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Comment by duckmysick | Hacker News Reader

I'm curious about rain screens. One of the pages says "Perhaps the single most important thing you can do for your walls is to include a rain screen, which is like venting for your walls."

How does that work exactly? Here's how I understand it: I would have to cover an entire building with some sort of a skeleton (layer) of wood, with an air gap in between. And it will cause the rain drops to slide down to the ground instead of the wall absorbing it.

Sounds expensive. Am I missing something?

Around here (mostly concrete or brick buildings with a thick exterior polystyrene insulation) water-repellent acrylic or silicone renders are more popular. Example: https://isomat.co.uk/product/marmocryl-silicone-decor-2-en/

I guess using both would be overkill. Not to mention attaching a rain screen with the outer layer of polystyrene would be more difficult.

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> How does that work exactly? Here's how I understand it: I would have to cover an entire building with some sort of a skeleton (layer) of wood, with an air gap in between. And it will cause the rain drops to slide down to the ground instead of the wall absorbing it.

You have external cladding to protect against bulk water, UV; it is generally metal siding, brick, stucco. You then have a >5mm air gap between that cladding and the next layer of your building:

* https://www.buildingscience.com/documents/insights/bsi-001-t...

This is because moisture will either get through the cladding, or the cladding itself will absorb water (brick, stucco), and needs a way to dry.

You don't need a lot of space behind the clapping, but you need something:

* https://en.wikipedia.org/wiki/Rainscreen

* https://www.dupont.com/products/tyvek-drainvent-rainscreen.h...

I think the exterior insulation is kind of taking the place of the cladding or rain screen here. I suspect it also as an air gap. But I suppose you could affix a rain screen to it, although I am not sure if that's worthwhile.

There's some more info here: https://www.designingbuildings.co.uk/wiki/Rainscreen

I hear the term "rain screen" fairly often to describe open cladding styles. Scroll down to "timber cladding profiles" and then "open cladding": https://www.thousandhuts.org/walls Rather than a more closed timber cladding, like shiplap or board-on-board, there would be a gap, maybe 8mm between the cladding boards. Rain can get in, but with an effective membrane beneath, it's waterproof. And there's always ventilation, so it dries out more quickly. If the wall is built for it, the membrane should be vapor permeable and allow the building to "breath".

I'm not sure if this is as appropriate for your local buildings if they have exterior polystyrene etc. Where I am in northern Scotland many of the buildings from the past several decades are timber frame inside + cavity + a wall of concrete blocks. Those concrete blocks are basically the cladding. They have the same renders as you linked on them. While a lot of people are skipping the blockwork and just cladding the building with timber or metal instead.

You basically want something so that if water / condensation / vapor manages to penetrate the siding (and it always will), it will DRY OUT vs absorbing into whatever the next layer is.

As others have described there are wraps. We just used firring strips made out of PVC. Others do it with ripped strips of pressure treated 2x4s.

You SHOULD still do it over the exterior insulation if it is there, so there is a space for things to dry out.

The reason you see old 100+ year old uninsulated houses standing is that, while they were crappy for retaining heat and combatting airflow, they did at least always dry out after they got wet. This is even more important in a modern home where all your materials are preventing that drying out, so you have to plan for it.

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