# Crawl Space Foam Board



## 01grander (Jun 25, 2012)

Hey,

I was going to be installing some foam board in my crawl space. I was going to be adding either 1 1/2" or 3" foam board in-between the joists(under the above flooring not exterior). 

Are you supposed to leave a gap? I was looking at the eps board and it says something about leaving a gap. I am just going to be cutting it to size and holding it in place with some cut wood that is drilled into the joists.

Does the spacing help? The EPS instructions(even though i am using foam board) seem to indicate that it actually helps with insulating. Didn't know if that is a property of that material specifically or if it can be applied to foam board as well.

Thanks,
Steven


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## Windows on Wash (Aug 30, 2011)

That is not best practice. If you are going to insulate the floor, you need to put the framing in the warm wall and cover the joists. 

This will keep moisture from condensing on them. 

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


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## 01grander (Jun 25, 2012)

I am not quite sure what you mean. 

I was talking about this with a gap between the foam board and the plywood.


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## Windows on Wash (Aug 30, 2011)

The gaps need to be sealed.

Please read the article that I linked in the previous post. 

Insulating your floor like that will not give you the performance you are looking for.


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## 01grander (Jun 25, 2012)

Windows on Wash said:


> The gaps need to be sealed.
> 
> Please read the article that I linked in the previous post.
> 
> Insulating your floor like that will not give you the performance you are looking for.



Sorry, I was viewing your post on my phone and didn't see the link. 

I just realized I might not be using the correct terms. EPS seems to not have any foil but polyisocyanurate does and I assume that is what the article is referring to in the drawings.

Are you suggesting the foil faced insulation or can I accomplish the relative same concept with the pink foam board. I wasn't very impressed with the r-value of the foil board insulation although most of my concern was air permeability which either would help. 

I assume you just attach the board below the joists and screw it into the bottom of the joist itself?

Interesting article. I've seen so many people install it like the picture I posted. I would try the fiberglass batt hybrid approach but considering the humidity in the south, I'm not sure it wouldn't start growing mold.


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## mikegp (Jul 17, 2011)

Either way you need to use spray foam in all of the gaps around the foam and between one piece to the other. As it sits, air is just going to travel around your boards. As far as leaving a gap, you can since air is a good insulator, but if you get condensation behind the gap you will have mold.


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## 01grander (Jun 25, 2012)

mikegp said:


> Either way you need to use spray foam in all of the gaps around the foam and between one piece to the other. As it sits, air is just going to travel around your boards. As far as leaving a gap, you can since air is a good insulator, but if you get condensation behind the gap you will have mold.


What do you mean by "air is just going to travel around your boards"?

I originally was going to install it like the first image I posted. I since have read the article that was posted and was now going to install the isocyanurate on the end(bottom) of the joists and I was planning on using some cans to fill in the seams and some tape. 

I'm not really sure how to completely get rid of any moisture so it seems mold to some extent is inevitable if I seal the ends with spray foam.


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## 01grander (Jun 25, 2012)

Windows on Wash said:


> The gaps need to be sealed.
> 
> Please read the article that I linked in the previous post.
> 
> Insulating your floor like that will not give you the performance you are looking for.



Would you suggest the R-Tech or the foil type rigid foam board insulation? I do not have much of a gap in between the ground and my joists so I want to get whatever would handle moisture best. I do plan on using foil tape and spray foam for the seams and gaps.


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## curtd (Oct 25, 2013)

Since 1963 the floors in our crawl space have not been insulated and there is no wood rot anywhere (and our crawl space has a dirt floor. Dirt floor not covered with plastic). Why fool around with something that works. Per Windows on Washs' links he gave above, i would insulate the perimeter walls rather than the floor


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## 01grander (Jun 25, 2012)

curtd said:


> Since 1963 the floors in our crawl space have not been insulated and there is no wood rot anywhere (and our crawl space has a dirt floor. Dirt floor not covered with plastic). Why fool around with something that works. Per Windows on Washs' links he gave above, i would insulate the perimeter walls rather than the floor


I think he was just suggesting that I not install it like the first picture I posted. The article he posted seemed to indicate that you would want to insulate below the end of the joists. Basically parallel to my floor but just on the opposite side(underneath the house). I had planned on tacking on sheets of insulation below the joist. Am I misunderstanding something? I have had people telling me different things(not here) and I just need to do something. 

In the winter, I can feel air coming through my flooring.


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## Windows on Wash (Aug 30, 2011)

01grander said:


> Would you suggest the R-Tech or the foil type rigid foam board insulation? I do not have much of a gap in between the ground and my joists so I want to get whatever would handle moisture best. I do plan on using foil tape and spray foam for the seams and gaps.


The foil faced will have a lower vapor permeance rate so if you're worried about moisture getting up and into the assembly that is the better choice.


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## Windows on Wash (Aug 30, 2011)

curtd said:


> Since 1963 the floors in our crawl space have not been insulated and there is no wood rot anywhere (and our crawl space has a dirt floor. Dirt floor not covered with plastic). Why fool around with something that works. Per Windows on Washs' links he gave above, i would insulate the perimeter walls rather than the floor


I don't think the question was about whether or not it would rot and there are plenty of systems that work for hundreds of years from the sustainability standpoint. Homes that were extremely drafty didn't have any issues with mold mildew or a lot because they could dry out to either side. 

That all being said, I wouldn't want to be forced to pay utility bills on one of those older homes just like I would prefer to have a car that gets 35 miles per gallon as compared to an old single four barrel carburetor that gets 7 miles a gallon.


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## 01grander (Jun 25, 2012)

Windows on Wash said:


> The foil faced will have a lower vapor permeance rate so if you're worried about moisture getting up and into the assembly that is the better choice.



Thanks for all the comments and putting up with my lack of knowledge. 

I am just trying to avoid freezing to death this winter. I have a 2200 sqft house and I am a single guy. I typically just huddle in my room with a space heater to avoid gas bills but the cold from the floor was too much when it hit 12 degrees.


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