# Insulating between floor joists in crawl space



## Nealtw (Jun 22, 2017)

If the white on the concrete is an indication on how much moisture is entering the space, that venting you had before was not enough and might be the cause of the fiberglass to be ugly. More venting or power vent or dehumidifier might be the order of the day. You might consider vapour barrier on the ground run up all the way and sealed to the top of the concrete as that would not interfere with your venting.


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## Kory Beam (Nov 15, 2014)

Thanks, Nealtw.

The efflorescence on the back foundation wall has been there since I bought the place about 5 years ago. I believe it was from snow sitting against that area, and snow melt from the roof saturating the ground in that corner during winter weather. It has since been re-roofed, gutters, and a french drain to direct any water away from saturating the areas around the exterior foundation. The crawl space seems to stay dry now. Not sensing any moist smells/humidity. The original fiberglass batts were funky and torn up from rodents getting into it, so I thought it would be best to remove and replace with something fresh. The old pink stuff was original to the cabin ('73).

I have certainly considered a vapor barrier over the dirt floor and up the concrete foundation. Need to look into that more...


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## Kory Beam (Nov 15, 2014)

Does the insulation trick I proposed in the first post seem like a fine way to stuff the joists and get the required fill/R-value for 2x8 floor joists?

Is Roxul a decent/suitable insulation for the floor joists in a crawl space?


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## Marson (Jan 26, 2018)

Kory Beam said:


> Does the insulation trick I proposed in the first post seem like a fine way to stuff the joists and get the required fill/R-value for 2x8 floor joists?
> 
> Is Roxul a decent/suitable insulation for the floor joists in a crawl space?



I haven't used Roxul personally, but I don't see why it wouldn't work. The main thing about floors is to get good contact of insulation to subfloor. You don't want an airspace there. 



I wouldn't rely on friction fit to hold those batts in place. Over time you will have batts falling out. What I have done is pull masons string taut across the bottom of the joists, and staple the string to the bottom of each joist so that it holds the batt up. Do this on 16" centers or so, going back and forth. There are also wires you can get that wedge between the joists to hold up the batts, but I haven't used them.


The other point I would make, and people might disagree with me, is that it is actually OK to compress fiberglass in a joist cavity. Higher density insulation is a good thing, to point anyway. That's why an R 21 batt is denser than an R 19 batt, even though they both are made for a 5 1/2" cavity. The most important thing is to eliminate or minimize any voids in insulation in the stud cavity.


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## Nealtw (Jun 22, 2017)

I like the string idea, I have seen almost anything put up but mostly they fall down when they get heavy with moisture, letting you know you have other problems.


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## Kory Beam (Nov 15, 2014)

Out of curiosity...

Would cobbling polyiso or xps Foam board to the subfloor in the joist cavities be a bad idea? Like might cause moisture trapping issues and mold at the subfloor?

Was thinking that 1-2" foam board cobbled in those joist cavities, then sealed with can foam (like done at rim joist).... then fill rest of joist cavity with the 5.5" r23 batt insulation (roxul). Would this be a moisture trap no-no? Also wondering if that would be a problem for getting termite inspection if sometime in the future If I decide to sell the house?


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## Nealtw (Jun 22, 2017)

Kory Beam said:


> Out of curiosity...
> 
> Would cobbling polyiso or xps Foam board to the subfloor in the joist cavities be a bad idea? Like might cause moisture trapping issues and mold at the subfloor?
> 
> Was thinking that 1-2" foam board cobbled in those joist cavities, then sealed with can foam (like done at rim joist).... then fill rest of joist cavity with the 5.5" r23 batt insulation (roxul). Would this be a moisture trap no-no? Also wondering if that would be a problem for getting termite inspection if sometime in the future If I decide to sell the house?


A plywood subfloor is considered the vapour barrier so having another in contact and sealed should be fine.
Not sure about termite inspections.


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