[Greenbuilding] Radiant Floor under wood

Thu Sep 7 20:20:27 CDT 2006

I have seen in a passive solar book where they use a sand bed with pex 
tubing under a concrete slab for heat storage and distribution.
    Don't do it. In the building science world one example of an 
expensive stupid mistake is to put sand under your concrete. The typical 
detail would be soil, vapor retarder, sand, concrete. This is what Joe 
Lstiburek (a good Canadian eh) calls a "tampon", sorry, his description, 
but accurate. So before you pour the slab you wet the sand so it doesn't 
wick the moisture out of the concrete too fast causing curing problems. 
We then usually dump lots of extra water on the slab to cure it, and in 
the rainy season leave it exposed to rain. The sand will absorb lots of 
water, the problem is it takes forever for it to dry back out. Of course 
the only place it can dry to is into the house, increase the moisture 
level and you end up with moisture problems like mold, rot, etc.. 
Lawyers get involved, you get sued, it costs lots of money to fix. It 
has happened to large national homebuilders in the sunny dry central 
valley of California. So please don't try this at home. The least 
expensive fix is epoxy on the whole floor and slipping metal under the 
plates.
    How to build this right? soil, sand or gravel, rigid foam (I think a 
must for slabs even if no radiant heat), vapor retarder, concrete.

Keith Winston wrote:

>I'd heard about the sand idea a while back, and did some research. It 
>seemed simple, good thermal mass, etc.
>
>Alas, dry sand has very poor heat conductance, .35 W/mK vs. concrete's 
>1.28, Stainless Steel's 16 and copper's 390. Sand has decent thermal 
>mass, measured (one way) as volumetric heat capacity, 1.3 (10^6 J/m^3K) 
>vs concrete's 1.9, copper's 3.4, water's 4.2. Taken together these give 
>you the diffusivity, the rate at which you can pour heat into or extract 
>heat from a substance. Dry sand has a diffusivity of 28 (10^-8 m^2/s), 
>vs. concrete's 66, water's 14 (good for a liquid), and copper's 11000. 
>The other complaint I've heard is that sand might settle, and reduce 
>contact between the floor and material, but honestly most screeded 
>concrete jobs I've seen weren't done nicely enough to assure contact of 
>the floor with the concrete.
>
>Anyway, there is a fix. Just install a leaky system. Wet sand does MUCH 
>better, better than concrete even: conductivity of 2.7, vhc of 2.6, and 
>diffusivity of 100. Woo hoo!
>
>Seriously though, this concept of saturated sand as thermal mass has 
>intrigued me, I'm trying to figure out how to install a mess of it in a 
>house to do annualized energy storage, and still assure you won't end up 
>with a humid, skanky house. I like the idea of a flooded subfloor, so 
>the sand has a structural raison d'etre also (it supports a concrete 
>slab. Still thinking it through, though... In combination with solar, a 
>heat pump, evaporative or night-emissive cooling, it might be possible 
>to make a VERY low-energy house. Is it possible to do while minimizing 
>material use, that is, a bigger picture view? Given capillary action, 
>how important is leak protection? But DON'T install it as per the 
>tongue-in-cheek comment above.
>
>As for concrete: concrete is heavy. Often people use a "gypsum 
>concrete", which is comparatively lightweight, in this application so 
>they don't have build a massive structural floor to hold it up. But the 
>lightweight concrete is expensive.
>
>Warmly, Keith
>
>
>Clarke Olsen wrote:
>  
>
>>  Concrete is a much better conductor than sand. You could pour concrete  
>>over
>>  your tubing and screed it off at or below the top of your sleepers.  
>>You don't need
>>  to fill the 9", just encapsulate the tubing.
>>  Clarke Olsen
>>
>>On Aug 21, 2006, at 4:58 PM, josephbecker at riseup.net wrote:
>>  
>>    
>>
>>>I am looking for some science and guidance on what to do with the  
>>>spaces
>>>between sleepers and tubing on top of a sub floor and beneath the  
>>>finish
>>>floor of vertical grain red oak.
>>>
>>>Our plan is to put 3" sleepers every 12" leaving 9" spaces. Our  
>>>flooring
>>>man suggested filling the voids with sand instead of leaving air in  
>>>order
>>>to prevent a hollow sound. The owner does not mind a hollow sound
>>>
>>>Would sand help spread the heat or inhibit the spread and trap the  
>>>heat?
>>>Best Energy,
>>>Joseph Becker
>>>
>>>
>>>
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