I'm wondering if anyone out there has come across specific language for the attachment of fiberglass insulation over an unconditioned crawl space.  I realize there are various ways to accomplish proper installation where the insulation material makes contact with the floor diaphragm.  Personally I prefer to see the use of nylon twine as a means of attachment and I'm wondering if anyone happens to know of any recommended spacing guidelines for the attachment points of a zig zag pattern of twine when used for support of floor insulation.

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Seems like a good question for one or more of the insulation manufacturers. When we "twine" insulation, we run it straight instead of zig-zag and generally on 16" centers.

Generally wire supports are used for this purpose:

But there is a danger in leaving the bottom of batts and floor joists exposed in an unconditioned crawl space in cold climates, since the underside of the floor assembly will be radiantly-coupled to the ground beneath and remain very close in temperature. If the crawl space is damp (and which one's aren't?), there is a high probability of mold growth on the joists and condensation on the batts.

bsi009_figure_03.jpg

So a far better approach is to place rigid foam board under the insulated joists to thermally isolate the entire floor assembly from the cold crawlspace floor and to create an air/moisture barrier as well. This is explained in Building Science Inc BSI-009 New Light in Crawlspaces http://www.buildingscience.com/documents/insights/bsi-009-new-light...

bsi009_figure_07.jpg

Those "lightning rods" rarely make for a good installation in my opinion. They depend on over-compressing the insulation in order to be securely installed, and if the insulation isn't over-compressed, they soon fall out.

In some cases I like rigid board under the floor, but in many houses there isn't an access door big enough to put full 4' sheets into the crawl, so you end up cutting the stuff in half or thirds.

I've never liked the "lightning rods" either. I prefer wood strapping.

But you can always jack the house up several feet onto cribbing, install the foam board, and drop it back onto its foundation.

Robert, you can save time by simply jacking one edge of the house. Lift it enough to slip all of the foam board into the crawl space, then lower it.

I like the idea of jacking several feet, though. When I retrofit a foundation to my old house, I lifted it just above head level so I could walk around under it, excavate, set forms, pour piers, do all of the mechanical work, etc. It was extremely convenient. It's back down to about 30" now, a crawl space once again, but most of the work is already done.

I like some headroom, too.

raise3

I appreciate the feedback on the floor installation procedures.  Just want to remind you that what I'm looking for are guidelines such as one might find through Resnet or Energy Star.  Personal opinions?  Not so much.  Thanks again though.

I don't think you'll find any such recommendations from either of those sources. There are best industry practices, building science evaluations, insulation manufacturer's recommendations, and the extensive personal experience (what you deride as "opinions") of professionals in the field.

I have considered this issue several times as well. I am thinking of using House wrap or the reflective bubble wrap as support/vapor barrier attached to the underside of the joist. Both are able to fit into the crawlspace areas. My concern is trapping moisture in the floor system. I would be happy with an "experienced opionion" or two.

House wrap is exactly the wrong thing to use if there is any dampness in the crawlspace, as it allows water vapor through but traps liquid water. Bubble foil would have the advantage of being a vapor barrier and radian barrier to decouple the joists from the ground temperature, but it would also trap moisture above.

I've used filter fabric if there was not a significant moisture source below (such as on cantilevered porches that got enclosed and insulated), which is not a vapor barrier but will breath if moisture does enter the cavity.

I guess my thinking was to isolate the floor cavity from the crawlspace with the barrier. The drawing above witht the rigid foam board makes sense but the issue is getting it in and installing it as stated above. I suppose the bubble foil might not prevent condensation from forming which could allow water to form above. How much moisture is likely to make its way throught the floor assembly from the inside? 

That depends on the permeability and leakiness of the floor above - will mop water or spills seep through? - and on the RH below. If the crawl space has a dirt floor with no VB or has any leakage from the outside, then you can bet the RH will be close to 100%.

To effectively decouple the floor assembly from the ground temperature requires both enough insulation and a radiant barrier, such as foil-faced foam board. Bubble Foil is a relatively useless product and simple Kraft-backed builder's foil works just as well in most applications.

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