[Cob] straw clay in floors

[Feile Butler]

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


































T:    +353 (0) 71 930 0488 

M:   +353 (0) 86 806 8382

E:   feile at mudandwood.com 

W: www.mudandwood.com





----- 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
> 
> 
> _______________________________________________
> Coblist mailing list
> Coblist at deatech.com
> http://www.deatech.com/mailman/listinfo/coblist
>