Rockwool between colombage vertical beams?

When we renovated our house one wall in the lounge was exposed which had mud and wattle between the vertical beams. The mud was removed as it was in a poor state and we then had the open beams which we thought were a lovely idea. :roll_eyes: Big mistake - draughty and cold! We’ve a temporary solution - thin roof insulation tacked on the back - the silver stuff - and kraft paper on the lounge side which for a while sort of looked ok. It doesn’t now.
I’m wondering whether we could shove rockwool between the beams? But that raises a number of questions:

  • I don’t like fibreglass insulation because of the loose fibres when I cut it, but I’m not sure rockwool would be any safer?
  • would it be possible to use lime render directly onto the rockwool once it’s in place?
  • I’ve seen 100mm thick rockwool advertised on the internet. Is there any thinner? 70/75 mm would be better - most of the beams are about that thickness.
    As always, grateful for any advice / suggestions. Thanks.

You could cut some 50mm thick rigid insulation or insulated plasterboard to go between the beams (whats on the other side?), wedged in or fixed with angled screws, coat with pva and then lime render. Rockwool will not be rigid enough to render, likely it would just make a horrible mess

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We have 100 mm rockwool insulation in our internal walls in a space just short of that depth. It compressed well when we then fixed plaque over it.

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I have no idea whether this would work, but I watched someone build a straw bale house and if my memory has not failed, they cut and stuffed straw bales (the rectangular ones) between joists and then hurled lime mortar at it.

@JaneJones is on the right track. You could try horsehair plaster if you want something ‘authentic’, not sure if it is available in France but you can buy it in the UK. Gives an authentic, rough, depending on your plasterer’s skill, finish. Then paint. Or not.

Cuisine ceiling .pdf (1.2 MB)

I wouldn’t advise you use Carlite Bonding, which has amazing properties and is easy to use, but if it is applied directly to the underside of the floorboards it will fall off after a short time due to the boards flexing.

A narrow passage way which one day will probably be a way into the garage through the house.

Not available in France, thankfully

How narrow and is it internal?

About 90cm. It’s internal but the other wall is to the outside and it goes from the back wall of the garage to a two storey space that used to be a barn (hence draughty and cold).

Sorry Susannah I’m obviously causing confusion using the word “beam” - I probably should say posts. They are about 20 cm apart

Given that the posts are only 20cm apart and if I really pack the rockwool in I thought perhaps I could render it. Cutting plasterboard is quite beyond my capabilities, especially since the gaps between the posts are anything but straight

Doesnt really matter if there are gaps down the edges of the placo because you will fill them with render. If you’re going to render, it should be put onto a reasonably solid substrate which is why directly onto rockwool probably wouldnt work.

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Do bear in mind that Rockwool only gives its full insulation properties when it is not compressed. Once it’s squished it becomes much less effective as an insulator.


I thought the rockwool would have the additional benefit of insulation, which I’m assuming the placo won’t have?

Rockwool slab insulation is used when rendering and a render mesh is emeded into the render for additional strength. This is not the soft loft insulation you may be thinking of.

What about chaux chanvier (sp) Jane? Could then be a diy project.

Other than improved insulation what do you want to achieve?
Do you want to still see the posts?
You suggest the posts are about 3 inches square?
Trying to insulate the post wall and keeping the posts exposed on both sides severely reduces how you can insulate it even with the best insulation avialable as the timber posts have minimal insulation value.
The corridor you mention at 900/3feet wide suggests that it is to get from a to b so I would consider constructing an insulated wall tight up to the posts of about 100 mm thick which would allow the posts to be featured in all their glory on rhe side you prefer which I imagine is not the corridor.
Yes your passage would be 100mm narrower but still walkable.
I think from your previous comments that what I suggest maybe a professionals job rather than something you might have a go at but you never know.

Lime plaster is more reliable when polyfibres are used as horse hair etc breaksdown from the caustic nature of the lime.

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Yoh can get placo with various thicknesses of insulation bonded to it. Easily cut with a saw

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I would dry line it from behind plasterboard on a metal framework put the plasterboard cut to size in first and then the frame afterwords you can screw it from the right size later Another way use tongue and grooved boards behind the beams

Gosh, @SuePJ ! Reading all the posts of excellent advice, seems to me a bit dangerous for you, up a ladder, hands raised, with head back for a longish time. Rather than risk an accident, might it not be best to call an artisan? You are now a well informed client, so wool pulled over the eyes is less likely.

I don’t mean to doubt your abilities and courage but I tend to look for dangers


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I was going to suggest sheeps wool insulation, which you can get on rolls. It’s much less irritating to the skin that other products and easy to work with. We have 500mm of it in our loft. However, if horsehair is not good with lime render then sheeps wool probably won’t be as well.


Thanks for your concern Susannah. I agree, in an ideal world I would always use artisans. Unfortunately we live in a money pit and I’d hoped this might be a simple job I could do myself.

If I were to use sheep’s wool, any thoughts anyone on what I might be able to face it with on the lounge side so that the front face of the beams still show?

This is the piece of wall alongside the bit I want to insulate - it’s rendered

And this is a wall in the dining room which was done sometime in the past - I think it’s probably crepi