External insulation for conventional building.

Hi, all.

I have an unfinished building built with fired  porous clay bricks (named porotherm here). I would like to add exterior insulation.
Is there any system to make it insulated with 12 -15 centimeters of straw? Something like that straw gets infilled inbetween the walls and some outside barrier, maybe just wood boards or something.
I saw similar insulation with blown cellulose or wood fibers and on the outside there are wood fiber boards..

Does anyone have a suggestion, idea or a working example of something like that?

Looking forward to answers,
Klemen

Do you want to create a frame on the outside and use that frame to hold insulation and then clad the frame?
What is the weather like in your location?

I feel that if I wanted to do something like this with straw, this is how I would do it.

1) gather lots of straw bales.

2) lay the brick down for the bales to sit on.

3) Build a frame to hold the bales

4) stack the bales against the exterior walls.

5) Install board and batten exterior walls using the frame build to hold the bales.

Cob could be used instead of the board and batten.

This thread might offer some suggestions:

https://permies.com/t/217040/sawdust-insulation-plywood-breathable

I am looking forward to what other folks suggest and seeing how this project works out for you.

First, thanks for replies.

The weather here is so that we get about 1500mm of rain per year and about 75% of rain comes in the colder third of a year. We do get snow, temperature falls to -10C. It's southern alps, but we are not under the influence of adriatic sea.

I would make some frames to the outside of existing walls so that it would hold the insulation in place. Something similar to this system:
http://www.greenmaterials.se/steico-wall.html

But i would change the infill material with something strawlike.
I imagine that using only straw it might compress in time. But if i would add some clay slip, just to make the straw dirty  i could lessen the compression problem- i wouldnt compress that lightstraw at all. Also the "dirty" straw wouldnt be interesting to insects.
On the outside i could just put some 1x4 to hold lightstraw in place or maybe i wouldn't need to if i add some "anchorss" inside strawclay.
I added a sketch of what would it look like. The strawclay would be 15-20cm thick.

Would i even need tyvek/vapour barrier?

Great sketch!
What you have there is not unlike  a Larson Truss wall.
https://www.greenbuildingadvisor.com/article/larsen-trusses

A Larson truss doesn't expand the footprint of the building,and it can be added to an existing structure, but that also means relying on on the trusses to transmit the weight of the insulaton, sheathing,  strapping, etc, to the wall that sits directly on the foundation.

As to the vapor barrier, in your sketch the tyvek is part of a rain screen, right?
Some kind of water shedding surface should be at that layer of wall, even if it's not tyvek.

Hi, William.
The tyvek functions here as vapor barrier and secondary rainscreen. Over it come wood boards to protect it from the elements - right part of sketch.

I agree with William--great sketch!

Question: What's your seismic activity like?  

If there's some risk of a significant earthquake where you're building you'll want to take that into account in your design.  

Both plastered straw bale and plastered straw clay walls are quite heavy, and that needs to be considered when you think about which wall elements are taking on the in-plane and out-of-plane forces from an earthquake.  By "quite heavy" I mean that a typical square foot (as viewed while standing in front of a wall) of plastered straw bale wall weighs between 45 lbs. and 55 lbs., and the walls are between 15" and 24" thick.  A straw-clay wall is usually about 12" thick to meet energy code requirements in the United States, and these walls are even heavier per square foot.   For comparison, a stick framed wall with 6" studs and batt insulation, exterior sheathing and siding, and interior drywall weighs about 12 lbs. per square foot.  

If you don't plan to plaster the straw bale or straw clay walls you could deduct 15 lbs per square foot per side assuming 1" thick plasters.

If the brick wall is reinforced it may lend some shear to the wrap material--whatever you decide--but you'll want to confirm this with an engineer.  Assuming the brick wall has suitable engineered sheer, attaching wood fiber panels to the exterior, along with the lightest weight siding you can find might be your best choice if you want to avoid figuring out ways to supply supplemental shear to handle the extra weight.

Another concern I have about the straw-clay is that it takes quite some time for the material to dry when it is exposed to air on both sides.  The general guide here is one week per inch of wall thickness, so your proposed 15 cm to 20 cm thickness might dry in as soon as 8 weeks if it could dry to both sides.  If the straw clay is packed against the brick wall I'd be concerned that the dry time would be much, much longer as it can dry to one side only, and possibly too long before microbes became impatient and decided to eat some of the straw.

Building with more natural materials is awesome!  But done without regard to structural issues can result in beautiful, natural, and also unsafe structures.

Jim
Many Hands Builders

Jim Reiland wrote:I agree with William--great sketch!

Question: What's your seismic activity like?  

Another concern I have about the straw-clay is that it takes quite some time for the material to dry when it is exposed to air on both sides.  The general guide here is one week per inch of wall thickness, so your proposed 15 cm to 20 cm thickness might dry in as soon as 8 weeks if it could dry to both sides.

Hi.
Seismic activity is not a problem here.
I thought the drying is meant for 1inch per week from each side... I did have a concern about this, too.

At present my wall is 30cm thick. Adding 20cm extra is quite a lot for me.  Comparing the R value of 20cm lightstraw to 12cm mineral wool is quite a difference :-/.
Also the easiness of attaching it compared to Larsen trusses or similar is quite a difference, too.

If no seismic activity, then all you need to deal with in terms of lateral forces acting on the wall assembly would be wind.  

My experience with waiting for straw-clay to dry is that the 1 week per inch of wall thickness requires optimal drying conditions.  When the material gets packed into the wall it's quite wet, and that moisture comes out more quickly if external conditions are warm, dry, and windy.   For a 12" wall, sometimes three months isn't enough, and we have had to wait an additional couple of months, or even into the following year before applying plasters.

One of the advantages of modern insulations like rock wool is that they insulate better per given thickness.  There are plenty of trade-offs (cost, carbon footprint, recyclability, toxicity, etc.), but it's hard to beat rock wool for giving you a maximum R-value to pack into a relatively narrow cavity created by cantilevering Larsen trusses from your existing brick walls.

Blown-in-cellulose would be right up there, too, as would blown-in-straw.  Check out Iso-Stroh, an Austrian company, at https://www.iso-stroh.ch

Good luck!

Jim
Many Hands Builders

Are there lots of sheep in your region? If wool is plentiful, it can be treated with relatively nontoxic additives like boric acid to make it unappealing to insects, and it's great, lightweight insulation. In lots of places (including here) it costs farmers more to shear their sheep than the wool is worth, which is a real shame given what a resource it is. We use the wool from our tiny flock for mulch, mostly, but I would like to start insulating the roof of the garage with it.

I agree with Phil about wool (and it really is a shame that it costs more to shear sheep than the wool is worth!).  I have used it for ceiling insulation and it's comparable to both blown-in cellulose and rockwool in terms of insulation value per inch.  We used wool from Havelock, a company in the U.S. because it was easy--already treated, ready-to-install.

Jim
Many Hands Buildders