[GSBN] using loose straw insulation in roof

Bruce EBNet bruce at ecobuildnetwork.org
Wed Oct 25 11:36:49 CDT 2017


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
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
>> 
>> Derek Roff
>> derek at unm.edu
>> 
>> 
>> 
>> 
>>> On Oct 25, 2017, at 4:34 AM, Rikki Nitzkin <rikkinitzkin at gmail.com> wrote:
>>> 
>>> Hi!
>>> 
>>> 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.
>>> 
>>> thanks!
>>> Rikki
>> 
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> 
> Dr John F Straube, P.Eng.
> jfstraube at uwaterloo.ca
> www.JohnStraube.com
> 
> 
> 
> 
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