[GSBN] using loose straw insulation in roof

Bohdan Dorniak bohdan at bdcoarchitects.com.au
Wed Oct 25 16:56:46 CDT 2017

Hi All

Graeme – I’d be worried about your statement about a “coating of clay” – in our fire test of rendered strawbales by CSIRO - done in 2002 – the earth rendered bale (render about 50mm thick – Frank Thomas did the rendering) started to show cracking in the render.

Wondering whether eventually they would burn? 

Other considerations is the weight factor and stronger ceiling structures (if using thicker coats of render).

That’s my 2 bob’s worth.



From: Gsbn [mailto:gsbn-bounces at sustainablesources.com] On Behalf Of Misha Rauchwerger
Sent: Thursday, 26 October 2017 8:04 AM
To: Global Straw Building Network
Subject: Re: [GSBN] using loose straw insulation in roof


I believe another issue is whether the attic space is vented or not, as well as the kind of roofing.  After the Oakland fires, I remember this being debated furiously, because the codes require attic venting, and going with something like polyisocyanurate rigid insulation without vents was controversial.  Obviously the fire danger will increase if air can mix with flammable insulation fuel (frieze block vents and ridge vents create a perfect means for fully combusting the attic materials) . If it is encased and covered with earth on the outside and inside (plaster on the ceiling), as in a living roof, we can decrease the flammability factor.


The Lobo fire that swept through our neighborhood recently came right to our friend's straw bale house with only superficial plaster damage on one corner.  Of course there are far too many factors to say it was saved because of being straw bale or not.


Then again, if you get a fire like just swept through Napa and Sonoma Counties, all bets are off...




On Wed, Oct 25, 2017 at 1:38 PM, Graeme North <graeme at ecodesign.co.nz> wrote:

This list is a fantastic resource for learned info and is greatly appreciated I can tell you.


The fire thing is scary - there is no requirement in NZ for single dwellings to have fire resistant ceiling materials, but we do need to have smoke alarms near bedrooms and escape routes.

The really scary thing is that the fire people here advise that a dry house with an open attic with light timber rafters or trusses for roof framing will burn from end to end in 60 secs.

Goodness knows what adding an accelerant like straw (without a good clay coating) might do. I would advise against it.


I would use straw coated with clay  - pretty much a LEM mix - if my structure allowed it to happen easily and I was considering it for my own house but the work required and extra weight decided against it 

and again recently I considered it on another design but the considerably increased extra depth of the roof structure to accommodate it, along with the extra cost, work, and weight, decided against it again so I went with wool insulation in both cases.


I am also not convinced that cellulose/paper insulation with borax retains it fire rating over time - I have seen examples of old (only a few years) cellulose insulation that would not ignite when new,  ignite readily with a match and it seems to keep burning quite happily.













On 26/10/2017, at 5:36 AM, Bruce EBNet <bruce at ecobuildnetwork.org> wrote:



Don’t you love it when someone else chimes into these discussions ahead of you, and says everything you wanted to say?  


I sure do.  Thank you Derek and John;  what they said.


Bruce King

(415) 987-7271 <tel:(415)%20987-7271> 

bruce at ecobuildnetwork.org





On Oct 25, 2017, at 7:56 AM, John Straube <jfstraube at uwaterloo.ca> wrote:


I would echo Derek’s concerns exactly. Loose fluffy straw burns very quickly and you may as well say you lost the house.
Adding clay slips will increase the fire resistance to the point is acceptable, as will dense bales with some sort of clay slip top, but once you do that, you have a heavy and labor intensive roof insulation.
Cellulose with 20%+ borate treatment is inexpensive, gives good R-value, is widely available, and is very good at fire resistance.

On Oct 25, 2017, at 10:00 AM, Derek Roff <derek at unm.edu> wrote:

I have a different concern about using straw packed at low-densities in the roof.  I think that the fire risk increase is much more of a problem than the decrease in insulation value.  If you have seen flakes of straw or loose straw burn, you will be aware that they are much more flammable than standard bales.  Straw flakes are probably about half the density of a building bale, and loose straw might cut the density in half again.  

Losing the roof in a fire usually means losing the whole house’s integrity and value.  While enclosing the flakes or loose straw for the roof in plywood, for example, would help somewhat with fire resistance, I recommend against design choices that depend on a few things going right to avoid a catastrophic failure in a fire.  

An additional consideration that has been mentioned here before, is that while bales may be relatively inexpensive, placing them in the roof requires increasing the size and number of the roof’s structural elements, which will likely raise costs more than the amount saved by using bales instead of other insulation materials in the roof.  Using lower density straw diminishes that problem to some extent, but straw is still likely to be significantly heavier than other kinds of roof insulation, for an equivalent insulation value.  


Derek Roff
derek at unm.edu

On Oct 25, 2017, at 4:34 AM, Rikki Nitzkin <rikkinitzkin at gmail.com> wrote:


I don’t know if you all remember, but not long ago I asked about the MAXIMUM density of a SB before it begins loosing thermal properties… most people agreed that we should not worry about a bale being too dense.

Now I ask about the opposite question: is there a minimum density?

The other day we were talking about using flakes of straw to insulate a roof. One of the builders insisted that is was important that the insulation cavity was filled with highly compressed straw, and another said that as long as the cavity was properly filled (leaving no big holes for air to circulate) that the density of the infill is not important, as the straw (loose or dense, but enclosed in the insulation cavity) impedes the circulation of air, and that is what insulates.

Can any of you technicians refer me to studies about insulation properties and how they relate to density? or at least clarify my doubt: Is it important to highly compact the insulation in the roof? and why… so I can explain it better.


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Dr John F Straube, P.Eng.
jfstraube at uwaterloo.ca

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