[Strawbale] Convection in SB walls

Dave Howorth dave at howorth....uk
Thu Dec 9 23:58:15 CET 2010

Thanks everybody for your very useful replies to my crazy suggestion.
I'll try to read all the references tomorrow. Let's hope google's Czech
to English translation works well - there are no pictures to help me :(

To save having to look them up I think Jenik was referring to

I absolutely agree that the first step is to plaster both sides of the
bales so there is no gap and so the wall is airtight. Well, that's the
second step; the first step is to make sure the bales are dense and the
wall is compressed and gaps are stuffed!

But I think I've read things that suggest convection within bale walls
does take place, though it's less significant. I'll try to dig them out.
It must certainly be less significant than transport through the straws.

I think dipping all sides of the bale in clay slip (rather than just the
two sides) will produce a thermal bridge that I suspect would be worse
than any elimination of convection. Certainly worse than paper. I can
see some point in dipping the two sides, though perhaps not enough to
overcome the difficulties generated by the mess.

> My guess is it makes it worse, in the wall. It is not a pleasant
> thought for me, as I have insulated this way (using cardboard or
> paper) one wall already. It is better to let the air flow along the
> wall, as it makes no harm. The more convection cells spanning the
> full width of the insulation layer, the worse.

To make sure I understand: you mean that paper/cardboard will leave a
gap at the edges that forces a large convection cell?

> The airflow/convection barrier should be perpendicular to the heat flux, 
> like panes in the window. This is easy in roofspace insulation (two 
> layers of small bales or many sheets of straw from jumbo bales). I'm doing 
> it on walls as well, using one decimetre thick sheets from large bales put 
> into a light wooden framework.

I see the rationale but I believe there's also some value in limiting
the height of windows for example, to reduce the intensity of convection.

Thanks again for all the valuable replies.

Cheers, Dave

PS  The phenomenon of convection within walls presently rejoices under
the name "thermal bypass". Mark Siddall published a review article in
Green Building magazine last year.

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