The process by which heat energy in the form of light (usually IR unless the substrate is hot enough to glow in the visible range) is emitted more strongly by warm surfaces and absorbed by other materials especially those of low IR reflectivity (think matte black finish). Radiant heat transfer does not require a medium. Foam insulation materials, such as spray foam insulation, are opaque to thermal radiation, like most solid materials.
I don't know if it's common or uncommon, but installing a layer of rigid foam above your roof sheathing in order to interrupt thermal bridging is always a good idea, and is preferable in all respects to a roof assembly without any rigid foam above the sheathing. If you decide to install rigid foam above the roof sheathing, you shouldn't use closed-cell spray foam under the sheathing; either use open-cell spray foam (which is vapor-permeable) or a fluffy insulation like cellulose, mineral wool, or fiberglass (any of which will allow the roof sheathing to dry to the interior).
Excellent kit for insulating a house. Slow rising foam takes longer to insulate an area as you must do it in layers 1" thick. Any thicker can overheat the chemicals and prevent a proper cure and off-gassing. Recommend purchasing a second suit and nozzles so you can use it multiple days to achieve the thickness desired for the highest R value. Seals up and insulates much better than traditional fiberglass. The sound suppression is noticeably different. Used it to insulate the ceiling above our bedroom. The sound is remarkably more quiet than the fiberglass insulation we had before. The only down side is the weight of the containers. Using a smaller kit would be easier to handle and move around when spraying a ceiling in an attic. A word of caution, use plywood that spans at least 2 joist per container when spraying in an attic to hold the weight.
I live in Baton Rouge LA with a very old and drafty house. There is no blockages in the walls between the crawl space and attic. Lots of critters just come on in. I would like to used closed cell sprayed under house to warm up floors and block moisture. I would like to spray closed cell into attic, but am afraid of enclosing attic due to moisture build-up. Can I spray closed cell against the attic floor in same way as installing bats, thus leaving my venting the same as it always has been. Other ideas? Thanks.
The problem was that the installer was doing his first spray foam job ever, and the thickness of the insulation varied from zero (visible roof deck) to about 9". Unfortunately, good average thickness doesn't cut it. The coverage needs to be uniform because a lot of heat will go through the under-insulated areas. (See my article on flat or lumpy insulation performance.)
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