[GSBN] Heat storage capacity of wall systems.

[John Straube]

Well insulated walls with more thermal mass can often perform slightly better than those without thermal. But the improvement is very dependent on the variations of exterior temperature, solar exposure, interior temperature swings, etc. Thermal mass within a building is far more useful and its benefit slightly more predictable.

The heat capacity of basalt (and most rocks) is about 0.8-0.9 kJ/kg C, say 0.85
The heat capacity of cellulose insulation is about 1.4 to 2.0 kJ/kgC. I think the higher end is valid
The heat capacity of straw is harder to find. It should be in the range of 1.4 to 2.4 as it is a wood-based material, and the best resource I have shows 2.0. But some older results show lower results for reasons I cant understand.

So if we use a low density rockwool batt of 32 kg/m3 we get a volumetric heat capacity of 27 kJ/m3/C
IF we are talking about higher density exterior sheathing insulation, say 70 kg/m3, then 60 kJ/m3/C
If we use cellulose at 4 pdf (64 kg/m3) and 2.0, then 128 kJ/m3/C.
IF we use straw, and  value of say 7.5 pcf=120 kg/m3 and 2 kJ/kgC, then we have 240 kJ/m3/C.

So,  for a wall of the same thickness of insulation 
-dense pack cellulose would have a greater thermal mass than rockwool, easily twice as much, maybe four times as much. 
-straw of modest density would have a thermal about twice that of a cellulose wall, and as much as 9 times that of a batt insultaed rockwool wall (not a good idea at the thickness we are talking).

Hence, although enclosure thermal mass is not a huge factor, the fact that cellulose and straw have many multiples of the thermal mass of rockwool will likely make a modest difference to energy and maybe a more significant difference to comfort.

John



> On May 2, 2019, at 06:37, Lars Keller <larskeller at gmail.com> wrote:
> 
> We have a discussion in Denmark where some people argue, that if you compare two walls with similar insulation values, one being insulated with mineralwool, and one with wood cellulose or paper cellulose, then the wood cellulose option can retain / contain more heat thatn the mineralwool solution.
> 
> The advantage of this for the wood cellulose is, that this solution is then capable of absorbing more heat when there is eg more solar influx, and later release the heat into the room again, thus creating more comfort. I assume that this is a result of the cellulose option being heavier than the mineralwool option.
> 
> I would like to hear thoughts about whether my understanding is correct.
> 
> I assume straw would share the benefit of the cellulose option.
> 
> Does anyone know if we have / there is numbers to back this up ?
> 
> Best, Lars
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Dr John F Straube, P.Eng.
jfstraube at uwaterloo.ca
www.JohnStraube.com