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: 1446

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

Photos

Loading…
  • Add Photos
  • View All

Latest Activity

Profile IconTirso De Ubago, Hamed Ghanbari, Bob Eldredge and 1 more joined Home Energy Pros
2 hours ago
Hamed Ghanbari replied to Brian's discussion Everything you need to become a energy auditor for sale in the group Energy Auditing Equipment for Sale, Trade or to Purchase
"Hello, I am interested to buy a Minneapolis Blower Door DG700. How much is your unit? Thanks. "
5 hours ago
Hamed Ghanbari joined allen p tanner's group
Thumbnail

Energy Auditing Equipment for Sale, Trade or to Purchase

Discuss the pros and cons of the equipment you are interested in prior to purchase. Post equipment…See More
5 hours ago
Colin Genge updated an event
Thumbnail

Measuring HVAC Performance webinar at Online

February 23, 2016 from 11am to 12pm
If you install, service or test HVAC systems you will want be there for this special webinar.…See More
9 hours ago
Colin Genge updated an event
Thumbnail

Testing for IECC Code Compliance for duct and house leakage webinar at Online

February 16, 2016 from 11am to 12pm
The International Energy Conservation Code (IECC) is in use or adopted in 47 states, the District…See More
9 hours ago
Jan Green posted a status
"I'm honored to be selected to speak at the 2016 RESNET Conference right here in sunny Scottsdale 2/29-3/2/16 with SRP & others!"
10 hours ago
Joe Konopacki posted an event

Duct & Envelope Tightness (DET) Verifier at Temperature Equipment Corporation

February 26, 2016 from 8:30am to 4:30pm
The Duct & Envelope Tightness Certification (DET) was designed by the Midwest Home Performance…See More
11 hours ago
Diane Jackson liked Don Hynek's discussion Superinsulation retrofit is Complete!
12 hours ago

© 2016   Created by Lawrence Berkeley National Laboratory.   Powered by

Badges  |  Report an Issue  |  Terms of Service