[GSBN] Straw Bale Insulated Ceiling
John Swearingen
jswearingen at skillful-means.com
Tue Feb 4 16:31:00 UTC 2014
We haven't done a bale-insulated roof in quite a while, for the two major
reasons that have been brought up already--the difficulty of sealing gaps,
and the added weight of the structure--since we're in earthquake country,
we pay a premium for any weight added high in the structure.
I think it would be possible to carefully detail a truss system that could
make it easier to fit bales tightly, particularly if you were willing to
shave the bales to a standard width, and developed a technique that would
fill any gaps with straw/clay. Gaps could negate all the positive benefits
of the straw insulation, and an effective air/vapor barrier is important.
Sealing the gaps is also important for fire protection. I think Derek's
risk assessment is overstated--the increased vulnerability of straw
shouldn't be considered any greater than that of conventional materials
when turned horizontally and put at the top of the building. The enormous
heat blasted at the wall in a fire test is comparable (I imagine) to what
would be encountered at a ceiling. In Mongolia, the builders insulated the
roof of a medical clinic, then installed the chimney, and of course a fire
started rather early in the life of the building. Since it was a remote
location, fire crews took a very long time to arrive. The roof was
eventually heavily damaged, but before the fire crews arrived the staff was
able to safely remove patients, furnishings and expensive equipment while
the bales were simmering. So in this case bales in the roof gave
additional and vital protection.
If you did have a leaky roof, you might not know about it until a lot of
damage was done to the bales--in a conventional roof there would quickly be
drips on the floor to alert the occupants to trouble.
One reason to use bales in a roof hasn't been mentioned--sound insulation.
Low-frequency sound, say from trucks on a highway, can easily be
transmitted through a conventional ceiling, which can act like a drum. The
weight of the bales dampens that noise very effectively (an earthen roof
does the same).
John
.
On Tue, Feb 4, 2014 at 6:29 AM, Derek Stearns Roff <derek at unm.edu> wrote:
> Catherine Wanek and I visited several houses in Switzerland and Italy,
> which had bales in the ceiling. The houses were beautiful, and the owners
> were happy. However, I wouldn't do it. Some of the virtues of bale walls
> become liabilities when bales are put in the roof. The square, blocky form
> of the bales makes it very hard to get a great fit and fill around roof
> framing elements, even on a flat roof plan. With a normal gable or hip
> roof, the problems of fitting the bales to the roof structure increase.
> For a more complex roof, with multiple ridges and valleys, as is the
> design norm for custom houses, it's very hard to get a continuous layer of
> bale insulation, and labor costs soar.
>
> The weight of the bales means that the roof structure will be larger and
> more expensive. Building a flawless air barrier in the ceiling becomes
> even more important, because the stack effect drives moisture laden
> interior air through any discontinuities in the ceiling air barrier, and
> into the insulation. The ceiling/roof assembly is unlikely to allow for
> drying to the outside, so moist air moving into the ceiling is likely to
> accumulate in the bales to a larger extent than it would in the walls.
> Roof leaks allow gravity to bring water into the insulation from the top.
> Leaks in the roof are always a risk, but straw is more vulnerable than
> many types of roof insulation, and the moisture risks to bales are more
> problematic in the roof than in the walls.
>
> Fire risk is also greater for bales in the ceiling than it is for the
> bales in the walls. The flames and hot gasses seek the highest point,
> focusing their destructive energy on the ceiling/roof structure. I've
> never seen any figures, but I would guess that if you took a fire-tested
> vertical wall section and turned it horizontal, so that it becomes the
> ceiling/roof assembly, the fire rating would be significantly less in that
> position. In the fire tests of bale walls, we have seen that the plaster
> fails at some point, but the wall continues to resist the fire for a while,
> protected by the surface layer of charred straw, which is consumed fairly
> slowly. In a ceiling, it is likely that the charred straw would drop away
> more quickly, and add some fuel to the blaze.
>
> Given that using straw insulation in the roof is likely to cost more, be
> harder to do properly, increase the size and cost of the roof's structural
> framing, and be more vulnerable to moisture than other choices, I favor for
> the other choices. Cellulose is one of my favorites.
>
> Derek
>
>
> On Feb 3, 2014, at 3:44 PM, Van Krieken wrote:
>
> I used strawbales in the roof, with a special atention to fill the voids
> between the bales (its quite time consuming). We sealed the bales with a
> hidrated (or aerial) natural lime mixed with expanded clay aggregate.
> You may see some pictures here: http://strawhouses.carbonmade.com
>
> All the best
>
> Jorge Van Krieken
> Portugal
>
>
> On Mon, Feb 3, 2014 at 4:09 PM, Danny Buck <dannycbuck at msn.com> wrote:
>
>> Our experience here in New Mexico was that the roof structure gave us
>> a lot of voids between bales that were very challenging to pack. If it is a
>> true attic with a flat open surface to work off of, that would be different.
>>
>> The surface of the bale that is accessible to air (oxygen) is best sealed
>> with a plaster.
>>
>> Danny Buck
>> Daniel Buck Construction, Inc.
>>
>> ----- Original Message -----
>> *From:* Feile Butler <feile at mudandwood.com>
>> *To:* Global Straw Building Network <GSBN at sustainablesources.com>
>> *Sent:* Monday, February 03, 2014 2:57 AM
>> *Subject:* [GSBN] Straw Bale Insulated Ceiling
>>
>> Hi All
>>
>> I have clients who are considering insulating above the ceiling with
>> straw bales. They have a large attic which could easily accommodate the
>> bales and plenty of outbuildings, so they don't need the attic for storage.
>>
>> I have never detailed a straw bale ceiling before, but would like to
>> explore the idea.
>>
>> One of my main concerns is fire. Is it enough to encapsulate the bales
>> within plasterboard/clayboard on both sides (top and bottom)? If we want to
>> deck out the floor above the bales, would it be enough to coat them in a
>> lime or earth plaster before building up the floor above them? Is there
>> any data out there about the behaviour of straw bales in ceilings that I
>> can show the building control officer, particularly with regard to fire?
>>
>> Any advice on or experience of straw bale insulated ceilings would be
>> very welcome.
>>
>>
>> Thanks
>>
>>
>> *Féile Butler*
>> *MRIAI B.Arch Dip. Arch Conservation Grade III*
>> *Mud and Wood*
>> *Grange Beg, Skreen, **Co. Sligo, Ireland*
>>
>> *T:
>> +353 (0) 71 930 0488 <%2B353%20%280%29%2071%20930%200488>*
>> *M: +353 (0) 86 806 8382 <%2B353%20%280%29%2086%20806%208382>*
>> *E : **feile at mudandwood.com*
>> *W:www.mudandwood.com*
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>
> Derek Roff
> derek at unm.edu
>
>
>
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--
John Swearingen
Skillful Means Design & Construction
2550 9th Street Suite 209A
Berkeley, CA 94710
510.849.1800 phone
510.849.1900 fax
Web Site: http://www.skillful-means.com
Blog: https://skillfulmeansdesign.wordpress.com
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