without a rigid thermal break, you have to have a vapour barrier, and hard to seal a vapor barrier to the structure all around in that area.

Hence recommending rigid insulation.

Neal is right on both counts - foam needs to be covered without something non-flammable and rigid must be sealed.


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## DeeEyeWhy (Jan 19, 2016)

Hey guys, piggybacking on this thread because it is somewhat related. I was removing this old pegboard and found this hole at the top of my foundation wall. Can I just stuff batt in there or should I use spray foam insulation?

Thanks


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