# Crawl space ceiling/floor insulation with Roxul



## Windows on Wash (Aug 30, 2011)

You can. You should still use some rigid foam to back the joists and seal the seams. This will help with the thermal bridging and any condensation issues that could develop over time. 



http://buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces


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## Nealtw (Jun 22, 2017)

The sub floor is considered the vapour barrier and is on the warm side. 

Think of a attic upside down air below insulation to allow drying. 

I think the rock wool is the right product I would fill the space to the bottom of the joist. then i would apply a window screen for insects and the a wire mesh to help hold the insulation in place and keep critters out of it. 

The dirt below should be covered with a 10 mill sheet poly and sealed to the foundation if you have one.


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## Kory Beam (Nov 15, 2014)

Nealtw, the Roxul batts are R23 for 2x6... I don't think the batts are thick enough to completely fill a 2x8 joist cavity. Is it necessary to fill to cavity to the bottom of the joists?


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## Windows on Wash (Aug 30, 2011)

Depending on moisture conditions in the crawl, the framing ends can be condensing locations. Please see the link to reference more effective means of insulating the space. 

Besides the thermal bridging of the R-7 weak points of the framing, the foam provides a more effective air barrier (less important with Roxul, but still of some benefit).


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## Windows on Wash (Aug 30, 2011)

Kory Beam said:


> Nealtw, the Roxul batts are R23 for 2x6... I don't think the batts are thick enough to completely fill a 2x8 joist cavity. Is it necessary to fill to cavity to the bottom of the joists?



See the picture below. If you have an air/insulation barrier over the insulation, it is ideal to leave an air gap under the subfloor to allow for some warming of the floor.


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## Windows on Wash (Aug 30, 2011)

Depending on the glue used in the plywood, it will be a Class 2 or Class 3 vapor retarder and will allow for drying. Depending on the foam and details on the seams, there will be next to zero moisture coming from the earth and into framing.


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## Kory Beam (Nov 15, 2014)

If I am only going to be insulating with rock wool batts in the joist cavities (no rigid foam encapsulation of joist assembly), I should make sure that the insulation batts should be in full contact with the subfloor, right? Since these are R23 batts for 2x6, they would not completely fill a 2x8 joist cavity.


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## Windows on Wash (Aug 30, 2011)

If they are tight fitting on the joist spacing, I would still like them hang down to be flush with the joist bottoms. 



After that, I would cover the bottoms with some sort of wrap or plastic to hold the insulation in and create and air barrier.


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## Nealtw (Jun 22, 2017)

Windows on Wash said:


> If they are tight fitting on the joist spacing, I would still like them hang down to be flush with the joist bottoms.
> 
> 
> 
> After that, I would cover the bottoms with some sort of wrap or plastic to hold the insulation in and create and air barrier.


Would you cover the insulation in that attic with plastic or some sort of air barrier?


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## Windows on Wash (Aug 30, 2011)

Depends on the application.


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## Nealtw (Jun 22, 2017)

Windows on Wash said:


> Depends on the application.



Mobil homes have fabric so it can breath,


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## Windows on Wash (Aug 30, 2011)

Homes and insulation don't need to breath and you don't need to vent through the bottom of the insulation in a crawl space. The vast majority of moisture is coming via connection to the exterior climate and soil at that point.


Attic doesn't have the same challenges.


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## Nealtw (Jun 22, 2017)

Windows on Wash said:


> Homes and insulation don't need to breath and you don't need to vent through the bottom of the insulation in a crawl space. The vast majority of moisture is coming via connection to the exterior climate and soil at that point.
> 
> 
> Attic doesn't have the same challenges.


That is why you put the poly on the ground.


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## Windows on Wash (Aug 30, 2011)

I can forward your concerns to the Building Science corporation if you like. 



And we put poly on the ground as well..when we are conditioning a crawl in conjunction with sealing the rims and insulating the stem walls. 



I fail to see the point in it with a vented crawl. The relative humidity in the air will usually be consistent with what is in the air at that point.


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## Nealtw (Jun 22, 2017)

Windows on Wash said:


> I can forward your concerns to the Building Science corporation if you like.
> 
> 
> 
> ...



The RH in the air is how much different that the air venting the attic.


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## Windows on Wash (Aug 30, 2011)

Is that a question or a statement? 

Where do you think more moisture is being lost from the house? Through the crawlspace floor or through the ceiling plane into the attic? 

Do you have the same space limitations in the attic as you do in the crawl where you can't insulate and thermally uncouple the trusses or framing from the inside of the home? Seems to me that you could insulate over the top of the framing much like the recommendation of foam to the OP here. 

Last question...of what benefit is to this to the OP? He has the link to the BSC which is a bunch of guys/gals that are smarter than you or me.


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## Nealtw (Jun 22, 2017)

Windows on Wash said:


> Is that a question or a statement?
> 
> Where do you think more moisture is being lost from the house? Through the crawlspace floor or through the ceiling plane into the attic?
> 
> ...


 Every time some one sends me to building science because they can't find the answer, I find a lot of does and don't with out an explanation that answers the question. So if you have something to prove find the explanation and quote it, then we can talk about that.


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## Windows on Wash (Aug 30, 2011)

Every time that I send someone to the BSC, it is because they have the technical explanation, data, and imagery to support the positional approach that I reference in my posts. 

If the DIY OP is that interested in learning and putting in a bit of the work to understand the basis for the recommendation, he can go there and see what research that I have done previously to arrive at the conclusion of my recommendation. 

One of my great political heroes said it best....


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