[Cob] straw clay in floors

[Graeme North]

HI all

Interesting note from Feile regarding a damp proof membrane and Irish conditions- in NZ we generally have very wet ground conditions
and we leave out a DPM - damp roof membrane -out of sold floors built on the ground at our peril.

Yes we put anti-capillary drainage systems under floors, but there is still water vapour coming out of the ground and sometimes its at quite high pressures too. 
Indeed, even with suspended timber floors with heaps of underfloor ventilation, we still sometimes need to cover the ground under the floor with plastic that is weighted down to help keep houses drier as moisture coming up out of the ground can help building s stay damp.   

Personally I prefer a dry floor and reluctant though I am to use plastic, a sheet of 0.025mm polythene is easy, cheap and effective - and it also stops radon coming up if that is of concern. 
And its a really really big problem if you don’t have a DPM in there and then find you need it.  
And if you are the designer or builder then you are also facing a very uncomfortable liability.

Yes people have espoused that there is no need for a DPM under an earth floor here,  and lived to regret it in NZ too.

In a dry climate with low rainfall and low humidities the story will be different.

cheers

  

 
Graeme 
Graeme North Architects
49 Matthew Road
RD1
Warkworth 0981


www.ecodesign.co.nz




> On 24/03/2017, at 11:53 PM, Feile Butler <feile at mudandwood.com> wrote:
> 
> Hi Robert
> 
> I have no experience of light straw clay under a floor, but I would be concerned about the amount of time it would take to dry out and whether it could stay dry. In Ireland, we are pretty sensitive to wet ground conditions - it may not be such an issue where you are. But I would wonder how well it would cope with the beating a cob floor takes curing construction ... and depending on your earth/straw ratio how it would do with compression over time.
> 
> Is there a reason you are adding the plastic vapour barrier? In Ireland, people often think that the building regulations state that there must be a moisture barrier. In fact, the regulations state that moisture ingress from the ground must be prevented. So having a drainage system under the building (and no plastic barrier) can meet this requirement. I am not a fan of introducing plastic to earth. 
> 
> With the LECA, we only use the larger grades, 4 - 8mm and 10 - 20 mm. Smaller than that and the voids are too tightly packed - the capillary break is gone. But we would also have a layer of drainage stone under the LECA.
> 
> Another product that we use is Expanded Recycled Glass. The brand available in Ireland is Technopor, but I don't know what is available in Spain. The advantage of this product is that it can double up as the drainage layer, minimising the depth you need to dig down for your floor. It can take a pounding (important for making the cob floor) and it won't compress over time. Also, I think its thermal resistance is slightly better than LECA. It's not cheap though. I like it as it is a recycled product.
> 
> 
> Kind Regards
> 
> 
> Féile Butler
> 
> MRIAI B.Arch Dip. Arch Conservation Grade III
> 
> Assigned Certifier - Accredited PSDP
> 
> Executive Board Member of Earth Building UK and Ireland
> 
> 
> 
> Mud and Wood
> 
> Roots Architecture
> 
> Grange Beg, Skreen, Co. Sligo, Ireland
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> 
> ----- Original Message ----- 
> From: "ROBERT ALCOCK" <ralcock at clientes.euskaltel.es>
> To: <coblist at deatech.com>
> Sent: Tuesday, March 21, 2017 10:59 AM
> Subject: [Cob] straw clay in floors
> 
> 
>> Hi all,
>> 
>> We're deciding what kind of insulation to put in the subfloor of the new cob/straw bale houses we're building in northern Spain (http://abrazohouse.org/the-place/buildings/el-manzanal/)
>> 
>> The options we're looking at are either (a) vermiculite or expanded clay, or (b) light straw clay.
>> 
>> Does anyone have experience of using light straw clay in a subfloor (underneath a cob floor)?
>> 
>> We would be using a plastic vapour barrier, perhaps with the addition of a gravel drainage layer on top of the plastic.
>> 
>> Of course we are concerned about the moisture hazard. Any advice would be more than welcome!
>> 
>> Yours in cob,
>> 
>> Robert http://abrazohouse.org
>> 
>> 
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