Err, what? The link is to a reprint (with many duplicated paragraphs - perhaps it had to meet a word count target? :) of a long-ago article from PH+. "This article was originally published in issue 8 of Passive House Plus magazine." - that was back in 2013 or 2014 - I can't see a date on my copy. From memory I think Medite Smartply OSB3 was the previous name before it got rebranded as Smartply Propassiv. There's only that product on their site now, anyway.

I don't think anybody's ever claimed, let alone proved that 11mm OSB is airtight. The school project specifically talks about taping the joins too, so I'm not sure what your 'no tape' is about.

Old gems, re-issued to subscribers as 'The Thursday long read', worth reading anyway.

Propassiv is the coated version of Smartply made by Medite, part of https://www.coillte.ie/about-us/our-story. Shipped from Cork, for S/Central England this foreign import has less 'airmiles' than the UK alternative Scottish Sterlingboard, as well as 'no added formaldehyde' glue, poss because Irish trees already have more natural formaldehyde than Scottish ones.

Peter Warm and now I see Architype both say that OSB3 has adequate, if not complete, airtightness. Personally I supplement it with blown-in cellulose, belt n braces, and I use gapfilling glue and screw instead of tapes, and around windows expanding tape (Compriband) plus silicone sealant into sized gaps, instead of tapes.

Instead of WarmCell, I'd specify Icynene.

Have you ever found WarmCell settles, at all (i.e. 5 years later do a TI inspection)?

Posted By: fostertomhygroscopicity

I had to look that up!

Why would you want to insulation to absorb moisture? Part of the attraction of Icynene was that any moisture would just pass through (as I understood the physics of it).

Posted By: borpin[hygroscopicity]...
Why would you want insulation to absorb moisture?

You want a material that can adsorb moisture (take in water in it's gaseous state) rather than absorbing it (soak up liquid water) - and desorb it (reverse the process), as many natural insulation products do. In other words, as Simon mentions, it buffers moisture vapour.

The benefits are that this:
- reduces the risk of condensation (by reducing the build-up of moisture at the interface of the insulation and a colder surface, where condensation tends to occur).
- Improves thermal comfort (keeps the air within a comfortable relative humidity range for longer)
- improves air quality (at <40% RH viruses are present in the air for longer, respiratory infections are more likely, and the skin, nose & throat can become irritated; at >60% bacteria and mould thrive)
- tends to reduce heating and cooling requirements (adsorption of moisture releases heat, desorption takes heat, though research into quantifying this in buildings still seems to be limited)

https://www.diffen.com/difference/Absorption_vs_Adsorption

also

https://en.wikipedia.org/wiki/Double_layer_(surface_science)

and http://www.iust.ac.ir/files/fnst/ssadeghzadeh_52bb7/files/Israelachvili_J.N.-Intermolecular_and_surface_forces-AP_%282003%29.pdf if you want a real in-depth exploration

aBsorb = a liquid or gas fills up the pore volumes in a different solid or liquid

aDsorb = a liquid or gas sticks onto the surface of a different solid

Both happen more in porous materials, which have larger pore volumes and larger surface areas, eg dead wood can absorb and adsorb water into its little tubes.

Adsorption is more interesting because the surface properties of the solid can be tuned to adsorb specific liquids or gasses, and release them again at useful moments.

Nature might have done the tuning already, EG wood can absorb/release water vapour in response to the Relative humidity (instead of the Absolute humidity) which gives it useful RH buffering and wicking properties.

Synthetic materials can also be tuned, eg those little sachets of silica gel can adsorb moisture and release it again when heated slightly.


Clay is like sand, only the grains are smaller. The spaces inbetween the grains can absorb and adsorb liquids and gases. Likewise concrete, plaster(board), lime mortar all buffer humidity to some extent.

Posted By: WillInAberdeenClay is like sand, only the grains are smaller. The spaces inbetween the grains can absorb and adsorb liquids and gases.

No, clay is like mike1 said, lots of platelets. Sand is three-dimensional grains.

Likewise concrete, plaster(board), lime mortar all buffer humidity to some extent.

Not particularly true. Concrete yes; the rest no. See the conservation physics link I posted.

I'm quite partial to the principles of house building design as described by the Conservation Physics site. Indeed that's the approach I took in designing my house. I do, however, think some of its conclusions have been superseded by more recent research into moisture buffering materials.

So, I'll just jump in here to suggest that the moisture buffering abilities of the materials is down to pore structure. I'll also suggest that following more recent research the moisture buffering performance of clay might not be as superior as suggested. I've linked to the below paper before. See page 48 for sorption isotherm curves, but will quote a couple of things:

"Clay and lime presented mainly macro-pores, which have an average diameter of around
125 nm. Gypsum also had macro-pores of a significant bigger size (365 nm average),
but it also presented micro-pores, as shown in Fig. 3-2b. Gypsum plasterboard had a
more accentuate micro-pores presence and a significantly higher average pore diameter
(631 nm) than standard gypsum. Overall, the gypsum and plasterboard showed a
significant higher pore volume than clay and lime. Due to the more complex pore
structure of gypsum and plasterboard, both vapour and liquid transport take place
into the materials, when exposed to a RH and vapour pressure gradient, while in clay
and lime only vapour transport occurs. Water vapour transport can take place in
the macro-pores and its driving potential is the water vapour pressure, whilst liquid
transport takes place in the micro pores, where the driving force can either be the
relative humidity and the capillary pressure."

"Hygric properties and moisture buffering capacity of coatings (clay, lime, gypsum and
plasterboard and hemp-lime) were determined experimentally. Density, porosity,
water vapour permeability, sorption capacity and thermal conductivity were measured
and compared with moisture buffering, calculated performing the NORDTEST
procedure in a climatic chamber. The laboratory test showed gypsum stored and
released more humidity than the other materials, due to the presence of micro-pores,
and the consequent activation of liquid transport together with the water vapour
transport in the macro-pores. A direct correlation between moisture buffering and
hygric properties was found, which highlighted a significant correlation between
porosity and the dynamic sorption capacity."

"When temperature and RH vary simultaneously, the response of the materials showed
that moisture buffering performances were effected by temperature, but clay and
gypsum responded differently to the variable environmental condition, due to their
different pore structure. Gypsum with its complex pore structure, increased its
moisture buffering, due to the higher impact of temperature on liquid transport. The
RH and temperature gradient activated capillary transport and surface diffusion,
when gypsum was subjected to simultaneous square wave variation. On the contrary,
the moisture buffering capacity of clay was reduced probably due the combined effect
of the saturation of macro-pores at high RH and impact of low temperature on the
hygric properties."

The paper does look at performance of woodfibre board in the test room.

It's clear from this phd dissertation that the moisture buffering performance of the materials is not just intrinsic but also depends on outdoor humidity as well as indoor temperature and humidity patterns of variations.

Please don't shoot the messenger, but I do encourage a read: https://researchportal.bath.ac.uk/en/studentTheses/understanding-moisture-buffering-effects-in-the-indoor-environmen

"Repulsive electrostatic forces also control the long-range swelling of clays in water.
Most naturally occurring clays are composed of lamellar aluminosilicate sheets about 1 to
2 nm thick whose surfaces dissociate in water giving off Na þ , K þ , and Ca 2þ ions, and when
placed in water they can swell to more than 10 times their original volume (Norrish, 1954).
The swelling of clays is, however, a complex matter and also involves other forces at
surface separations below about 3 nm (van Olphen, 1977; Pashley and Quirk, 1984;
Kjellander et al., 1988a, b; Quirk, 1994)."

... from the Israelachvili book I linked to. The sheet-like structure of clay is fundamental to how it behaves.

Clay swells in water. Sand does not. Clay buffers moisture. Sand does not.

Even the well-known gardening test to distinguish clay soils from sand etc uses the different behaviour to work.

Let's go back to the beginning

Posted By: djh

Posted By: WillInAberdeenClay is like sand, only the grains are smaller. The spaces inbetween the grains can absorb and adsorb liquids and gases.

No, clay is like mike1 said, lots of platelets. Sand is three-dimensional grains.

We still haven't managed to convince you of that apparently so here's some pictures of various types of sand:
https://smallpond.ca/jim/sand/micrographs/
and here's some of clay:
https://ceramicsweb.org/micrographs.html

As you rightly say, clay particles are a lot smaller but do you really still insist that they are similar?

And then right at the end you say:

Posted By: WillInAberdeenClay does the same things, only more so, because it has finer grains (I may have mentioned that) and so each kg of clay contains many more m³ of adsorption surface

So these 'same things' are why building regs have the same regulations about building on clay and on sand, right? Oh wait, they don't! They treat them completely differently because sand and clay behave completely differently. And you even confirm that yourself:

Gardeners will tell us that it's not only the total ad/absorption capacity that matters (clay is good), it's also the ability to move water in and out quickly (sand is good). Mix them together for happy plants.

Clay holds water. Sand drains well. Same behaviour, obviously.

Take a lump of clay. Roll it flat. Do the same with a lump of sand. Good luck with that. Leave them to dry for a week. Pick up the sheets of clay and sand. Again good luck with that. Put them in a kiln and heat to about 1000°C. Take out the clay tile and the heap of loose sand. Sand and clay behave the same. Only in your dreams.