# Walls and floors, Roxul or cellulose ?



## jklingel (Dec 21, 2008)

Glad to hear you are keen on cellulose, but the installer is one I'd not work w/. If he does not know about 3.5 lb density, then he's new at this or does not study. You can apparently tell when you are blowing it in, as the resistance gets tough at 3.5 and VERY tough at 4. That is what I read; I won't dense pack any till this summer. However, fat lot of good that will do you, unless you are there w/ the guy. Tough deal. I'd go w/ the Roxul, as much as I am sure DPC is better insulation (if done right). I'd also vote Rox in the floor. Is the "crawl space" enclosed? Conditioned? Open? I'd sure air seal the bottom of the joists, too. You can always add foam next year when your big bonus check arrives.


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## Gary in WA (Mar 11, 2009)

I'd use foam board strips on the joist bottoms to thermally break it from the ground temperature: http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces/

Gary


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## Arky217 (Aug 18, 2010)

GBR in WA said:


> I'd use foam board strips on the joist bottoms to thermally break it from the ground temperature: http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces/
> 
> Gary


GBR, I read the article at your posted link.

Are you saying that foam strips on the bottom of the joists will keep condensation from forming on the bottom of the OSB, even though the crawl space is completely open.

I don't see how that is going to change the temperature of the bottom of the OSB. 

It seems to me that:

In winter, I need to keep warm moist inside air from migrating through the subfloor and joist insulation and condensing on the inside of the OSB that's covering the bottom of the joists.

In summer, I need to keep warm moist outside air from migrating through the OSB and joist insulation and condensing on the bottom of the subfloor which will be colder due to air conditioning.

Just not sure the best way to accomplish this.

Would painting the subfloor with a vapor retarding paint help the winter problem ?

Not sure of any thing to do for the summer problem. Even if I cover the joists with foam boards (expensive for me), I would still have to cover them with OSB to keep out rodents.

I suppose I could add foam boards on the bottom of the OSB to keep it warmer to prevent wintertime condensation from forming on its inner surface, but I went over this with my wall. 
And was informed that no exterior foam boards were necessary to keep the OSB warm in my climate zone (western Arkansas).

Wouldn't that also hold true for the floor since the crawlspace is completely open ?

Could you go into more detail on how adding foam strips between the bottom of the joists and the OSB will help ?

Thanks,
Arky


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## Gary in WA (Mar 11, 2009)

The foam strips would insulate the joists from the colder plywood/housewrap, whichever you would install. Strips would act as a thermal break in the temperature differences of the cold ground air and the warmer joists (1.25 R-value per inch for solid wood).

Here is another similar explanation of the temperature transfer in an attic: http://www.buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion

Gary


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