What do y'all say about I Joist Insulation

[Erby]

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My understanding is that insulation is supposed to be installed in contact with the sub floor.

I commonly find it installed in I-Joist floors hanging on wires supported by the bottom flange of the I-Joist, leaving a gap (as seen in the picture above) between the insulation and the floor.

What do y'all say about htis.

[Marc]

The gap allows the development of convective air currents within that area but I can't see how that would be detrimental.

Convective losses through fiberglass insulation is going to happen regardless of whether there is a gap above it or not.

Marc

[Jim Katen]

Read all about it here:

https://buildingscience.com/documents/i ... 2520floors

[Erby]

Yeah, I read that last night too!

However, Joe's solution includes rigid insulation or drywall on the bottom of the joists. That ain't going to happen in a crawl space here. They don't even fully insulate the ends of the joist bays.

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Nice crawl space though. I wish all of them were like this.

[Scottpat]

I'm in the "do not insulate" the subfloor camp!

I have found more wet/moldy subfloors in homes that have insulated subfloors than I care to remember. Most likely it is due to improperly installed insulation and leaking air ducts/boots for floor registers. But in homes with no insulation I seldom if ever find wet subfloors or damage associated with condensation.

In the South we just do not need to insulate the subfloor. I know folks will not agree with this but, I just don't see the energy savings that will justify the cost and risk associated with insulating the subfloor in our part of the country.

[resqman]

Disagree with Scott. He said someone would...

I insulated half the crawlspace of a 1200 sq ft ranch in Atlanta Ga around 1986. Just never got around to insulating the other half.

I could physically feel a temperature difference when walking between rooms. Don't recall energy savings but sure did make a big difference in comfort while living in the house.

[Marc]

It depends on climate for one thing. In coastal Louisiana, the dominant mode of heat transfer on a home with a crawlspace is radiation, not convection or conduction. So I recommend leaving the structural floor un-insulated so that the floor can radiate to the cool earth during the summer months.

Marc

[David Meiland]

The gap allows the development of convective air currents within that area but I can't see how that would be detrimental.

The issue is that the air gap above the insulation communicates with the rim joist around the entire perimeter of the floor. When it's cold outside, a lot of heat is lost at the perimeter. I can't remember the DOE's exact numbers, but insulation with an air gap above it suffers significant performance degradation.

[Jim Katen]

The gap allows the development of convective air currents within that area but I can't see how that would be detrimental.

The issue is that the air gap above the insulation communicates with the rim joist around the entire perimeter of the floor. When it's cold outside, a lot of heat is lost at the perimeter. I can't remember the DOE's exact numbers, but insulation with an air gap above it suffers significant performance degradation.

If the gap is open at the ends, the performance degradation is 100%.