[GSBN] Heat storage capacity of wall systems.

Thu May 2 23:05:48 UTC 2019

Derek and Lars

I agree – the role of insulation is to prevent heat gain – particularly here in Australia where temperatures can get to 42 degrees C in summer.

That is the beauty of straw bale construction. This year in South Australia we’ve had some very high temperatures. Temperatures taken indoors of 2 straw bale houses has shown that they reached only 29 degrees C.

Maybe European climatic conditions require other thermal techniques?
Bohdan Dorniak

Architect

From: Gsbn [mailto:gsbn-bounces at sustainablesources.com] On Behalf Of Derek Roff
Sent: Friday, 3 May 2019 12:20 AM
To: Lars Keller; Global Straw Building Network
Subject: Re: [GSBN] Heat storage capacity of wall systems.

Perhaps I am misunderstanding the scenario, but I’m skeptical, Lars, that the difference in heat storage capacity of different kinds of insulation is very important to the building performance question.  In the example that you quote, of wall insulation absorbing heat from solar influx when available, and later releasing that heat to the room, an important distinction is whether the scenario assumes sunlight falling on the outside of an insulated wall, or on the inside of an insulated wall.   

When sunlight heats the outside of an outside wall, we need enough insulation that very little of that heat ever makes it inside the building.  Super-insulated buildings are designed to prevent heat movement from outside to in or inside to out.  Therefore, we can’t expect useful heat transfer into the living space from sun shining on the outside of an insulated wall.  The wall is designed to prevent precisely that.  

If sunlight is coming trough a window or skylight and shining on the inside of an insulated wall, then there is some hope that this solar energy will help thermal performance of the house.  The surface of the wall being struck by the sunlight will heat up to above the average ambient temperature in the room, and then (immediately) begin to release that heat through radiation, conduction, and convection back into the room, tending toward temperature equilibrium.  

Several problems come to my mind in considering trying to store useful solar heat in an insulated wall.  The first is that I wouldn’t expect very much sunlight to strike the inside surface of an insulated wall.  Light coming in through windows and skylights will mostly strike the floor, furniture, and interior (uninsulated) walls.  As I imagine a normal house, I wouldn’t think that even ten percent of the sunlight entering the building envelope during the course of a day would touch the inside of an insulated wall.  For a larger, commercial building, the percentage would be even smaller.  

Next, the insulated wall is coated with something.  Clay, I hope, but certainly something less insulating and of higher density than the insulation itself.  Assuming a clay coating, for the moment, it will heat up above average room temperatures when the sun strikes it.  As a dense, fairly conductive material, it will store a fair amount of heat, and change relatively little in temperature.  That is what we want it to do.  Therefore, the clay plaster will ‘protect’ both the air in the room and the insulation behind it from experiencing the higher temperatures that might otherwise occur from the solar influx.  That’s a good thing for comfort in the room, since it moderates temperature swings.  So now we have a slightly heated clay surface in contact with the room air on one side and with the insulation on the other side.  Certainly, the clay will conduct some heat into the insulation that it touches.  But temperature differential (delta T) is small, and the insulation wants to resist that heat flow.  Therefore, we can’t move very much heat very far into the low-mass insulation.  This gives us a small temperature change in a small amount of mass within the insulation, which equals low heat storage.  On the room side, the slightly heated clay wall surface will give up some heat to the room through radiation, conduction, and convection.  That is what we want, and that is where most of the heat will go.  So, what percentage of the heat stored in the clay plaster is transferred first to the insulation behind it, and then later, re-transferred to the clay plaster and returned to the room?  I’d be surprised if it was over one percent.  

If these guestimates are anywhere close, then the difference in heat storage capacity of different kinds of insulation won’t make any significant difference.  To summarize:  1) We can’t get much solar influx to heat the insulation, because little sunlight strike the inside of an insulated wall; 2) The surface coating of the wall will have more mass and greater conductivity than the insulation, and it will be the surface that is struck by the sunlight, therefore, it will do almost all of heat storage that might happen in the insulated wall; 3) Delta T will be low between the wall surface and the insulation behind it, minimizing the heat transfer to the insulation, and therefore minimizing the contribution to heat storage made by the insulation; 4) Differences between heat storage by different types of insulation will be insignificant in overall effect, because of all the preceding factors.  

Am I missing something important?  

Derek

Derek Roff

derek at unm.edu

On May 2, 2019, at 6:37 AM, 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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