Masonry Water Repellent: is it really breathable?

[Helen Foster]

Hi all,

Does anyone have an opinion on the efficacy and most importantly, vapour permeability of products such as these:

https://www.mikewye.co.uk/product/masonry-water-repellent-acs/

I've always found this supplier in particular to be trustworthy and generous with their time and advice, so I'm not questioning their honesty, but I'm nervous about using something that claims both to repel rain ingress at the same time as not compromising vapour permeability. Has anyone used it/can explain the chemistry of it - can it achieve both? I'm re-pointing our north facing gable wall with lime putty mortar, hoping it will stop rain ingress (there are quite a few bricks which have lost their face), but whilst the scaffolding's up I'm wondering whether to apply something like this too - I've always stayed well away from such products in the past. Thank you

 

[livias]

Rain comes in drops ... so big lumps
Water vapour is very fine
It is the difference in size which the product takes advantage of. Thats the theory anyway. Whether it is true is beyond me!

 

[Flyfisher]

Yes, that's my understanding too. Thus, breathable roof membranes and Gortex (tm) jackets.

But it all seems rather qualitative rather than quantitative, which makes comparisons very difficult. At least, I've never seen a 'scale of breathability' with the positions of various paints and mortars clearly marked along it.

This is an interesting read, though it does warn "There is no defined European Standard as to what constitutes breathability and there are at least 8 different ways to measure the breathability of a material. Until the industry adopts a defined standard this will always be an issue, however when it comes to paint the most accepted and adopted standard is SD Value, it helps that it is probably the easiest to understand, interpret and compare."

http://cornishlime.co.uk/information/breathable-paints-explained/

 

[Me!]

explain the chemistry of it - can it achieve both?

It's more physics than chemistry but it is done at a molecular level. Water vapour has molecules that are basically free to move around individually, whereas water liquid has molecules that are attracted to each other with sufficient force to stop them drifting apart. So if the paint has holes in it that just are big enough to let individual water molecules through then it is breathable, and if those holes are not big enough to let several molecules through at the same time when they are all holding on to each other in a liquid form, then it is waterproof.

If you try a Goretex coat, that works the same way. Rain drops run off the outer surface even when driven by a storm, but you don't get hot and sweaty inside as moisture from your skin can still evaporate and go out through the breathable membrane.

 

[88v8]

I can sort of understand this when applying just one coat of sealer or paint, but how on earth can it work when subsequent coats are added? I mean, surely the later coats block the pores of the undercoats? This sealer, it says to apply coat on coat to saturate. Hmmm.

Ivor

 

[Helen Foster]

Thank you all for such informed feedback - the comparison to a goretex coat makes a lot of sense to a non scientific person like myself. However, Ivor's comments comments about the use of repeated coats also has me wondering. Either way, i'll follow Flyfisher's link to Cornish lime to read up further and make a decision based on this research. The video on Mike Wye's site shows how the brickwork is completely unchanged visually after the application of the stuff, so I'm wondering about maybe only applying to the worse affected areas, as the rest of the brickwork is doing its job of keeping the rain out.

Many thanks to all

 

[kreyszig]

@Ivor -
Bit of background:
The dried coating as a material will have an inherent porosity (ratio of void space: total volume) and nominal pore throat size.
They are not necessarily equivalent; a low porosity material can have very large pores (but fewer of them) and vice versa, a high porosity material can have very small pores.

For this reason porosity is not equivalent to permeability (the ease with which a fluid/gas passes through the material). The permeability is also not dependent on the dimensions of the material; whether the material is 1 m or 1 micron thick the permeability will be fairly constant (ignoring gravity, compression effects, etc). The time taken to pass through the material will depend on the thickness, of course.

The important bit:
Permeability is dependent on the connectivity of the pores, but this shouldn't alter as a material property when you add further layers.

A fairly good analogy is thinking about a layer of (dry) sand; there is a lot of void space between the grains which is maintained if you pour more sand on top - the new sand doesn't block all the void space of the sand underneath it.

 

[CliffordPope]

A fairly good analogy is thinking about a layer of (dry) sand; there is a lot of void space between the grains which is maintained if you pour more sand on top - the new sand doesn't block all the void space of the sand underneath it.

No, but sand isn't a liquid, and the new layer will stay as a layer. Paint will soak into the previous layer to some extent - that's how it bonds. It's like the difference between a layer of plain tissue paper and one that's been treated with dope varnish.

Also I'm not convinced by the Gorex raincoat example. It may have the two properties of vapour permeability and water repellence, but I don't see how it can be both at the same time. How does water vapour force its way out through a layer of water? The rain will itself be producing water vapour on the outside - why doesn't that travel inwards? Why does the outward vapour migration exceed the inward? After all, however sweaty you may be on the inside, the rain on the outside must be even more humid, surely?

From bitter experience of water coming in through a newly repointed stone wall, using the correct lime mortar, vapour permeability is irrelevant if the mortar or stonework is porous to shear liquid water running down the outside.

 

[kreyszig]

A fairly good analogy is thinking about a layer of (dry) sand; there is a lot of void space between the grains which is maintained if you pour more sand on top - the new sand doesn't block all the void space of the sand underneath it.

No, but sand isn't a liquid, and the new layer will stay as a layer. Paint will soak into the previous layer to some extent - that's how it bonds. It's like the difference between a layer of plain tissue paper and one that's been treated with dope varnish.

Also I'm not convinced by the Gorex raincoat example. It may have the two properties of vapour permeability and water repellence, but I don't see how it can be both at the same time. How does water vapour force its way out through a layer of water? The rain will itself be producing water vapour on the outside - why doesn't that travel inwards? Why does the outward vapour migration exceed the inward? After all, however sweaty you may be on the inside, the rain on the outside must be even more humid, surely?

From bitter experience of water coming in through a newly repointed stone wall, using the correct lime mortar, vapour permeability is irrelevant if the mortar or stonework is porous to shear liquid water running down the outside.

Fair point sand isn't a liquid.not the best analogy. The idea of the pore sizes is he useful bit.. The dried layer is impermeable to liquid water. It is also impermeable to liquid paint. The pores don't get blocked.
Goretex: water vapour is a gas. It moves through goretex through osmosis from regions of higher concentration of water vapour to lower. Liquid water cannot pass through goretex, it is a semipermeable membrane in that regard. The amount of water vapour being produced by liquid rain is negligible compared to that inside the coat, so it migrates outwards. If it were more humid outside then you would indeed be seeing water vapour moving in towards the wearer.
As for the layer of liquid water, that is interesting. If the outside of the coat were submerged in water completely, vapour on the inside would not be able to move via osmosis. But when exposed to rain the jacket is never completely covered in liquid water, so the vapour can migrate outwards.

All of which is moot if, as you say, the material is permeable to liquid water, through!