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

Fri May 3 15:11:12 UTC 2019

There is a lot more “known” about this than is hinted at in the many interesting comments.
Heat capacity is one thing. The adsorption and desorption of water vapor from cellulose/straw/hempkrete is another.
The thermal conductivity (R-value) is another.
The reason the metrics like the decrement and phase shift have not been a focus in in the last decade or two is that we have much better and more accurate tools that use hourly (or minute by minute) computer models that account for all of those factors and more (eg vapor permeance, capillary conduction, evaporation and condensation)
WUFI is one such program that I have a lot of trust in, as we have compared its results to both laboratory and field test. We have measured temperature and RH (and heat flow to a lesser extent) of walls made of straw bale (in the field, in Canada and California) and cellulose (numerous locations) and been able to predict and understand performance using WUFI once the proper material properties (see above) and environmental conditions (hourly temperature, sun humidity wind rain) were input.

What we did not do was to try use these models to predict the thermal and hygric mass benefits to comfort or energy use in buildings. Partly because it rarely matters compared to other features and partly because there are so many variations of building exposure, operating cycles and comfort demands as to make generalizations difficulty.

I do know, for sure, that a well insulated building that is airtight and has not too many but excellent quality windows uses little space conditioning energy and provides great comfort and resiliency.
In hot weather, insulation is less important, airtightness remains so, and excellent shading are the key features.  Thermal mass helps during cold weather, and helps during warm weather and is really really helpful in hot dry climates where is gets very warm during the day and quite cool at night.

But I dont think we need to look to magic, or assume poorly understood science to understand our experiences and practical good performance. We may be missing some material properties, and we certainly are missing detailed and proper studies and measurements of building performance. But I dont think that claiming that natural materials have special properties is that useful to getting them used more widely. I can always use more fiberglass to get the same performance as cellulose. Or use phase change salts in drywall inside a building to get high thermal mass behaviour. The real  questions are more like: to meet a certain performance level, can I use an 18” straw bale instead of 24” of fibreglass? Can I use 16” of hempcrete to equal that 18” strawbale?   This was closer to what Lars asked originally. I dont know those answers, but I think we could get close to answering them by doing good studies with what we know already.  But in the extreme we do know the answers: if you build a straw bale wall and then use a lot of west-facing double glazed windows with little shading in Denmark, it will overheat in summer, and no realistic amount of thermal mass will help. the answer lies not in the material properties of the opaque wall, but the design of the building for the climate and occupancy.

john

> On May 3, 2019, at 04:50, <asbn at baubiologie.at> <asbn at baubiologie.at> wrote:
> 
> Dear Lars
>  
> What an interesting discussion. I followed only partly, but some of the comments and analyses are really great for me, too.
> E.g. I never calculated the water in natural fibres as main heat storage capacity, as mentioned by Barbara.
>  
> I just want to additionally mention, that the temperature differences between straw bale houses (or those insulated with cellulose) are not just an effect of the heat storage capacity due to higher masses and water content, this is also (and I would say mainly) the effect of the moisture-storage capacity of clay plasters and the cooling effect due to evaporation, when outside or indoor temperatures rise.
>  
> Liebe Grüße / kind regards
> Herbert
> 
> asbn – austrian straw bale network
> 
> ESBA – European Straw Building Association
> 
>  
> 
>  
> Von: Gsbn [mailto:gsbn-bounces at sustainablesources.com] Im Auftrag von Rene Dalmeijer
> Gesendet: Donnerstag, 02. Mai 2019 21:29
> An: Global Straw Building Network
> Betreff: Re: [GSBN] Heat storage capacity of wall systems. Hempcrete and Decrement Delay
>  
> Lars 
>  
> I once had the occasion to visit 2 identical pre-fab wood frame houses. It was very hot 35+ The cellulose insulated house was comfortable the the glass fiber batt wasn’t. Which shows in practice what the theory tells. I found it hard to accept that the big difference was only due to the phase shift effect. It really makes a substantial  difference, mainly in the summer at least with substantial dinural temperature swings. 
> 
> Sent from my iPhone
> 
> On 2 May 2019, at 19:35, Tom Woolley <tom.woolley at btconnect.com> wrote:
> 
>> 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 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
>>>  
>>> 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 
>>> www.reginc.com
>>> 
>>> 
>>> 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
>>>  
>>> 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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