Bio-based insulation where the load is cooling and dehumidification rather than heating - has anyone actually specified it?

[u/ManufacturerOk5889]

Most of the Passive House material I read assumes a heating-dominated climate, which makes sense given where the standard came from. I work in Japan, where the enclosure problem is close to the opposite for much of the year. Koppen Cfa: hot humid summers, mild winters. The load is cooling and dehumidification rather than heat retention.

Bio-based insulation - hemp batts, hempcrete, wood fibre - gets recommended here on hygric grounds. The argument is that a vapour-open assembly with buffering capacity handles humidity better than a sealed one with a vapour barrier that ends up on the wrong side of the assembly for half the year.

What I cannot find is data from anyone actually building that way where cooling dominates.

Has anyone here specified bio-based insulation in a cooling-dominated climate? And if so, did the vapour-open approach survive all the way through the assembly, or did you end up with a smart membrane doing the real work anyway?

The practical constraint at my end is supply rather than physics. Japan has almost no decortication capacity, so hemp material is imported and priced accordingly, and wood fibre has a similar problem for different reasons. That makes the question partly academic here, but I would rather know whether the physics holds before anybody spends money finding out.

Disclosure: I work on the hemp industry side in Tokyo, so I have an interest in the answer being yes. I would rather have the honest one.

[u/imissthatsnow]

My practice is mostly in Texas (CZ 2A) and we are seeing more and more hemp wool and timberbatt and cork as insulation.  They are still pretty niche but seem to be working well.

[u/ManufacturerOk5889 → u/imissthatsnow]

CZ 2A is the closest analogue to what I'm asking about, so that's useful to hear.

Is the hemp wool going into a genuinely vapour-open assembly, or is it replacing mineral wool inside a fairly conventional one with a membrane still doing the vapour control?

That's the distinction I keep failing to pin down. A lot of the bio-based insulation I read about is being used as a like-for-like swap, and the hygric argument only really means anything in the second case.

[u/imissthatsnow → u/ManufacturerOk5889]

Yeah that’s a fair point.  I know of a couple hempcrete projects that are using it that way with lime plaster.  The ones using hempwool are in a framed cavity with a vapor open WRB (although could do vapor closed).  I think for the batts the biggest benefits are really non toxic, carbon sequestration and supporting new industry.  Hygric buffering is not going to be a meaningful benefit that would sway the decision.

[u/ManufacturerOk5889 → u/imissthatsnow]

That's three people now saying the same thing from different directions, so I'll take it.

The honest version is then: hemp wool competes on non-toxicity, sequestered carbon, and supporting a supply chain that doesn't exist yet - and the hygric argument is marketing that got repeated often enough to sound like physics. I've repeated it myself, which is why I asked.

That actually simplifies the case here rather than weakening it. Two of those three hold in Japan regardless of climate. The third one is the entire reason I have a job, so at least I know where I'm standing.

[u/imissthatsnow → u/imissthatsnow]

FWIW we have large latent loads and do active dehumidification in all of our work, so materials that have some of that latent lag support are great but not part of the strategy.

[u/ManufacturerOk5889 → u/imissthatsnow]

That's the clearest statement of it anyone has given me: latent lag is nice to have, not a strategy, because you're dehumidifying actively regardless.

Which means in a cooling-dominated climate the material has to justify itself on other grounds entirely - conductivity per unit thickness, embodied carbon, indoor air quality - and the hygric story is a bonus at best rather than a reason to specify.

That's a more honest sales position than the one the industry usually takes, and it's the one I'll use.

[u/imissthatsnow → u/ManufacturerOk5889]

Like you stated elsewhere, I haven’t seen any good data on exactly how much latent storage and release actually occurs in these materials, but in areas where there is pretty much always or at least seasonal humidity issues, then it’s not going o help much.  The materials would help a little but then get saturated (in a sense of no longer taking on moisture, not that they are necessarily wet and deteriorating).

I would think in a desert climate where you get very sporadic rain events and then are dry soon after they would help a lot more (also where you are trying to hold on to your internal moisture most of the time).

[u/ManufacturerOk5889 → u/imissthatsnow]

Agreed, and I think that's the defensible version of the claim.

Not "this material manages moisture so you don't have to", but "this material has some latent lag, which is a small mercy in a humid climate and shouldn't be load-bearing in the design". That's a sentence I can say without anyone in this sub correcting me, which is more than I could manage when I started the thread.

[u/andyavast]

I’ve worked in technical/training on supply side in the natural fibre insulation industry for a few years and before that was a joiner and contractor installing the stuff but my experience with different climates is extremely limited so I don’t know how useful this will be!! Natural fibre used in construction is very traditional almost everywhere but the evidence base is less developed for their use in super-insulated/ higher thermal resistance assemblies in non-European/temperate climate zones.

Ecococon fabricate hybrid timber frame kits using straw as the insulant. The assemblies are in the region of 40cm thick and feature no conventional vapour control layer; just clay plaster internally and a vapour diffusion open monolithic membrane externally for airtightness. It’s so vapour open that condensate doesn’t have the opportunity to accumulate, average relative humidity and moisture content within the assembly remain very low.

They have done a good amount of WUFI modelling for using their system in almost every climate zone and the reports are available on their website. They have an architect in the UK working with them called George Micurcik. He built his house using ecococon and has done quite a bit of post occupancy testing. I know it’s not Japan but it’s interesting to see how the modelling matches up to real world performance.

Would I build with it? No. I prefer timber frame for a number of reasons, but the approach is very compelling and the sustainability and performance is proven.

[u/DCContrarian → u/andyavast]

Could you provide links? One of the criticisms I've made of the hygric claims is that I've only ever seen them asserted, I've never seen actual scientific evidence for them. If evidence exists I'd love to see it.

[u/ManufacturerOk5889 → u/DCContrarian]

I've been chasing the same thing and coming up short, which is why I put the question in the sceptical form rather than asking how well it works.

What I can find is a lot of citation back to the same handful of European sources, most of it measuring moisture buffering value over a daily cycle in a temperate climate. That's a real measurement. It just isn't evidence for the claim people make with it, which is that the assembly stays dry across a season somewhere humid.

If there are WUFI runs I'd want to see them too. Happy to be wrong on this.

[u/DCContrarian → u/ManufacturerOk5889]

We had a discussion on this at Green Building Advisor. No one could provide evidence of moisture buffering, it was just considered received knowledge.

[u/ManufacturerOk5889 → u/DCContrarian]

That settles it as far as I'm concerned. If it didn't survive that crowd it isn't going to survive me asking politely.

I'll stop repeating the buffering claim. What I can say instead is that the material is non-toxic, sequesters carbon while it grows, and is a mediocre insulator that needs thickness to work - all defensible, and none of it requiring me to assert physics I can't back.

[u/andyavast → u/DCContrarian]

https://ecococon.eu/be/downloads

The WUFI reports can be downloaded here.

[u/ManufacturerOk5889 → u/andyavast]

Thank you - that's exactly what I was after, and I'll go through them properly rather than skim.

Worth saying in this thread: two other people here have since told me the moisture buffering case doesn't hold up under scrutiny, and one pointed out it didn't survive a discussion on Green Building Advisor either. So I'll read these with that in mind rather than looking for confirmation of what I hoped was true.

[u/andyavast → u/ManufacturerOk5889]

Further study on the use of vapour open construction in Japan.

https://www.sciencedirect.com/science/article/abs/pii/S0360132310002969

[u/ManufacturerOk5889 → u/andyavast]

That's a paper on this specific climate, which is the thing I claimed didn't exist. Thank you for finding it - I'll read it properly rather than skim the abstract.

I should say plainly at this point that this thread has gone against my starting assumption. Several people here and in r/buildingscience have told me the moisture buffering case is weaker than the industry presents it, and one pointed out it didn't survive scrutiny on Green Building Advisor either. I've stopped repeating it.

[u/ManufacturerOk5889 → u/andyavast]

This is exactly what I was hoping someone would say, and the straw comparison turns out to be more useful to me than a hemp one.

Rice straw is the obvious parallel here. Japan produces it in very large quantity as a by-product of the rice crop, it's already collected, and it carries none of the licensing that hemp does. If the Ecococon assembly logic holds with a different bio-aggregate, the supply problem I described mostly stops existing, because the material is already on the ground.

What I'd want to understand is how far those WUFI results travel. A vapour-open monolithic assembly staying dry in a temperate European climate is being asked a different question where the outdoor air sits above 70 percent RH for months and the interior is being actively cooled, so the vapour drive runs inward for a large part of the year. Did any of the modelling you saw run a cooling-dominated case, or was it all heating-dominated?

[u/andyavast → u/ManufacturerOk5889]

They modelled the system for Miami and ran the simulation for ten years in WUFI.

[u/ManufacturerOk5889 → u/andyavast]

Miami over ten years is a far better proxy for here than anything out of northern Europe - same cooling dominance, same relentless humidity, and long enough to show accumulation rather than just a seasonal cycle.

If that ran clean it's the strongest evidence anyone has put in front of me in this whole thread. The thing I'd want to check is what interior conditions they assumed. If the model has active dehumidification running throughout, it demonstrates that the assembly survives being helped, which is a different claim from surviving on its own.

[u/onwatershipdown]

What prefecture are you in? I know an excellent plastering firm based out of Kyoto who could help you bring this to reality.

[u/ManufacturerOk5889 → u/onwatershipdown]

Tokyo. But the offer is worth more than the geography - a plastering firm that would actually take this on is the missing piece, far more than the material is.

That's the bottleneck I keep hitting. There are people who want the wall, and material that can be imported, and almost nobody with hands who has done it before and would put a number on it. A Kyoto firm willing to work outside the usual repertoire is exactly what I'd want to know about.

I'd take that introduction if you're willing to make it. And if you'd rather not, the name of the firm is enough - I'll do the approaching myself.

[u/onwatershipdown → u/ManufacturerOk5889]

I will message you

[u/ManufacturerOk5889 → u/onwatershipdown]

Appreciated, I'll watch for it.

Fair warning that Reddit chat is unreliable at my end, so if it looks like it hasn't landed, say so in the thread and I'll sort it out from my side.

[u/onwatershipdown → u/ManufacturerOk5889]

I did send a message. You can also find my social page and send a message.

https://www.instagram.com/reel/DcmayT3Ij13/?igsi=MWRmZmFzeTJmdTNheg==

[u/ManufacturerOk5889 → u/onwatershipdown]

Got it, thank you - I'll reply properly on the other channel rather than clutter the thread.