[Greenbuilding] Radiant Floor under wood

[Keith Winston]

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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-- 
Keith Winston
Earth Sun Energy Systems
3927 Madison St.
Hyattsville, MD 20781
301-980-6325
keith at earthsunenergy.com
www.EarthSunEnergy.com