[GSBN] Straw Bale Insulated Ceiling

Tue Feb 4 14:29:43 UTC 2014

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<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<mailto: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<mailto:feile at mudandwood.com>
To: Global Straw Building Network<mailto: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<tel:%2B353%20%280%29%2071%20930%200488>
M: +353 (0) 86 806 8382
E : feile at mudandwood.com
W:www.mudandwood.com
_______________________________________________
GSBN mailing list
GSBN at sustainablesources.com<mailto:GSBN at sustainablesources.com>
http://sustainablesources.com/mailman/listinfo.cgi/GSBN

_______________________________________________
GSBN mailing list
GSBN at sustainablesources.com<mailto:GSBN at sustainablesources.com>
http://sustainablesources.com/mailman/listinfo.cgi/GSBN

_______________________________________________
GSBN mailing list
GSBN at sustainablesources.com<mailto:GSBN at sustainablesources.com>
http://sustainablesources.com/mailman/listinfo.cgi/GSBN

Derek Roff
derek at unm.edu<mailto:derek at unm.edu>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.sustainablesources.com/pipermail/gsbn/attachments/20140204/aa53b300/attachment.htm>