Sealing Stone

[RJH]

I've just had the front of the house sandblasted - looks fine, right mess to clear up though.

Anyway, while the scaffolding's still up I was thinking about sealing the stone - this was a service the sandblaster bloke said he could offer, but not until it gets warmer.

As it is I think it looks OK, but I wouldn't mind having it a shade darker to match next door, and it does look a little friable close up.

Any recommendations for a suitable product, or applying this time of year? I'd obviously rather get it right than end up having to sandblast the whole lot again ;-)

[tabbypurr]

Have you checked the SPAB guidance on this?

NT

[RJH]

No, but thanks for the tip, I'll take a look.

I did consult with the council (the house is in a conservation area) before doing it but just went round in circles. They effectively wanted an assurance relating to what it would look like after it was cleaned. Which of course I wouldn't know until it was cleaned. So we sort of left it at 'if you make a mess of it, repaint it to how it was'. And I trusted the work to a local contractor.

[harry]

ISTR there is a possibility of damp getting into the house if you seal the outside of a stone wall. Rising damp if the DPC is not very good.

[RJH]

It's only a stone skin (albeit over a foot). The inner leaf is brick. The DPC is old but fine - no rising damp anyway. The main problems to the front are condensation. Anyway . . .

. . . I wouldn't look to seal it completely, more 'fix' or 'consolidate'. Or something.

[tabbypurr]

What did the SPAB leaflet say?

NT

[Stuart Noble]

So called masonry sealers are no such thing these days, but are more often fine dispersions of solids that don't form a continuous film

[RJH]

I can't see anything readily accessible in their various FAQs and publications, and certainly no direct reference to my issue. I'm sure they may take on my question as a 'case', although their focus is more conservation areas I think. Thanks anyway, worth a look.

[tabbypurr]

l the outside of a stone wall.

OK, I'll try & quickly sum up the general position in conservation work. Da mp moves very slowly from interior to exterior through porous stonework and evaporates. When one seals the outer layer, it builds up in the stone, and breaks the stone surface up when it freezes. Thus sealing it causes damp & often decay, and is not appropriate.

NT

[Muddymike]

That looks to be the same type of stone as my house. We had expanding foam cavity fill installed a couple of years ago. Big difference.

Mike

[RJH]

Noted - thanks very much. I've noted that effect in the bay walls beneath the window. I'd guess the paint trapped moisture, and the moisture pushed the paint away.

I've since found the stuff I had in mind:

I really don't know if it allows moisture out and manages to repel water. The stone does seem to 'wet' quite readily. Anyhoo, any ideas that manage to achieve a degree of balance welcome.

[tabbypurr]

eal the outside of a stone wall.

. Damp moves very slowly from interior to exterior through porous stonework and evaporates. When one seals the outer layer, it builds up in the stone, and breaks the stone surface up when it freezes. Thus sealing it causes da mp & often decay, and is not appropriate.

Balance between what? What's the use in a product with negative outcomes? M ost people have no grasp of how such walls handle water, and seem to think they should apply chemicals to keep the rain out. They don't work like that at all.

NT

[RJH]

Balance between stopping rainwater soaking in on the outside (thereby reducing the thermal performance of the wall) and allowing moisture from the inside to escape. I can't imagine any product will do both perfectly.

I've no real idea how the wall works - hence my question.

[tabbypurr]

seal the outside of a stone wall.

rk. Damp moves very slowly from interior to exterior through porous stonewo rk and evaporates. When one seals the outer layer, it builds up in the ston e, and breaks the stone surface up when it freezes. Thus sealing it causes damp & often decay, and is not appropriate.

s? Most people have no grasp of how such walls handle water, and seem to th ink they should apply chemicals to keep the rain out. They don't work like that at all.

Rain soaks into stone very shallowly, and soon evaporates. If you want bett er thermal performance you need insulation.

NT

[The Natural Philosopher]

Or a lot more stone. 3 meters of stone has the same insulation properties as 3" of celotex :-)

[Stuart Noble]

I really don't know if it allows moisture out and manages to repel

I've no experience of stone, but rain certainly goes straight through some brick walls. The "sealer" referred to above is claimed to be breathable. How can that be? Round and round we go

[Vir Campestris]

I have a breathable waterproof jacket.

Raindrops fall off it, but warm moist air can get through. (and there are two completely different breathable jacket technologies)

Andy

[tabbypurr]

It's meaningless really. 'Breathable' has become a fashionable coating desc ription, all it means is it's not totally airtight, which in practical term s means more or less nothing. A pillow is 'breathable' in that sense, but i t doesn't mean you can really breathe through it.

NT

[Chris Hogg]

Not followed this thread, but if a material is porous but water repelling, might it not reasonably be described as waterproof but breathable? I'm thinking of a waxy type coating that could be full of fine holes to let air pass, but because it's hydrophobic, being waxy, it won't allow water to penetrate those holes. Of course, it depends on the size of the holes, but below a certain size I would think it would be effective.

[The Natural Philosopher]

Its not meaningless. In fact it has a fairly precise technical meaning in the context of buldings

Resists water a lot, but water vapour very little is generally the meaning.