Thompson SFP House Detail

Cross-section of frost-protected shallow foundation for 12" truss-wall framing system.

Comment

You need to be a member of Home Energy Pros to add comments!

Join Home Energy Pros

Comment by Robert Riversong on September 6, 2012 at 10:52am

I forgot to include the BSC link: http://www.buildingscience.com/documents/insights/bsi-003-concrete-...

The other consideration is the very high global warming potential of XPS, for which a new analysis by Alex Wilson and friends shows becomes worse over its lifetime at more than about R-15-20.

http://www2.buildinggreen.com/blogs/insulation-keep-us-warm-not-war...

http://www2.buildinggreen.com/content/global-warming-potential-insu...

Comment by Robert Riversong on September 6, 2012 at 10:47am

David,

This represents a balance between minimizing heat loss from a radiant slab into the ground and allowing sufficient downward heat loss to maintain the foundation as frost-proof (as well as moderately earth-coupling the house to the ground in the event of prolonged periods without supplemental heat, such as power outages or winter vacations).

Most proponents of superinsulated building would put more than R-10 under the slab, particularly a heated slab, with Passive House folks sometimes using as much as R-40. So I consider 2" XPS a minimum for this application, which includes FPSF, radiant and solar thermal mass slab.

Comment by David Williams on September 6, 2012 at 9:48am

Extruded (of course).  I also like the way that you achieve the R15 edge requirement.  I have modeled energy loss comparing 2" XPS with 1" XPS in well drained + soils climate zone 6. and have reservations about the added cost of 2" under the slab.  Knowing that you only get 1 shot at getting this right, I wonder what you thoughts are and where you would draw the line between 1" & 2".

Can you offer a link to the Building Science Corp reference?

Comment by Robert Riversong on September 6, 2012 at 9:18am

David,

Yes, that should have been noted in the section. I use a Tu-Tuf 4 mil laminated vapor barrier that is designed for sub-slab applications and is highly tear-resistant. All seams are taped, as well as all seams of the XPS (not EPS) insulation board.

It is a bad idea to put any "reservoir" layer of sand between the vapor barrier and the slab - Building Science Corp addresses this - as it is likely to create moisture problems. The slab finisher loves it because he can power trowel the slab and get home to his Budweiser sooner, but a vapor barrier does not cause slab cracking or curling.

Comment by David Williams on September 6, 2012 at 8:56am

Do you have a moisture barrier below the slabs' EPS?  I have found that a 1" layer of sand, placed over the EPS helped to protect both the insulation and vapor barrier.  This may have also provided some amount of moisture control during the curing phase.  I have had concrete contractors exclaim that slabs poured over EPS are very prone to cracking because of the way the the moisture escapes form the curing slab.

I have not had these cracking issues, on the radiant heated slabs that I have done over the past 30 years, but I have seen them in slabs that did not have proper edge insulation. 

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

Scott Katznelson posted a discussion

Database of zip code or county by climate zone

HI.  I'm looking for a database that could help me assess the average energy use of a home (per…See More
5 hours ago
Hal Skinner replied to Hal Skinner's discussion The 'Touchy - Feely' sales pitch. in the group Radiant Control Coatings
"The 2nd bestsales tool I ever came up with.  aI will be happy to send anyone a test strip like…"
11 hours ago
Patrick Michaelyan joined Hal Skinner's group
Thumbnail

Radiant Control Coatings

A group where people who work with radiant control coatings can share their success stories and let…See More
14 hours ago
Joseph Lamy posted a blog post

After 70 years on the fence, knotty LED light fixtures hide the Ductless in splendor

After the first week, the ductless heat pump and I became so close that could never even think of…See More
17 hours ago
Profile IconHome Energy Pros now has Member Map
18 hours ago
Profile IconLaura Spencer and Ann Griffin joined Home Energy Pros
18 hours ago
Tom White posted a video

The Animals Save the Planet - Energy Efficient Penguin

"Energy saving bulbs use one fifth of the energy of ordinary bulbs." Help the animals save their planet and ours, use energy saving bulbs! http://fartingcow....
21 hours ago
Laurie DiDonato posted an event
Thumbnail

Hybrid HERS Rater Training at Online + Manchester, NH or Berlin, CT

September 22, 2014 at 9pm to October 10, 2014 at 7pm
Online Component- September 29th-October 10thChoose from 2 Classroom Sessions: October 27-30 –…See More
yesterday

© 2014   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service