# I want to be sure



## Maintenance 6 (Feb 26, 2008)

If you are using faced insulation, what is the purpose of the air gap? You don't want two vapor barriers in the same insulation sytem. An air gap allows for convective currents which you don't want. You plan to place a foil faced board at the cold side and seal the seams which creates yet another vapor barrier. And foil is the most efficient so if any vapor is going to condense in the system it will happen right at the foil membrane. Poly on the ground is OK, but doesn't really do much in a vented crawl space.


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## joecaption (Nov 30, 2011)

Change out your old style vents to automatic opening, there only about $17.00 a piece at Lowes or HD. In most cases I can change one out in less them 15 min. It opens when warm and closes when cold with no power, No way you can forget to open or close them.
I'll make a guess the area under you house is lower the the grade outside.
Unless you raise that grade by adding sand it will always be a swamp under there.
It's very important to have that 6 mil. vaper barrier. I live in a very low lying area and seem to have the same conditions as you.
Without that barrier the insulation will stay wet and fall down, also the insulation hangers will rust and fail.
And no there is no need for an air space between the floor and the insulation. You also do not want to form a double vaper barrier.


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## gcan (Dec 23, 2011)

R-13 won't fill the joist cavity and I mentioned the air gap at the top becasue one of the recommended sites posted it but it did not say if the insulation was faced or not which is why I asked the question.
I live in Alabama and the cabin is in TN and for some reason Lowes and HD in both cities only stock R-30 in unfaced insulation except for the Roxul which is over double standard insulation.....but if that is the only way it will work then I can do that.

So should I put the paper against the house floor? should I pull the paper off since it will have the poly sealing it in?

I agree on the auto vents and will plan on doing them once I get past some of the other projects.


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## gcan (Dec 23, 2011)

Here's the picture that I'm using as a guide


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## Windows on Wash (Aug 30, 2011)

gcan said:


> Here's the picture that I'm using as a guide


What is the point of the air gap in this application?

Where did you get the picture?


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## gcan (Dec 23, 2011)

Wow - not sure why the air gap...see the link below the picture I used is figure #7



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

thanks for the help


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## Gary in WA (Mar 11, 2009)

As the BSC article brought out,* radiation* heat transfer is stopped by the foil-facing. 

*Convective* heat transfer is stopped by the seam-sealed foam board (high density).


That just leaves* conduction* = air gap to floor sheathing; http://www.buildingscience.com/documents/digests/bsd-011-thermal-control-in-buildings


Tape all seams, it has to be air-tight....


Just be certain to foam board/canned foam the rims first, if no foam on exterior; http://www.buildingscience.com/documents/information-sheets/crawlspace-insulation/

I would also partition the floor joist cavities over the bearing beams with foam board or the existing solid wood blocking and caulk to divide the floor space in to segments, in case of an air leak somewhere…


Gary


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## gcan (Dec 23, 2011)

will R-13 faced work in the floor cavities if I pull the paper off? unfaced for some reason is hard to find in my area, except R-30.

Using Roxul cost over 2X as much as kraft insulation.

In the rim cavity does the foil go toward the rim joist or face out to the crawl space?


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## Gary in WA (Mar 11, 2009)

R-19 for your area if in Zone #4: http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_11_sec001_par001.htm

http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_11_sec002.htm

You should be fine with the R-13. The asphalt paper facing is a little iffy because your location doesn't warrant it. Zone 3, or 4? It is a vapor retarder, *not barrier* as some call it. It is moisture variable with wetness; http://www.buildingscience.com/documents/reports/rr-0004-air-barriers-vs-vapor-barriers
Just pull it off, if concerned, to be safe. 

Good to cover the dirt with poly. 

Gary


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## gcan (Dec 23, 2011)

Great - I up it to R-19 FG and pull the craft paper off


thank you for the response


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## Gary in WA (Mar 11, 2009)

You're welcome, glad we could help!

Gary


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