We are seeing this more and more... 9" of insulation flush on the bottom of a 12" or 14" floor joist cavity.  No contact with conditioned floor, enclosed and ventilated crawlspace.  They typically have 3" to 5" air gap above the insulation.  From underside it appears to be grade 2 and possibly grade 1!   Does any one know how poor these really perform?

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Hi Kent,

Cutting corners is a practice many have perfected.  But that air gap above the insulation may not be all that bad.  Emphasis on "may", as there are other details that need to be done correctly.  Joseph Lstiburek did a good article on this, attached below.

What's your climate zone and what does enclosed means if it is also ventilated.  Is the crawl floor covered and sealed?  If it truly needs to be a vented crawl space that floor above needs to be covered from below with an air barrier.  What else is in the crawlspace?



Thanks for the feedback Bud! 

We are in Climate zone 4 - marine & radon zone 1. It's a foundation wall with venting 1/150.  Crawl is covered with 6 mil visqene with good overlaps & sealed.   Insulation is open to crawl with no rigid on the bottom side.  These homes do get frequent 10- 20 mph winds.  2-3 weeks of freezing and 2-3 weeks of high temperatures - moderate temperatures otherwise.

Lstiburek's article implies there isn't too much loss unless exposed to wind and big temperature swings. 

First, to clarify, we are discussing a short basement, ie a crawlspace??  Your reference to 1/150 for venting I would normally apply to attics.  Not sure that number is applicable to a below the house crawlspace, minimal stack effect and limited high/low locations for vents.

In a humid climate I probably would not vent a crawlspace.  Seal it up and insulate the foundation walls and make it part of the conditioned space.  This avoids summer humid air entering and finding a cold surface to condense on.  But, if they go with a vented crawlspace, then the crawl ceiling needs more than exposed fiberglass.  If the house is air conditioned, that same humid air will end up condensing on the bottom of the floor, right through the insulation.  Exactly what needs to go there is debatable.

But your original question is about how effective the insulation is in this configuration.  Joe covers it in his paper, the limited air flow.  In a warm over cold configuration, convection is naturally suppressed.  Depending upon the location of those vents and the amount of air flow the loss of r-value will be limited.  But, there are better ways of insulating these locations.

And, as for the radon, they should at least have a passive system installed ready to be upgraded to active if needed.


Thanks Bud.  Yes these are short foundation walls and access under is usually an issue.  Unvented & enclosed crawlspaces have been tried here (Pacific Northwest) with conflicting results and the consensus is to stick with vented crawlspaces.  The passive radon system is required and also ready to be made active if deemed necessary.

I appreciate Lstiburek's article and it makes some sense but I'd like to hear from actual results.  The beer reflection gives me hesitation, as that has often resulted in breaking out in handcuffs.  Logic and alcohol are not good mixes!

I think if it were my house I'd have the insulators get the insulation in contact with the subfloor...basically -  just fill the cavity.

Haven't seen anyone in this area do the rigid foam on the underside yet- probably too moderate of a climate to make it cost effective.

One of the distinctions Joe makes about the gap above (which I agree with) is a "warmer floor".  Floors are hard to heat because of the natural stratification above.  Leaving the gap allows for some heat loss into that space and a slightly warmer floor.  But, that cavity should be as sealed as possible.  For simplicity and for less overall heat loss, filling the cavity might be best, especially since you are not in an extremely cold climate.  I'm just sure how to evaluate the moisture flow.

As for Joe's humor, that is his writing style, his experience is extensive.  As for some having tried the sealed and unvented approach, I would suspect they did not condition the resulting space below, a must.  If fully conditioned, it would be no different from a conditioned basement. 





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