I’m not a big fan of bales in the roof as you likely already know. They are so heavy and there are several areas of concern in regards to using them above head.
1. The frame needs to be drastically increased in size and/or spacing to support the extra load.
2. Plastering over head is VERY difficult and tiring.This can be alleviated by using planking in place of plaster for the finish.
3. The R-Value gained is not anything better than what you can get with regular insulation materials (either blown in or batts and either natural materials like cotton or wool or conventional ones like fiberglass).
Personally, I stick with light weight insulation materials in the roof and leave the bales for the walls; however, if you want to use bales, you can use an assembly that looks something like this:
– 2×6 tongue and groove planking over beams/girders (to be engineered per the loads and spans)
– Double layer of drywall as described for boiler rooms, etc.
– Vapor barrier as required. I’m not sure what options you actually have for this and may end up with plastic sheeting as the only option.
– Bales dipped in lime plaster to provide fire protection for the bales themselves.
– 1″ Plywood
– 1.5″ air space
– 2×2 wooden nailers secured with 18″ (min) panel screws to the 2×6 wood planks and the beams below the bales.
– 1″ Plywood roof decking material.
– Living Roof Assembly with 2×6 Rim
– Underlayment and pond liner wrapping over the edge of the rim boards.
– Chicken wire over rim board edges to secure the soil and living roof in place. Be careful not to puncture the pond liner inside the rim boards. Nail the wire to the outside of the boards only and fold all sharp edges in before installing.
This system can be done, but it will be very expensive in the cost of wood or steel framing required to support the extra loads of the bales and the living roof. You will definitely need to have an engineer design this frame. Once again, I think you’re better off sticking with conventional insulation materials in the roof, especially if you plan a living roof as that is heavy enough on its own.
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Interesting line, I enjoyed it very much. Keep up the good work, Clayton Fresta
As I look at this, I think a roof would almost be better with loose fill chopped straw inside of a 2×6 roof framework. It just has to be kept dry. The weight of bales within a roof has to be a very serious consideration. Loose fill should insulate just as well. I also like the agriboard panels which have been advertised to be able to span 24 feet which have OSB on the outside of the compressed straw.
The issue is that loose straw does not offer the same fire resistance or Rvalue as bales do. I agree with your thinking though and prefer to keep bales out of the roof entirely at this point. Straw SIPs are a better choice as you note.
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thanks Andrew Morrison for this good work!
I’m doing a research on using straw bale on the existing steel sheets to provide Thermal and Acoustic insulation for my bachelor degree but it seems there is no publication about using straw bale in roofing.. But as I found your website I hope I will learn more. If there are any tips, journals or articles which can help, you can please share with me.
Thanks
Thanks for writing. Most people don’t use bales in the ceiling/roof because they are too heavy. You need the thickness to get the Rvalue and that thickness is too heavy for roof structures. Good luck and let us know what you discover!
What do you think about using straw bales on the outside of a metal domed building (like a quonset) for insulation? If you were to “butter” the building side of the bales before stacking them and then covering the exterior entire surface with plaster and then a roofing rubber membrane to seal out water. Or something along these lines.
I see the idea, but am worried about the moisture issues. The exposure of the bales is 100% in a scenario like this so any failure in the membrane could/would cause major damage to the bales. On the other hand, if the membrane remained 100% in tact, the risk of moisture build up in the wall is still high because the bales would be sealed from the outside. To me, it is too risky.
I personally think common sense should prevail here. Firstly you would not use a full bale in the roof and secondly I have previously seen a completed straw bale roof insulation. Granted it was smaller bale sections and they had rough rendering covering them. It would provide a better form of insulation than bats as they are completely sealed and you only have to look at the results of bush fires in this country to be aware that this is a fantastic idea. If the rendering is done properly it should retain the same fire rating as a strawbale wall.
The big issue is the weight. You need a specific depth of straw to provide the insulation value as straw, per inch, is not that dramatic as insulation. As you get thicker in straw, you get heavier too. Further, the plaster/render adds a LOT of weight. This all requires more framing to accommodate. As with everything in building (and in life), there is a balance that can be struck, but you need to decide where you want to add and remove detailing.