Welcome to the DoItYourself Forums!

To post questions, help other DIYers and reduce advertising (like the one on your left), join our DIY community. It's free!

Heat loss through concrete slab foundations


I_Will_DIM's Avatar
Member

Join Date: Apr 2010
Posts: 65
CAL

12-22-10, 08:30 PM   #1  
Heat loss through concrete slab foundations

What percentage of heat loss goes out through the floor.
Are there many slab foundation homes in really cold climates? I'm guessing not.
I used to live in a raised wood foundation home and as soon as I moved to a slab foundation home I noticed how it just did not retain heat well even thought the ceiling and walls were insulated.

 
Sponsored Links
Bud9051's Avatar
Member

Join Date: Oct 2008
Posts: 9,764
ME

12-23-10, 05:19 AM   #2  
It all depends upon how the concrete floor was made. If they used 2" rigid insulation and continued up the foundation walls with moisture protection and insulation, a slab home can be very comfortable and efficient. In floor radiant heat added to the above will eliminate any sense of cold floors.

For older homes with less slab insulation, some do well and others are always cold. Much depends on the floor covering and heating system.

The hard part is, improving an older poorly insulated slab is not easy. If radiant heat is added, the lack of insulation will make it expensive. Tearing up the slab to add insulation would be a major project and rarely cost effective.

As for the percentage of heat loss, it varies considerably with each home, but heat loss through a floor to mother earth below is far less than heat loss through exposed concrete surfaces.

Bud

 
tom ligon's Avatar
Member

Join Date: Jan 2011
Posts: 2
VA

01-06-11, 05:55 PM   #3  
Sounds like we're thinking along the same lines. I've been stumbling all over the place trying to find useful data regarding heat loss thru a gravel bed, and I'm coming up short. There are apparently some sophisticated computer codes out there for calculating this, for a price, but raw data on conduction thru dry gravel is conspicuously absent.

Stone is fairly conductive of heat, with R values typically in the .1-.2 range, but dry gravel should not be as bad, particularly nice angular crushed stone with a fair amount of air space and lots of point contact of the pieces. I'm guessing the R may be in the .6 per inch range, but it is really hard to come up with hard data. When I found this post I was sketching up a guarded hot plate design to run my own test, and hoping to heck it does not come to that.

If I knew then what I knew now, there would be rigid foam at some level down in my gravel bed. Evidently it can take the weight of typical floors with no problem, which I would not have guessed. But I deliberately went for a lot of thermal mass.

My particular installation is a radiant heat floor over concrete, over 60 tons of dry crushed stone. The concrete block walls around this are insulated with R13 foam, and the depth of the crushed stone varies from about 3 ft at the north side of the building to more like 7 ft on the front side. That results in a respectable R value even on the back if my guess of 0.6 per inch is right. Because this application is for heat going down, there should be less heat loss as the air cells between the rocks don't convect.

But the key to my design is that the floor is supposed to be a heat storage medium. This is actually pretty common in certain types of solar design, and I am finding data on using masonry and gravel beds to store heat so that, for example, AC systems can run in the night when rates are cheaper. This technique is starting to be used as an energy efficiency

Finally, I just found one heat loss estimation guide that puts the U value of concrete floors at a very modest 0.7 W/M^2-C. For my 67 square meters of floor, at 68 F with a deep sink temperature of 50 F (a 10 C difference), I'm probably losing about 470 W thru the floor, or about 1600 BTU/Hr.

But even then, it is not clear to me the heat is totally lost. The heat source is solar hydronic panels and ain't costing me or the environment anything. I have 15000 BTU/Hr available, for about 4 hours per day. Second, when the sun goes down, I should get some of that back. The nature of this little log cabin is that its extreme thermal mass holds temperature for a week at a time. If I can warm that floor the place will stay comfortable. All it really needs to do in the winter anyway is to keep the place from freezing.

This winter will be the big test. Anyway, there's another 15000 BTU/hour yet to be installed, sitting behind the shed.

 
drooplug's Avatar
Member

Join Date: Aug 2009
Posts: 4,940
NJ

01-06-11, 06:15 PM   #4  
Where are you located? I think you need to consider that your heatloss is going to be lateral as well. Depending on your location, the soil to the side of your foundation is going to be much colder than the soil under your foundation. My in-laws house is slab on grade with in floor radiant heat. The heat that escapes through the side of the slab melts all the snow next to the house.

 
I_Will_DIM's Avatar
Member

Join Date: Apr 2010
Posts: 65
CAL

01-07-11, 03:35 AM   #5  
I'm in Southern California, near the beach. In Winter it's usually 40 - 50 degrees at night.

 
tom ligon's Avatar
Member

Join Date: Jan 2011
Posts: 2
VA

01-07-11, 08:21 AM   #6  
The San Diego area? The soil there is sand and cobbles as I recall. It is not a great insulator, but you won't have a lot of heat loss, either. It drains well so should not be particularly wet unless you're a lawn sprinkler nut. The worst winter I recall when I was there was a cold, wet miserable 4-day weekend in maybe 1999 or 2000.

The slab probably does not have much gravel under it.

Summer AC use would constitute more of an energy conservation problem, but not bad if the thermostat is used judiciously.

Frankly, a carpet and foam pad would probably be enough in that climate, but for new construction some insulation under the concrete is cheap and typically a good idea. Based on the numbers I'm seeing, other surfaces, especially windows, are more promising energy conservation targets in that area.

 
Search this Thread