Sup.

I’ve got the task of restoring some traditional style trim and panelling on the inside of a large Bay window in an old house.

It’s been stripped back to brick as you can see and it’s single brick, no cavity.

The owner wants it battening and then the trim etc.

I’m concerned about damp effecting things but to be fair I know little about the actual fabric of buildings, I’m just a humble carpenter.

I would use treated battens, and then MR MDF to create the panels before timber trims.

My question to the panel is, should I be using some dpm as a layer between the brick and the new trim?

The owner seems keen that it’s able to breathe.

The batteningv would create a gap obvs, but should a dpm go over them to stop any moisture coming into the mdf/wood?

Thanks 👊

I think you want a Vapour Control Layer between the battens and the internal finish, with insulation between the battens.

If you apply a dpm up the face of the wall then moisture will get trapped on the inside and potentially damage your battens.

I’m only a lowly engineer mind, an architect will be better placed to advise.

When insulating loft conversion external brickwork walls we have had to pin 100mm dpm to tye back of the Battens before fixing to tye wall, as specced by architects.

You need to think where the inside air can get to, in relation to the temperature gradient in the wall. Air in the house could have quite a high actual moisture content, but because it’s warm, still a low relative humidity. If that air contacts the cold wall, it will chill, lose it’s ability to hold moisture and condensation will form. So you need to stop the internal air reaching a relatively cold surface.

The client would probably appreciate as much insulation as you can fit behind the panels. If it’s part of a larger refurbishment, you may be legally constrained to meet current building regulations, which might be tricky.

The client would probably appreciate as much insulation as you can fit behind the panels

I initially suggested it but he thought just battens would be best so it can breathe as I believe there may have been slight damp issues there before…

Man, why do I always get into these cans of worms? 😂

The distance from the brick to the face of the inner window frames is actually only 25mm so not very deep. I’d have to build out and create deeper window frames effectively.

I suspect before it was plastered and that was it.

just battens would be best so it can breathe

I think breathing is probably why it was damp before. It’s breathing the warm air and chilling it.

But, like twistedpencil, I’m only an engineer too.

Is there any way you can mitre up some MDF and fix it directly to the sash boxes? (I’m always a bit wary of fixing batten into skinny brickwork), helps with any cold bridging too.
Below the cill and to either side where it’s solid 9” go nuts, batten out what you like.
I wouldn’t worry about any DPM, any damp problems will most likely be caused by external issues.