Straw as in wall insulation ? disscuss please

Hello All
I would like to get folks thoughts on this idea

I was trying to figure out how to insulate a relatively thick timber frame wall about 7to9" and was wondering about using straw by cutting the bale and using 7to9" bale sections packed in rows into the wall to fill the entire wall, I worried about bugs, but i think that can be resolved by dusting with borax, there is the Obvious fire danger, but its a wood structure anyway so fires a bad thing either way

I would like folks input is there something im over looking that would let this not work.? no building codes to worry about

7to9" would provide roughly R12to15 if i have my calculations right, i live in Norther Arkansas so im factoring that in as well winders are not very brutal, but summers can be

Thank you all for your time and help

Cyric

A little surprised nobody has responded so will give my opinion, fwiw.

What are you going to seal the walls with? If straw is relatively loosely packed I'm not at all sure you will get much insulation value, especially if it isn't sealed as the air flow will simply pass between/around the straw stems.  Also it absolutely has to be kept dry.

If you look at the straw bale houses, they all have all the various chinks and hollows stuffed with cob or some such before they are plastered and that isn't just to make the plaster go on smoother. To stuff flakes of straw into a wall is likely going to leave a lot of gaps and spaces that the air will simply funnel through.

The other thing is that loose straw will burn like crazy. Though, come to think of it, the borax should help with that too, but still, wouldn't want to rely on it.

  People have had good success using straw mixed with clay tamped into the wall. It's called light straw clay, although some people suggest using wood chips/clay instead as that's easier and faster, depends on what you have access to I guess:))) In any case the clay holds the straw together and helps make a very tight barrier against air flow. It also helps a lot to fireproof it.

For that, you don't want to seal the wall until after you are very very sure the wall has dried so you don't get into mold issues, but after that..there are houses in Europe built like this that have been around for hundreds of years.

Thanks for the reply Pam
I kind of wondered about getting no response as well figured someone would weigh in for me

While i know its not to green of a building material, i already have quite of bit of OSB form a construction site that was slightly damaged and couldn't be used on the project for the outside wall sheathing that should seal things up tight on the outside then cover that with ceder slats that is milled locally. the inner wall is going to be a mix of various locally harvested timber as well.
what i had planned is to keep the bail sections tight and put it in the wall in rows and tamp it both vertically and horizontally to make it denser in the wall,  i have even thought about cutting the bails in half with a chainsaw to keep them compressed but im not so sue that would work..LOL

well the idea is to let the condensation find it's way out so..osb board will absorb moisture  but in the way you are planning to use it it sounds as though it would work. Certainly it would help the straw do its job. Main thing as far as I know is to avoid using concrete or plastic or other such impervious materials.

p.s.  I don't think I would use a chainsaw lol  I would guess the main result would be a very very dull chainsaw chain in very short order, dust and bits of chewed straw everywhere it would all fly apart as soon as you tried to move it anyway.

The flakes are likely as good as it will get short of coating them with clay slip first.
In any case, has to be a lot better than nothing. Best of luck!

Thanks for the Input Pam, i agree the chainsaw method would most likely end very badly, but it was a thought, i wonder if there would be a way to build the wall to allow some air flow without causing moisture problems

I don't know if your site gets driving rain at all, if it does it might be worth putting a breather membrane on the outside face of your OSB (keeping things drier until you put your treated battens and cedar on). It should stop rain getting into your straw through the osb but should let it breathe.

I have to admit 7 to 9" of straw seems quite thin to split a bale into, good luck with cutting them up, I had to split bales with a bale needle for a studio I built a could of years ago. The straw seemed to get everywhere, lots of sweeping up to do!

SherwoodOwl

thank you for the reply, We do occasionally get driving rain here (along with everything else imaginable, except lava no lava...Yet..)
Could you please give me some examples of this Breather membrane you are talking about so im sure we are on the same page.?

im shooting for the 9" thick, from my dealings with square bales and straw thats half the thickness of a bale, horizontally, and if i have my calculations right and i can keep things packed tight, it should provide better R value than is recommended for this area , But i guess i'll just have to see how things go

SherwoodOwl wrote:
I don't know if your site gets driving rain at all, if it does it might be worth putting a breather membrane on the outside face of your OSB (keeping things drier until you put your treated battens and cedar on). It should stop rain getting into your straw through the osb but should let it breathe.

I have to admit 7 to 9" of straw seems quite thin to split a bale into, good luck with cutting them up, I had to split bales with a bale needle for a studio I built a could of years ago. The straw seemed to get everywhere, lots of sweeping up to do!

Wouldn't the OSB just dry out as long as it wasn't covered? I have had some small bits sitting outside (they came in a load of scrap wood). They had snow sitting on them for several months, then got rained on for most of the summer and they're still fine; just used one of them as a base for a no hunting sign.

If they got entirely soaked from sitting in water then a different story but  if it's vertical most of the water should just run off and what does soak in should evaporate afterwards? I suspect the glues used now are somewhat more weather resistant than used to be the case.

OH!!  It occurs to me that there is likely  quite a difference what the OSB is rated for..I didn't buy this stuff but it's likely rated for outside use; if the OSB you have is rated for interior use only then it might be a very different story. In that case...dunno if it would be needed.

I did hear years ago that Canada sent a bunch of OSB to Guatamala as disaster relief after an earthquake and many of the people there now tend to think of us as well meaning and generous idiots. Apparently much of it was stuff meant for interior use and it quietly expanded then dissolved into its components in the Guatamalan climate. It wouldn't have been covered at all though.

Hi Cyric30, the sort of materials we use over here is Tyvek Breather Membrane or Kingspan Nilvent 17, they should let out moisture vapour and keep the rain from getting in. A reasonable company should have a good technical department you can send some details though to (I don't bother with the sales guys!). They should tell you if the membrane will do the job you want.

Pam, as I understand it you are spot on. Some OSB is only suitable for internal use and yes the external grade is usually fairly good at throwing off the water. Over here in the UK we have a damp & cool climate so it has its limits and I wouldn't want a timber structure to get too much of a soaking or it may have problems drying out. If the OSB is recycled you may not know what grade it is.

As I don't work in your part of the world it may be worth seeing if someone local will give some advice 

A good way to compress loose straw in a framed wall is to slip coat the straw first with earthen plaster. The slip will help the straw remain compressed as well as help wick away moisture once the wall is closed up. When using this method it is important to let the slip coated straw dry thoroughly before enclosing the wall (a few days at least-the longer the better). I like to finish the interior side of the wall 2" pine lathing strips - leaving 2" gaps between strips. Once the wall dries and firms up - remove the long straw sticking through the lathe by pulling and/or cutting them out - the wall is now ready for earthen plasters.

I'm glad someone started this topic. I have been thinking about this the last couple of weeks. I did NOT
want to use fiberglass insulation for my cottage under construction and was thinking of alternative insulation.
I have heard of "light straw-clay" before and seen it built in a small frame at a cob building course I took for 7 days
a couple of years ago.

My question is to Jeff, I have a 2X6 framed in cottage with OSB for sheathing as I am in the middle of construction.
From your post and I hope I'm not misunderstanding... is that you CAN leave the OSB sheathing on the outside and
still use the light straw clay in fill and earthen plaster on the inside without worrying about it staying too damp and being
able to breathe. Also, do I have to then place that breathable tyvek house wrap or would the black tar paper work?

I was thinking that I had to brace that the whole perimeter wall diagonally, then take the exterior OSB sheathing off
and hope nothing collapses. It seems that what you are recommending is safer and easier.

I just wanted to confirm. Thanks!

I've actually used straw to insulate a wall. I put 1 inch mesh chicken wire up on one side of the wall then put a 2 ft tall strip on the other side, stuffed it tightly, using poly bale twine to hold the two sides together at several points as I was stuffing. As each section was finished I added 2 more feet, lashing the two sections together. (Kind of like sewing them with bale twine.) Once the entire wall was done, I plastered two+ inches of adobe/straw plaster on each side. If you are using some sort of sheathing such as OSB on one side, use the chicken wire on the other and put nails in at intervals on your sheathing to tie the bale twine to. You will get a gently curving wall that is quite nice to look at. The tighter you get the bale twine and the more points you tie, the less it will curve.


Sorry no pix, this was in the pre-internet days and I didn't think to take photos.

Thanks Kim for your feedback and comments. Every little bit helps. Forget the pics, I can't believe you don't have any videos!

Nope no videos. Still don't have a video camera. I'm barely tech literate! Prefer the old ways.

I insulated a 20 by 20 barn framed with 2x6 and sheathed on the exterior with ply/osb and siding. Some tar paper and some tyvek type of outer moisture barrier. We made clay slip straw and tamped it down into forms. The forms were screwed to the wall in rises between 2 ft and 3 ft or so. We tamped with 2x2 scrap wood sticks. We had mixers on the ground and tampers on ladders and scaffolding. It is hard work for sure. A job for many hands. The forms can be moved up immediately after finishing a rise. It is unreal how much straw can be tamped down into a bay. The straw, after mixed, should have no shine to it. All straw should be coated and try to have very little clay clumping. The insulated walls look great and earthen plaster can be applied to the dried insulation without any other lathe or wire. Simple. Our walls took a while to dry, maybe two weeks or more. If exposed on the outside they would dry in half the time probably. Earthen plaster is very breatheable and will wick moisture from the straw easily. We watched this happen after applying a section of test plaster to a semi moist area of insulation. The plaster remained moist while wicking moisture from the insulation. It would be better to pound nails into the studs within the bay to give a hold to the insulation. Some areas that were not tamped as well shrunk and became a little loose within the bay. This problem was solved by adding more straw slip in afterwards. A little adobe mix helps a lot in tight areas and getting the straw smoothed down some for plastering.
I have no idea on the r-value but if you do a good job tamping it is probably a fine insulation for a well sealed passive solar home.
Maybe with the right design and using a rocket mass heater the r- value is not as important as once thought.
How far between your posts? That will be the main issue for you.
Best luck

Sorry to dig up an old thread.

I live in the tropics and was thinking of using rice straw for my wall insulation. My outer wall is sheet metal with a foil insulation backing and my internal wall is plasterboard with a foil backing. This is held up on a steel frame.

I have thought about packing the straw (which is a bit chopped up from the havesting machine) inside pastic sheeting to form bags about 4" thick which would then be placed inside the walls.

The only real alternative is fibreglass insulation, but I'd rather use straw as it free.

Any thoughts?

Straw (or any organic fiber) + high humidity = mold and/or fungus