Agglomerated Expanded Cork Insulation Boards: Breathability

[aockenden]

Hey I just wanted to share that I was originally quite skeptical that these agglomerated expanded cork insulation boards were going to be properly breathable, as I know some others on here have been as well, but about a week ago I took the leap and ordered enough to do my cold block-built home-office (from Mike Wye).

They arrived a couple days ago and I immediately tested their breathability by holding one of the boards up to my mouth, sealing my lips around the surface of the board and trying to take a breath in. I was able to draw air through the board without too much difficulty (although I got a few grainy bits of cork in my mouth for the trouble). I was pleasantly surprised by these results and it’s put my mind at ease that these actually won’t trap condensation behind them (especially when fully mushed into a contiguous backing coat of lime as recommended by the supplier).

So I’d say, fear not the agglomerated expanded cork board! I love that when used internally they don’t even need mechanical fixings as well. Saves the joints from all the hammer drilling into the masonry.

 

[Janela]

If you have the time to post some pictures and recount as you put them up it would be great.
Your previous posts on hemp and lime insulating plaster etc have been very interesting to follow.

 

[aockenden]

Here’s some for now, sry for the bad lighting but I’ve only just gotten out there and will be leaving again for Christmas stuff tomorrow.

Obviously we’re not done boarding the room out yet. Our process is as follows and seems to be giving good adherence of the boards to the wall.

1. Flatten and key-up the wall
2. Prime the wall with a sticky NHL 3.5 + water paste
3. Apply a fairly wet mix of 1 part NHL 3.5, 1 part fine hemp and 1 part sand to the backside of the cork board at about 1 CM thick with a notched trowel
4. Mush the lime-coated cork board onto the sticky lime-primed wall surface and firmly press / lightly fist-pound the board into place across its entire surface.


Officially they recommend using some proprietary lime-based backing coat product called “isovit”, but reading the materials sheet for that product it’s just NHL, some fiber, some finely ground cork aggregate, and a touch of cement. So we’re just approximating that with a homemade NHL 3.5 + fine hemp + sand mix and it’s going just fine. Adherence seems good.

 

[Robola]

What are the disadvantages of these cork boards compared to wood fibre boards?

 

[aockenden]

What are the disadvantages of these cork boards compared to wood fibre boards?

I think wood fibre is slightly better insulating but don’t quote me on that. I’ve also never worked with wood fibre myself so I shouldn’t make any claims either way. I just know these cork boards seem pretty legitimate.

 

[jamest]

I found the cork boards very nice to work with - I particularly like the smell and the fact that they are very light weight - though they do have a habit of shedding little granules of cork around the place. I confess to having used the Isovit stuff, but I wouldn't use it if I were doing it again - it's a little pricey for one thing.

I can't see any disadvantages compared to wood fibre really. Mike Wye and others quote the thermal conductivity of the cork as 0.037-0.04, which compares well to wood fibre (values similar). The one thing I have noted is that for cork a range is always quoted, whereas for wood fibre a single number is given (Mike Wye quote 0.04 for the Steico dry wood fibre boards, for instance). My guess is that the construction of the cork boards (pressing together expanded cork granules of various sizes) produces a slightly unpredictable pattern of voids and cork and so the thermal conductivity varies a little bit from place to place. Wood fibre is presumably more consistent.

The only thing worth noting is that cork boards are typically a little more expensive than wood fibre boards. If you have a large area to do, the added expense of the cork might become a deciding factor.

 

[Kearn]

I’ll add that woodfibre boards are a pain to plaster onto - not much key so you need a seriously sticky mix and expect half the plaster to fall off as you trowel on! Can get tedious and physically tiring. No idea if the cork has more of a key given then surface being less uniform?

 

[Lacan07]

I’ll add that woodfibre boards are a pain to plaster onto - not much key so you need a seriously sticky mix and expect half the plaster to fall off as you trowel on! Can get tedious and physically tiring. No idea if the cork has more of a key given then surface being less uniform?

Indeed, with wood fibre boards the first coat is always just get it on anyhow in order to apply the mesh which is what seals and flattens the wall. The hardest part is always the top of the wall as your looking to avoid down strokes.

 

[aockenden]

I’ll add that woodfibre boards are a pain to plaster onto - not much key so you need a seriously sticky mix and expect half the plaster to fall off as you trowel on! Can get tedious and physically tiring. No idea if the cork has more of a key given then surface being less uniform?

I haven’t tried plastering on-top of them yet but they aren’t overly smooth, and they’re not very spongy / bouncy, so I’m guessing they’ll take a coat of hemp-lime pretty well. I will report back on it once I try though…