Breathability vs Stain Blocking in Painted Timber

[Martin109]

I am planning a timber cladding ceiling, with wood fibre based insulation above in the 200mm or so of roofspace, and I'd like to finish it white or off-white, and I'd like to retain the breathability of the timber.

In terms of breathability, claypaint is excellent, having a SD rating of 0.02, and can apparently be applied to timber without a primer.  However, I'm concerned about tannins in the timber bleeding through this.  Normally, I would prime bare timber with a knot-priming and general stainkilling primer such as Zinsser B-I-N or Zinsser Bull's Eye 1-2-3 , followed by an undercoat and topcoat, but these latter coats are not breathable.

Are these two requirements - breathability and stain-blocking - mutually incompatible?

[Aunt Maud]

Yes.

[Asterix, the former Gaul.]

Why do you need the breathability of the actual timber?  Your best bet is to ensure the environment around the timber has ventilation, I'd have thought. 

[Martin109]

Why do you need the breathability of the actual timber?  Your best bet is to ensure the environment around the timber has ventilation, I'd have thought.

Well, I suppose it will have, as above the insulation is a semi-permeable membrane (rather than old-fashioned roofing felt) and tiles.

I was mainly thinking of the capacity of the ceiling (it's a bathroom) to absorb water vapour and then breathe it out again, as will be the case with that part of the walls constructed of hemp-lime, which has this capability.

[Aunt Maud]

The timber will swell and shrink, even with stain block on it. Will the clay paint stuck to the stain block ?

[Martin109]

The timber will swell and shrink, even with stain block on it. Will the clay paint stuck to the stain block ?

Good point.

It's probably got to be one or the other, hence back to the original question.

Maybe I'll just have to be satisfied with the planned breathability of the walls, and deal with the ceiling in the conventional, non-breathable way (ie prime, undercoat, traditional paint finish),

[andytheflyer]

Maybe I'll just have to be satisfied with the planned breathability of the walls, and deal with the ceiling in the conventional, non-breathable way (ie prime, undercoat, traditional paint finish),

Have you considered using one of the Kitchen/Bathroom emulsion paints that hold condensation whilst the bathroom is in use and then release it once the humidity levels have dropped?  You get damp walls for a while after a shower etc., but they then dry off without any mould, IME.

Limited range of colours though.

[Aunt Maud]

You could try a milk paint, as the Americans call it, which is lime wash mixed with casein. Or a soap finish is nice, which is soap flakes mixed in hot water and then rubbed on with a sponge.

[Martin109]

Thanks for the ideas...