[GSBN] Heat storage capacity of wall systems. Hempcrete and Decrement Delay

Thu May 2 17:35:30 UTC 2019

Lars

You have raised a very important debate which perhaps could be the subject of a scientific conference .
Maybe we should organise one!
What I find interesting is that no-one has yet referred to Decrement delay
https://www.new-learn.info/packages/clear/thermal/buildings/building_fabric/properties/time_lag.html

Thermal mass is important in terms of the thermal performance of buildings  but its effects cannot be fully understood without also considering decrement delay
The Concrete centre in the UK makes great claims about the benefits of the thermal mass of concrete for instance

However measuring thermal mass in itself is not helpful without considering the decrement delay factor

High thermal mass may not be beneficial if the time lag/decrement delay is too slow , as it is with concrete
Despite what August says I am pretty clear that heat release from concrete is not quick but extremely slow
This is why so-called night calling using exposed concrete floors is rarely heard of these days as the decrement delay in concrete means that it doesn’t warm up or cool down at the right time

One of the reasons we have become fans of hempcrete is that it evens out temperature fluctuations on a 24 hour cycle and so the temperature in a building with 30 cm thick walls remains constant whatever fluctuations are taking place outside, without any heat input
This is why it has become popular with the wine and food storage industry as they know they can store things in a space at a steady temperature of about 14 Degree C without any heating on cooling.

Sadly we have very little scientific data to back this up based on measurements at buildings.
Its just we know it happens from observation.
https://www.ukhempcrete.com/hempcrete-buildings-thermal-performance-and-costs/

My experience of any lightweight insulation materials such as cellulose or any of the synthetics is that they are poor at storing heat and their decrement delay is too short.
My own study that I am sitting in here is insulated with wood fibre boards and lightweight hemp flax quilt but on a cold day it loses heat far too quickly and heat has to be added

However our hempcrete house stays comfortable all day and night long with minimal heat loss because of its excellent combination of insulation and thermal mass and its near perfect decrement delay properties
One of the mysterious things about hempcrete is due to its hygroscopic properties. Hemp can take in and release water vapour and its absorption of water actually assists with its thermal performance. Water has good heat retention capacity

Strawbale in my experience is not as good as hempcrete in terms of decrement delay but if built well and depending on the plaster/render it can have good decrement delay properties

The pioneering work on decrement delay was done by Steven Szokolay in his wonderful book Introduction to Architectural Science first published in 2004
https://www.bookdepository.com/Introduction-Architectural-Science-Steven-V-Szokolay/9780415824989

Neil May of Natural Building Technologies in the UK, who sadly died recently at a very young age, was an expert on decrement delay but its hard to find what he published on this
This important report makes some references
http://goodhomes.org.uk/wp-content/uploads/2017/05/GHA-Critique-of-the-Green-Guide-to-Specification-051208.pdf

Simply comparing thermal mass without considering decrement delay will not give the full picture

Tom

Tom Woolley

On 2 May 2019, at 17:56, August Hasz <hasz at reginc.com> wrote:

Lars-

To do a comparison of this one single aspect of the insulation options, you need to compare the heat capacity (typically listed as energy stored per unit weight)+ the weight of the different materials. Resistance to heat transfer (R-vlaue or U-value) is typically the most important characteristic to consider. This is pretty straight forward math to do if you have access to the thermal properties of the materials being considered. With natural materials getting an exact reading can be a bit difficult because changes in moisture content (particularly for straw, wood and other organic properties) will alter the heat capacity, with and resistance to heat transfer significantly, but you should be able to find some general average values to use in a comparison.

The story of how this unfolds in buildings is a big more complicated though because we need that mass to be ‘active’ to be of use. For example, a similar thickness of some wood products will store as much heat as that thickness of concrete because of the difference in heat capacity, but concrete will release this heat more quickly, so if we are purely considering heat storage the concrete is likely superior BUT wood is of course renewable, has lower embodied energy and in many applications the increased resistance to heat transfer is a plus.

Is that helpful at all?

Thanks, August

August Hasz, P.E.
President
REG
Resource Engineering Group
Direct: 970.713.0984
Cell: 970.275.3603
hasz at reginc.com<mailto:hasz at reginc.com>
www.reginc.com<http://www.reginc.com/>

On May 2, 2019, at 6:37 AM, Lars Keller <larskeller at gmail.com<mailto: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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