Straw-clay attic insulation -- is it possible?

For insulation that would sit on the floor of an unheated attic, I would like roxul, but it looks unaffordable, and I'm very much not interested in fiberglass or foam.
I know the usual recommendation is cellulose, but I'm wondering if it would be possible to use straw, ie slip straw, light clay straw.

I understand that straw bales would be too heavy, and loose straw would be a fire hazard. Wouldn't clay straw be light enough and fire-resistant enough?

Couldn't I make blocks of it, say 2'x3'x12", dry them thoroughly, then install them in the attic?

If I should give up this idea, thanks for telling me why.

I haven't done this, or seen it done, but did discuss it at a workshop. I don't recall that anyone saw a problem with it, given that you make sure whatever thickness you use is going to be an appropriate weight.

Perhaps a bit of earthen plaster or extra clay at the end to tighten up the seams between blocks...

Here in France I have seen lots of older buildings with terra-cotta tiles in the Attic . I assume they have some insulating value

David

Hi Susana.

Strawclay has no issues per se but you have to check the weight.
I don't think it will be lighter than strawbale (if at all) but you could make it thinner maybe.
Take a look at this site (german) as they do roof insulation with strawbales (heavy).

So strawclay is ok since it's fireproofed and waterproofed by the clay.
You just have to pay attention to weight.
You can make a block as you suggested and weight it.
That should not be complicated and time consuming (except the drying).

it will be possible. but you would need to check how much weight your ceiling-structure can hold.

what s the density of straw light clay (SLC)? i read 400 -1200 kg/m³ for various light straw mixtures, but did not find the density for SLC.
compressed straw building bales will have a density of 80-115kg/m³.

clay has a density of 2000 - 2200 kg/m³, but you use just a small percentage of clay here.

so density could be less than 100kg/m³ if you pack it lightly. i think for ceiling insulation you can pack it very lightly and maybe add a thin layer of plaster or so to trap the air better, to reduce airflow through the blocks

you could always add another layer on top of your blocks. i like the idea of pre-made and already dry blocks.

weighing test blocks is a very good idea....


good luck, keep us updated and have nice and blessed easter holidays!

For data about straw clay mixtures (weight, density, proportions, etc) see this excellent resource by Doug Piltingsrud:

Thanks all for the replies.

@Dillon, that bit of encouragement was very much appreciated!
This is the first good news I've had in my building plan in a while.

@Ionel, the links are so useful, thank you for sharing them, just what I needed.

@Tobias, I will keep you updated, if I follow through on this.
If I can through reporting problems I encounter help other people avoid those same problems, great.
If I can report happy successes, even better!

Now I just have to find out if an extra ~6000 lbs in the attic is reasonable.

Happy easter / equinox everyone!

Thanks Ionescu for correcting my assumption that light clay straw would weigh less than straw bales.

The numbers I found, in case anyone else needs them:
Dry straw bales (moisture content below 20%) weigh 7-9 pounds per cubic foot (pcf)
Slip straw weighs ~13 lbs / cu ft
If I remembered where exactly I found those figures I would include the references.

thank you ... that means that the density of slip straw/ straw light clay is around 208 kg/m³.

i was thinking about what to use as a lightweight-insulating-mortar to glue blocks together and as infill for small spaces. kinda very light clay mixture but with small pieces in it. maybe hemp, fine straw, coarse saw-dust, wood-shavings... these mixed with a clay-slip. just enough clay that it will set.

Thanks Tobias for the suggestion of adding plaster, I'll think about that.
Plaster could probably be added later.
I'll be lucky to get the house enclosed before winter, so I appreciate having a plan that includes steps that can be put off till after the outer walls are finished.

I don't know how reliable the weight figures I found are, and of course it would depend on the exact proportions of straw and clay. I would be aiming for the lightest possible for this purpose.
Perhaps more research can make the numbers more definite. I've only just started to read up on clay-straw.

I like the idea of blocks being movable / removable, and thus would prefer not to cement them together.
But that's just general principle, don't make something immovable unless it needs to be, I don't have any particular reason for thinking they'd need to be moved.
Maybe 2 or 3 layers of 6" thick blocks with staggered seams? Would the lack of mortaring make that much difference if they were tightly packed?
And thinner blocks would dry faster too, a big plus. Maybe even thinner than 6", but then they'd need to be smaller so as to not fall apart when moved from drying place to final installation. And smaller blocks means more seams.

I'm a little slow with metric measures, I'm glad you're doing the conversions.

I don't think the issue is density as such as you obviously won't be using the same thickness.
How thick a tile to give the same heat retention as the thickness of straw bale would be the question to consider .

David

Susana,

The straw clay material is a sort of hybrid between pure insulation (straw - baled or not) and pure mass (cob for ex.).
It can be made into different grades closer to one extreme or the other.

What you want is closer to the insulative side.
That, you get by creating a yogurt (or thinner) like clay-water mix and dipping your loose straw in that then stuffing it into your wall or "block" forms.
After it's dry, there is not supposed to be much color change in the straw (because the clay is just a very thin film on top).
Basically, clay acts as binder (when it's wet) whyle building and as water/decomposing bacteria (and vermin) deterrent when it's dry.

You can get all scientific about it like Doug in the first video did or just hands on and dirty at it.
Your pick.

Here's another good video showing how it can be done, how it looks, how it behaves, etc. from the actual building site.

His mix mas a little more clay since it's for the wall, and a wall needs to withstand the FOOT test (see at the beginning of the video).

Is this attic vented and properly balanced or sealed? Was the attic originally designed for storage live loads? What type of floor and thickness on what ceiling joist spans & depths? Where is the location? Is this new or existing construction? If existing what insulation and r-value is in the ceiling and/or roof now? Some pics and a drawing would help.

Just running some quick numbers using 13 –lbs/ft3 and a 6”x23”x24” block does not add up. In Imperial units.

Volume = 2.0 FT3

Mass= D(density) x V = 13 lbs/ft3 x 2.0 ft3 x 32 ft/sec2 = 832 ft-lbs force @ CG

P (pressure) = F (force/area) = Distributed= 832/4 = 208 PSF. Best to apply a safety factor of 1.5 min= 312 PSF

At best the entire floor was designed for storage @ 20 PSF (4 SF x 20 LB/SF2 = 80 LBS(including people) not to be confused with joist span capacities in deflection, bending, shear at the rim joist/hangers.

There are ways to distribute the loads to bearing walls and joist blocking at a cost.

If the attic is not vented properly and sees high humidity it can rot the wood and cause failure and a safety hazard.

IIRC - The doc gets R= 1.7/inch optimized, most more like 1 @ 6” R-6 very low not worth the risk.

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_3_par037.htm

Clay slip best application is in a wall that is in direct contact with solid ground/interior space, and/or a ceiling/roof that has been designed to take the loads.

We did this (straw clay in the ceiling as insulation), but only last year and the year before, so I have no long term results to report back to you. And I wasn't personally involved, it was done by my close friends and former partners. Our climate is very dry, if that makes a difference. I think they used it in three houses, and in some made blocks to put between the joists, and in at least one of the places, used a thick layer of straw clay on the flat roof, below more earth and a water-resistant layer.

Thanks Ionel for another informative video.

@David, yes, required thickness is the question.
If the optimum insulative value of 1.6 or so per inch could be achieved, or more accurately if *I* could achieve it, then it seems it might be feasible.

@Rebecca, thanks for the personal experience. It's one thing to hear that it might be possible, very different to know that it has been done.
As a novice builder, not expert at anything and also underfunded, I pay close attention to the experiments, successes and failures of others, in addition to the the advice of conventional experts.

@Terry, thanks for chiming in, I greatly appreciate your casting an expert eye over my amateur speculations.
I didn't understand " 13 –lbs/ft3 and a 6”x23”x24” block does not add up." Do you mean my math is wrong? That wouldn't surprise me.
I had been imagining a block 6 inches thick and approximately 2 or 3 feet square.
If 13 lbs per cubic foot is incorrect, what is the correct number for weight? I didn't completely follow your calculations, can you dumb it down a little?

This is an as-yet-unbuilt timber frame, with a "cold" roof, which I understand to mean vented roof with insulation on the attic floor. I understand that the attic would need to be ventilated to prevent humidity build up. No plans or pictures yet. I'm at the stage where I'm trying to get a rough idea of the price of each element, before I can even settle on a size, 24x32 would be ideal but it might have to be much smaller. It will definitely be a simple square or rectangle, probably a 12/12 metal roof. Location northern VT.

I think you are saying that 20 lbs per sq foot is not unreasonable for some buildings. I'm just trying to get a "ballpark" idea whether an ordinary timber frame house could hold the amount of weight it would involve.

I don't have time tonight to read all the posts in thread. Strawbales can be used as attic insulation. Yes they are heavy, but that just means that your ceiling supporting members must be build strong enough to support them. Bales are pretty fire resistant, and a slip of clay on them would help prevent fire damage, insect problems, and if one ensures proper screening (with metal lath, or hamster wire) the area can be made rodent proof. I've seen both straw bale, and loose straw attic systems. Loose would be much more prone to issues, but could be lighter per insulation value. I have no experience with straw clay. I like the idea of straw clay pre-dried 'bricks', but can't say much about it.

If the optimum insulation value of 1.6 or so per inch could be achieved, or more accurately if *I* could achieve it, then it seems it might be feasible.

Not likely unless you have the processing equipment the doc has. Knock down his 1.6 by 30% = R1/inch.

As a novice builder, not expert at anything and also underfunded, I pay close attention to the experiments, successes and failures of others, in addition to the advice of conventional experts.

There should be no distinction here. Your best source of expert advice is more than likely local engineers, builders, universities, in your climate zone using the same building techniques and materials (slip, straw, etc). Local Architects and Production Managers should follow their builds and will be the first to get a call when things go wrong. Check around their reputation should precede them. A "design build" company that has in house engineering and trades would be a good source.

Structurally, it can be a cats and dogs comparison if the structure is not the same. In other words, advice from different climate zones using different allowed wood spans or species for example can be completely misleading. Don’t ask me why but Vermont has some of the most natural building talent in the country, plenty of good resources around you.

@Terry, thanks for chiming in, I greatly appreciate your casting an expert eye over my amateur speculations. I didn't understand " 13 –lbs/ft3 and a 6”x23”x24” block does not add up." Do you mean my math is wrong? That wouldn't surprise me. I had been imagining a block 6 inches thick and approximately 2 or 3 feet square

You welcome. Up to the point I chimed the discussion was on weight not what to do with it. My calculations took the weight using 13 –lbs/ft3 and a 6”x23”x24” block and shows that the weight is far too high for an attic that is designed for 10-20 LBS/FT3 or ~ 80 lbs of (local) weight which is code minimum. The same will result no matter what size of block. What I am referring to is putting a 4 ft2 box of household goodies in storage in your attic that is all it is really designed for of an existing home, or just for people to temporarily walk up there to do repair work. You can push that weight to several 80 –lbs boxes for example, to 250 lbs now you have used up all the “safety factor” allowed. What can happen is if the joist cannot take the weight in deflection over time they will permanently deflect or what is called “creep” or permanent deformation (no bouncing back to original shape) that can take years to occur and if you got lucky it may never. The risk is severe, once that occurs all the clay slip has to be removed, the joist replaced, reinforced properly, that is IF the ceiling did not collapse causing severe injury or death. If one notices too much deflection from cracked plaster, drywall joints, etc, it is best to dig out all the clay slip let the joist return to no deflection, add blocking. Think of taking a pencil at both ends and snapping it in half, the further you go out the easier it is. This is called deflection to a point of failure in bending from too much force or weight at center. At the ends where your fingers are joist hangers or nails for example in shear from vertical loads(if the weight is there) that can fail causing walls to collapse worse case. So the span tables in code are set up for 10 lbs/ft2 no storage, or 20 storage for the entire floor, not for putting large amounts of 13-lbs/ft3 weight all over the floor.

I'm just trying to get a "ballpark" idea whether an ordinary timber frame house could hold the amount of weight it would involve.

Let’s compare to your other choice mineral wool batts @ 2 lbs/ft3 @ R23/ 5.5 “, or cellulose @ 3-5 lbs/FT3 code "normal" design loads on interior cladding. Slip weighs over 6Xs @ 1/3 R. Due to the weight to design this correctly is going to cost more in structures. Make sense? You won't be following code, you'll need a structures engineer or someone knowledgeable to do all the calculations for your own safety.

Your min r-value in Vermont: R-49 : https://energycode.pnl.gov/EnergyCodeReqs/?state=Vermont You are in a cold wet building climate zone 6. 90 mph wind gust, 40 lb/Sf ground snow loads, not sure about seismic (SDC) since I don't know your exact location.

I had been imagining a block 6 inches thick and approximately 2 or 3 feet square.
If 13 lbs per cubic foot is incorrect, what is the correct number for weight? I didn't completely follow your calculations, can you dumb it down a little? If 13 lbs per cubic foot is incorrect, what is the correct number for weight? I didn't completely follow your calculations, can you dumb it down a little?

To put it simply it is not just the weight that matters which is way too high for a code ceiling, it is the distribution of weight or where it is located and how effectively it is transferred to load bearing walls/foundations.

This is an as-yet-unbuilt timber frame, with a "cold" roof, which I understand to mean vented roof with insulation on the attic floor. I understand that the attic would need to be ventilated to prevent humidity build up. No plans or pictures yet. I'm at the stage where I'm trying to get a rough idea of the price of each element, before I can even settle on a size, 24x32 would be ideal but it might have to be much smaller. It will definitely be a simple square or rectangle, probably a 12/12 metal roof. Location northern VT.

Ok, now we are getting somewhere. All the above does not apply to you, this was needed in the OP. If you had put this in an existing house it would have probably collapsed. Yes, I’ve done cost per square foot checks many times it is a challenge. Here I build @ $125/SF, timber framed $135 highly energy efficient net zero mid level finish per code. You might check what some in the area have built at. It will be a real challenge for a DIY novice to be build for far less since you will not get the discounts builders do, or for hired trades. Your ceiling/roof is where you’ll want to put a lot of $ as well as walls to take your HVAC and electrical installation and monthly cost down. Its pay it now or later.

In your climate zone if I were to do clay slip install I do a wet install between ceiling joist/screwed t&g or solid 1x boards before my roof cladding went on, and to get a good air seal. This may require tenting or tarping depending on time of year. The inner rooms would have access to the slip hygrothermal mass through the T&G or plaster. If is not all about high r-value in your climate. Additional form work and shoring may be required until dry. I’d dry it in @ 20% center moisture content. I’d use a 12”+ energy heel pack it tight, and monolithically tie it into my walls and foundations as continuous hygrothermal mass planes. I’d add 3/4" furring strips to mineral wool boards on rafters for and outer ventilation/ rain plain and outsulation board like mineral wool IS over them to stop thermal bridging. If I could not afford that I'd do another cast-in-place slip (clay or better lime is lighter) wet install layer at the rafters let it dry in. In the attic a 40/60 (lower to upper) cross sectional area split is a good rule of the thumb for proper ventilation flow….research this more. I’d use water and ice shield on my standing seam roof outside a ventilation gap. 12/12 is hard to clad and expensive in wood/clad and to span, 8/12 in plenty good. Put the saving in wood/clad into insulations. Plus high pitch cold attics have buoyancy issues, condensation at the ridge vent. Other options are a sealed attic with limecrete or hempcrete, or vaulted. Tray ceiling are also nice.

BTW: It is just not fire that is of concern it is also smoke spread. Mineral wool is zero for both. In other words, you make get out of the fire with a damaged respiratory track. Some not being code enforced don’t consider that until it is too late. Borax if applied correctly can help. You might search the net for more info on other materials.

I added some after thoughts to my last post: Here is a great source of info in Vermont. I've had some excellent conversations with some of the professors there: https://yestermorrow.org/

They have a permaculture, green roof, natural build, timber framing, net zero, design-build, etc, cert programs: https://yestermorrow.org/learn/courses Adds up to one of best states for natural building in the country proven, IMO. If you read some of the professors books or follow their builds which I have you will see the skill set is very high! No need to look elsewhere

Last I heard about a year ago they were starting to put conventional wet install @ 5 lbs/ft3 cellulose outsulation double wall stick framing(kinda like I described above but better and more costly) around strawbale/timbers to get their insulation values past IECC code min. Walls @ R-50, roofs at R60-70 for net zero builds. If I were you I'd seek their pro advise or take some of their courses before I attempted to design_build.

Maybe I'll shoot them a resume for a couple hundred grand a year No more Permies.com, nor apples or pies OMG now what?

@Roberto, thanks for more anecdotal evidence, much appreciated!

@Terry, thanks for practical suggestions. There may be readers of this thread who could benefit from them.
I'm sure the local well-known experts would give great advice and develop excellent designs.

Yes, I am familiar with yestermorrow's catalog, and I learned a lot from studying books and articles by some of their builders when I thought I'd be doing strawbale walls (a plan I haven't completely given up, but I'm leaning more toward straw slip or woodchip slip at the moment).
Yes their skill set is high and their work is enviable, and if I were looking for a company to hire to design/build/accessorize a fabulous latest-technology natural house, I would not need to look far.

Perhaps I haven't made it clear that I don't have a mortgage, nor the equivalent in cash, which means I am not the customer those design/build companies are set up to serve. It's great that so many people can have a good livelihood through natural building in a mostly-rural area. I'm happy for them. My project, however, will have to proceed in a more modest fashion.

Thanks also for the numbers on your pricing as a designer/builder. I'm curious what that includes. Is that for a "move-in ready" house including interior walls, kitchen cabinets, light fixtures etc? My real question is, how enormous is the difference between what I'm trying to do, and what is considered "ordinary". My plan so far is only for the house shell --foundation, frame, roof, walls, windows, doors, and rmh-- the rest will have to come later. What do you consider a reasonable price range for such a "barebones" house shell (excluding the rmh of course)?

I had originally put a 12/12 roof in the plan because that's traditional in my area, and it made sense to me for shedding snow. However I've been re-thinking that, maybe it wasn't just for snow, maybe those old-time owner-builders might have been more influenced by the ease of calculation that 12/12 offers than I need to be. Your suggestion of 8/12 sounds good, or maybe 9/12. I've been told that the ideal pitch at this latitude for solar panels is 7/12, but I think that's not quite steep enough, and anyway I'm not sure I'd want them up where it would be so hard to clean the snow and ice off them.

Do you use hempcrete? What I read about it a while back sounded appealing, but I thought availability here was a few years down the road.

Were you saying that mineral wool is bad because it encourages smoke spread?

I will study that paragraph, "In your climate zone ... " Thanks for the details on how you would do it.
Your point about information from different climates is so important. When I was researching strawbale, I saw so many beautiful houses in the hot dry southwestern US, but I knew I had to focus on info and examples from New England and Canada. As you said, Vermont does have a concentration of natural builders who publish. There's a surprisingly large strawbale building industry in Ontario, but they seem to spend their time building rather than teaching and writing.

I don't know if anyone at yestermorrow is teaching full-time, but I'm sure you're worth $200,000 for a couple of courses. :-)
If that doesn't work out maybe you'd want to bring that concept to your own area, why not teach what you know and gather others with complementary expertise? Yestermorrow seems focused on consumers, you could do the same and/or teach people already in the construction business, you already know the mind-set and speak the language. Not everyone wants to travel to vermont to learn. Being more centrally located would be an advantage.

Sounds like you are on a tight budget. Are you going with Timbers since they cost less than light wood 2xs there? Here they drive my envelope cost up 40% and are a luxury custom upgrade I offer clients. Are you being code enforced? If your budget won't allow you to get anywhere close to the min r-values there your best shot is knowing you will thermally bridge to try and break it and find the best use of thermal mass. Do you know how to do that? Do you have anyone that can do manual j calculations or a simulation model like BEOPT to determine your HVAC loads in BTUs/Hr to size your RMH(s)? You need the orientation for it.

It is next to impossible to get quotes without at least a plan drawing do you have that? # of rooms, zones, etc....

I'm waiting for CO to have more hemp hurd and the price drop. There are ways to make lime binders with far less drying times than earth, better for wet cold construction seasons.


So your hoping for 24' x 32' (768 SF) with one RMH? What are you thinking here an open loft, one bathroom? What SDC, what foundation?

Thanks Terry for your input. I found the answer to my question elsewhere, apparently the numbers offered, $25-35 per sq ft includes no construction at all, just a design on paper. Well worth it I'm sure especially if required by mortgager.
I didn't find the answer to my question about what you meant about smoke spread, but that is not a priority for me to research right now. Sizing the rmh is not a problem so I don't require manual j calculations nor a simulation model like BEOPT to determine HVAC loads, but thanks for asking if I know how to do it, that was very helpful.

Sorry, the $125/135 SF #s I posted are for a total finish home no profits, probably higher where your at.

I found the answer to my question elsewhere, apparently the numbers offered, $25-35 per sq ft includes no construction at all, just a design on paper. Well worth it I'm sure especially if required by mortgager.

Mortgage will more than likely want plans and drawings, and the appraiser will value on them. That seems way high, especially for only drawing I would expect 3D models and high definition images and perhaps videos for that. I'd do it but I don't have the time doing six now myself. Otherwise put an add here. This is the better software that does not cost us Architects as much:

https://chieftalk.chiefarchitect.com/index.php?/forum/15-seeking-services/

Mention you are doing Timbers since not everyone has those in their library that can cost more if they have to create them. Try AutoCad forums too. Look in the Gallery for "designers" (not Architects if you have your own design ideas), on the website for some awesome timber renderings, some of this stuff is amazing real.

Sizing the rmh is not a problem so I don't require manual j calculations nor a simulation model like BEOPT to determine HVAC loads, but thanks for asking if I know how to do it, that was very helpful.

For the rest of the readers if you are not meeting the min IECC insulation values especially in cold climates a rocket mass heater can be very inadequate needs alot of BTUs/yr to make up for low insulation. Any HVAC contractor should be able to do this fast and for free in most cases or the info is on line. A HERs rating can also qualify for energy efficient low interest mortgages with tax credits spread over some years can be money better spent than in constantly fueling a RHM or having to change it's design later. PV tied electrical boilers can make more sense than a RHM or as a supplemental active heating for a well insulated home, or ground loops.

Also be VERY cognizant of the smoke spread indexs and weights of materials don't want to wait until it is too late. I've read of fire departments and emergency services are refusing to go near some. I'd have to agree why should they risk their lives and lungs and also law suits from neighbors are gaining popularity. .

Good luck!

Later readers of this thread may be interested to know that some hempcrete builders use hempcrete for roof insulation.  

One author, iirc Magwood, says that the difference between hempcrete and "lightweight insulation" is 12.5-15.6 lb/sq ft vs 1.9 lb/sq ft in weight and R2-2.4 per inch vs R 3.5-5 per inch.

It may be a moot point by now, but I wanted to comment that I found some of the load calculations earlier in the thread confusing. Somehow a 6" thick layer of light straw clay at 13 pounds per cubic foot becomes a load of 200 or more pounds per square foot.

It does not matter what size the blocks of LSC are; at 6" thickness, the total weight will be around 7 pounds per square foot. A foot thick layer would be 13 psf, and 2 feet would be 26 psf, very easy to build for.

A point which Terry never explained about flame/smoke spread: mineral wool having a "zero" rating is the best possible; it never contributes to fire or smoke generation in any way.

LSC is obviously unrated, but my guess would be that depending on the density of clay in the system, it might have a below-average to above-average rating compared to other common building materials. The more clay on the outside, the lower the actual flame spread factor is likely to be. The less clay in the mix, the better the insulation value, so there is a trade-off.

Thanks for the clarifications Glen.

To clarify one clarification, I think that the number of ~250 lbs was referring to a hypothetical box of household goods. Looking at it now it seems that the point was to pay attention to *where* the weight is, ie "distribution of weight or where it is located and how effectively it is transferred to load bearing walls/foundations."

Seems like that could still use a bit of clarification. Does 20 lbs / sq ft not mean that you could put 20 lbs on every sq ft? Ah, that clarifies it for me. It was probably an awkward unclear attempt to explain the simple and obvious principle that you shouldn't think that 20 lbs / sq ft in a 100 sq ft space would allow you to put a single small-but-heavy 2000 lb weight at the point of weakest support.

Actually it was difficult for me at the time to focus on the technical info that particular poster was asserting because it took so much effort just to imagine a positive intention on his part, given the intense sneering tone his posts originally had, before the numerous edits he subsequently made (8 - 25 edits per post) that have resulted in the exchange now sounding fact-based, and even useful.

After calling me stupid clueless numerous times, stating confidently that my house was definitely going to fall down crushing small children *and* it was going to burn down while the firefighters stood aside and laughed, he went, I found out later, to a thread of which I was unaware, to complain that I hadn't thanked him enough!
https://permies.com/t/160/54594#455443
(also intensively edited to sound less hostile)

I'm glad that at the time I made the effort to not respond in kind, because I think people since then may have benefited from this thread. It's had over 20,000 views!

By the way, it's not unusual for hempcrete builders to embed a timber frame in the walls, ie the hempcrete completely surrounds the posts. I haven't heard of anyone doing this with slipclay, and wouldn't plan on it without further research, since lime and clay are very different, but if it can be done then it has a big advantage in eliminating thermal bridging thus raising the effective insulative value of the wall as a whole. Probably only for those without code or mortgage, and/or outside the US.

To return to the topic of the thread, apparently yes it is possible to insulate roofs/attics with light clay straw, IF the frame is strong enough for the weight, which probably rules it out as a retrofit on most existing homes. To use it on a building strong enough for it, one strategy might be to start with the advice in hempcrete books (there are only a few at this time) for hempcrete rafter insulation, then get advice from a *friendly* expert to adapt those methods to account for the differences between hempcrete and lechtleim.
Best of luck to all owner-builders.

If any of the folks who contributed to the above thread are still around, I would love to know how it went with insulating that attic/roof.  I'm interesting in doing something similar.  We're not trying to insulate against intense cold, just nighttime lows of 40-45 with occasional dips down to freezing.

Having built an ICF house (lost due to mortgage collapse of the market in 2008-9), I sort of traveled a long research path, wanted to do something more natural, etc... and settled with:  rock wool. Very similar to fiberglass batting, but some superiorities. See this Rock Wool Article. What I did like was that the rock wool is:

- Made from a higher % of recyclables
- More Sound deadening
- not hygroscopic, that is it doesn't pick up moisture from the air  (so low or no mold)
- Basically not flammable

Didn't like:

-- devil of a time trying to find a local source that didn't confuse rock wool and the golden fleece, pricing wise
-- harder to manipulate

My ideal would be something like "blown-in rock wool that I could seal in afterward" but I don't know if anyone has solved that one yet.

I can't give any actual construction experience with it, but I think for your case light straw clay would probably work well. As you have variable temperatures, the much higher mass relative to fiberglass or rockwool or cellulose would be a benefit, shifting nighttime losses to daytime for an average decent R-value in a mild climate. Do you know what a building code that applies in your climate requires for insulation R-value? (Never mind if you are subject to it or not.)

Hello.
Would you be able to give some feedback after these years?
Tanks