This is an offshoot from Hal's thread on the need for better classification of radiant barriers:

I didn't want to drift on his, so I have started new.

Using the sciences to our advantage.

I have a camp in the planning stages (similar to David Meiland's question on the cost of rigid foam insulation), on posts and enclosed on the bottom.  Here's David's

Mine is a heating dominant climate as well, in Maine and close to water, thus the posts will provide a better view and some protection from seasonal high water on rare occasions.

Having a living space over a cold space is a frequent problem in cold country and often results in cold floors.  Overhangs, porches, a bedroom over a garage and yes, my proposed camp are all examples.  But all of these have one design consideration in common, the warm is stacked over the cold, a configuration that does not support convection.  IMO, warm over cold needs some careful consideration.

Of the three modes of heat transfer, radiant, convection, and conduction, any time we can shut one or more down, we have reduced some major paths for heat transfer.  Now, picture the floor of my camp design, foil under the flooring, I-joists, and air sealed cavities with no insulation.  That's no insulation.  There will be some conduction via the bridging and some radiant transfer from the upper portion of the I-joist (which could be detailed with some foil as well), but no conduction through the cavity, virtually no radiant transfer, and because the warm is stacked over the cold (almost) no convection.  Add to this that any heat that does migrate down and warm the cavity air, the resulting convection would move the warmer air back to the top of the cavity.  It's ironic, but filling these cavities with fiberglass could increase my heat transfer as air is a very poor conductor of heat.

The benefits of warm over cold are not new as our cooling climates benefit from it when the ac is running.  Warm attic air does not migrate down to displace the cool air buried under our insulation.  In a heating climate, that cold air we vent into our attics goes directly to the lowest point it can access and pushes any warm air it finds up and away. 

Instinct says the floors will be freezing, but I can't identify the heat loss path.  And if there is any benefit to adding a radiant barrier to the underside of these floors, even with just a small air gap and lots of insulation, I'd like to be able to calculate it.

All comments are welcome, I think.


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Bud, I don't think you will get anything close to your R-23. For one thing, the increase in total R with depth of cavity is non-linear with quickly diminishing returns. For another, it's impractical to expect much value from a layer of foil on the bottom facing upwards as it will become covered with dust or condensation (unless the space is perfectly hermetically sealed in a dust-free, dessicated environment and impervious to water vapor diffusion). The thermal bridging will be substantial because of the large surface area of the TJIs facing the air space. And there's a reason that most of the R-value claims of the radiant barrier industry have been debunked - they have very limited value in the real world. 

Unless you can create a perfect vacuum in those joist cavities and turn the floor into a thermos, you're going to need some kind of insulation to make it energy efficient. There's no getting around it.


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