This is an offshoot from Hal's thread on the need for better classification of radiant barriers: http://homeenergypros.lbl.gov/forum/topics/different-classification...

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 http://www.linkedin.com/groupItem?view=&srchtype=discussedNews&...

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.

Bud

Views: 1330

Reply to This

Replies to This Discussion

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.

RSS

Home Energy Pros

Home Energy Pros was founded by the developers of Home Energy Saver Pro (sponsored by the U.S. Department of Energy,) and brought to you in partnership with Home Energy magazine.

Latest Activity

Home Energy Magazine posted a blog post
2 hours ago
Casey Gesell replied to Casey Gesell's discussion Looking for home improvement companies to join our dealer network!
"Hi Patrick, I'd be glad to have a conversation with you about the details on our program. What…"
5 hours ago
Amina Lang posted an event

19th International Passive House Conference at Congress Center Leipzig

April 17, 2015 at 9am to April 18, 2015 at 6pm
The Passive House Standard enables every building owner to benefit from the energy revolution, with…See More
7 hours ago
Don Hynek replied to Luis Hernandez's discussion Non-continuous exterior insulation
"This is a matter of far more than R-values. Average exterior temp, heating load vs. cooling load,…"
23 hours ago
Patrick Michaelyan replied to Luis Hernandez's discussion Non-continuous exterior insulation
"Don, I'd like to ask you specifically about your comments on putting the structure at great…"
yesterday
Patrick Michaelyan replied to Casey Gesell's discussion Looking for home improvement companies to join our dealer network!
"Casey, Time frame for relocating is late spring to early summer. I am considering both counties…"
yesterday
Dennis Heidner replied to Patrick Michaelyan's discussion Vinyl Siding with Integral Insulation
"The observation about rot is important.   I agree. One of my complaints with some of the…"
Friday
Jennifer Schenk added a discussion to the group Job Board
Thumbnail

Job Opening - Energy Programs Manager - Frisco, CO

Energy Programs Manager - High Country Conservation CenterJob DescriptionGeneral Description of…See More
Friday

© 2015   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service