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Insulation between Floor Joists


Do you recommend inserting insulation between the floor joists, underneath a
home over a crawl space that is exposed to the dirt?  If yes, do you install
it with the paper side down or up?  (I have a vague memory of you saying
that we should never put insulation up between the floor joists.)


Your vague memory serves you incorrectly.  Some do not recommend insulation
in a crawlspace in favor of allowing the subfloor and finished floor to

However, if the living space is air conditioned, the floor becomes cool.
When warm moist air enters the crawlspace through foundation vents, at a
minimum it can absorb into the plywood or OSB subfloor and then on upward
into a hardwood floor causing cupping.  Worst case warm humidity can
condense on the cooler subfloor when the subfloor reaches the dew point
temperature.  Conversely, in winter months, the floor can become cold and
impact energy consumption and cost.  

The paper side to which the fiberglass is glued with a tar-based adhesive,
serves as a vapor retardant barrier.  But if water damage occurs inside a
home, that vapor barrier can collect water on top, thus making the job
harder to dry.   

Not being as smart (the smarty) as I used to be, I Goggled installing
insulation in a crawlspace.  I came up with the following from the Johns
Manville website.  Seems to confirm what I thought.  

Crawl Space Insulation

A crawl space is an unfinished, accessible area below the first floor of a
building. Remember, the main purpose of insulation is to enclose the living
space in a thermal envelope. Therefore, if the space below a floor is
unconditioned, either the floor or the walls below it must be insulated.

There are two types of crawl spaces: vented and unvented. Vented crawl
spaces open to the outside, while unvented crawl spaces are part of the
basement. How and what is insulated depends on whether the space is vented
or unvented.

In a vented crawl space, the underfloor should be insulated, much as it is
installed in ceilings. This may be done either before or after the sub-floor
has been applied. JM ComfortThermR, Kraft-faced and unfaced insulation are
all possible products to use for this application. With Kraft-faced
insulation, the facing is generally against the sub-floor.

In an unvented crawl space, you should insulate the perimeter walls.

Of course, the dirt crawlspace floor is another source of evaporating
moisture and it should be covered with a 6-10-mil polyethylene vapor
barrier.  I think that may be part of the building code in your state.  

L. Jeff Bishop, SCRT Technical Advisor
New Email: 

Clean Care Seminars, Inc.
983 Tate Dr. Unit 1
Dothan, AL 36301
334.446.1847 fax


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