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Wall panelling advice please.

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Wall panelling advice please.

Postby Doug71 » 10 Jul 2021, 18:57

I'm doing some wall panelling for a customer and am after some advice/opinions please.

It's a large dining room that I am panelling out, I'm basically putting 25mmx50mm tile laths on the walls to straighten things up (dpc between the laths and the walls), covering it all in MR MDF then adding some strips of MDF to form the panels.

It's an old place with solid walls, they all seem dry but one of them is an external wall. I was going to put a coat of external varnish on the back of the MDF in case there was any damp but then I wondered if there was any stick on foil you could buy which might do the trick? Of course then I think well should I just put some Kingspan in the cavity which would also act as insulation?

The walls are really bent so in places there is a 75mm cavity between the back of the MDF and wall. As it's a solid wall I guess it should be allowed to breath although at some point it has been refinished with modern plaster.

I just wondered if people thought it was best to leave a cavity and just varnish the back of the MDF or if I should put some maybe 20mm Kingspan or other foil type insulation in the cavity keeping it off the wall?

I don't want to cause any problems with condensation etc in the cavity.

I originally wanted to build a stud wall with insulation in and a breathable membrane etc but the customer didn't want to lose the space.

Don't suggest using something other than MDF as it's already bought :)

Any advice appreciated.

Thanks, Doug
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Re: Wall panelling advice please.

Postby Mike G » 10 Jul 2021, 19:18

Hi Doug.

I would be cautious of leaving an unventilated void to an external wall. I think the most important thing you can do is to leave a top and bottom vent in each enclosed cavity. I guess that means that if the battens are tight to the wall, you'd need to have a vent top and bottom in each section, or a long continuous vent covering all of them. The alternative, which might be better, cheaper and easier, is to fill the entire gap with mineral wool/ fibreglass, and so be rid of the void altogether. Solid walls.......I'd be wary of introducing a vapour barrier anywhere.

What you do to the MR MDF is probably very secondary. Sealing it sounds problematic, though as that would be a form of partial vapour barrier, and solid walls really need to breath. If you could paint it both sides in a breathable paint that would probably work best with a solid wall. (By solid wall, I assume you mean 9" brickworkwork, rather than cast concrete or solid stone).
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Re: Wall panelling advice please.

Postby Doug71 » 10 Jul 2021, 20:39

Thanks Mike and yes it is a 9" brick wall, might be thicker actually I haven't checked.

Ideally I would be venting it but the panelling will be floor to ceiling so it's hard to incorporate any vents without them looking obvious. The good thing is (kind of) that the walls are really bent so most of the laths are packed off the wall so each section isn't really sealed, I could drill holes through the laths of any that are.

I'm liking the sound of filling the cavity but does that then rely on the MDF letting moisture through as I'm not sure how breathable MR MDF is?

Thinking about it I can probably get some vents in the skirting that will be out of site it will just be venting the top, could it vent in to the floor cavity above if I put some holes in the ceiling? It's an old place and everywhere will be draughty if you know what I mean.
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Re: Wall panelling advice please.

Postby Mike G » 10 Jul 2021, 21:44

As long as there is airflow, Doug, it doesn't much matter where it goes. Just be careful of insects if venting to the loft.

I don't know how vapour permeable it is, but the fact that the data sheet says it can swell up to 25% (depending on thickness) in high humidity situations suggests that it must allow vapour in. Because it is homogenous, it seems likely that if it can get in, water vapour can also get out again. This suggests to me that it is breathable, and that with the right paint you could use it with insulation against a solid wall as described above. If I were you I'd give the manufacturer a buzz and see what they say, because stuffing a bit of fibreglass in there would be a lot easier than ensuring good ventilation.
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Re: Wall panelling advice please.

Postby Doug71 » 29 Jul 2021, 19:37

Only just getting back on to this and still not sure what to do with the external wall :|

I spoke with Medite and they told me the Vapour diffusion factor of the moisture resistant MDF was....

Wet cup 16

Dry cup 25

Doesn't really mean much to me but a bit of googling tells me (I think) a brick is 9.5 so I guess the MDF is not as permeable as the brick :eusa-think:

I have had a couple of 3'x3' pieces of visqueen taped to the wall for a few days to see if any damp appeared behind them, it didn't which I guess is a good thing?

Probably going down the vented route but not sure if it's best to paint the back of the MDF or not?

Sorry for going on, just overthinking out loud!

Here's where I'm at with one of the internal walls

panelling.jpg
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Re: Wall panelling advice please.

Postby AJB Temple » 30 Jul 2021, 10:39

I had a very similar issue to yours Doug, in that I needed to panel (painted MR MDF) a wall that was solid dense blockwork with insulation on the inside and painted render on the outside. I opted for no gaps behind the panelling (filled with rockwool where needed) and it has been free of damp since installation two years ago.
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Re: Wall panelling advice please.

Postby novocaine » 30 Jul 2021, 10:54

where as we put a full height cupboard along one external wall of our living room that effectively segregated it from the house, no back just doors with the wall as the back. 1 year later and I had to add ventilation to the doors as it smelt of damp. I ran the small dehumidifier in there for a few months and it pulled out a few litres of moisture, not much at all but it was just enough to cause that damp smell. we've now had the cavity insulation removed from the wall, damp issues are gone, strangely enough. :)

I'd vent what you are doing, this would mean you don't need to paint the back of the MDF but if the front is painted anyway it's probably worth doing it still.
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Re: Wall panelling advice please.

Postby Doug71 » 31 Jul 2021, 10:58

Thanks guys.

I decided to paint the back of the boards with some breathable paint as guess it can't do any harm and the customer wants to go down the vented route.

Still struggling with venting the top, thinking of cutting the quirk off the mould that will be in the top of the panels and using track saw to slot the MDF, something like this

vented panel.jpg
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Adrian, you did comment in the past when I was planning this that the hardest part is making sure all the joints in the back sheets fall in the right place, I totally agree!
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Re: Wall panelling advice please.

Postby Cabinetman » 31 Jul 2021, 12:59

I can’t think of any way better than that Doug, no doubt that is above eyeIline? But will it be enough? Would you be able to do the same at the bottom?
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Re: Wall panelling advice please.

Postby AJB Temple » 31 Jul 2021, 21:12

I would try to make the vents (now that you have decided to do this) look like shadow gaps. This will require a little design innovation but will be very discreet.
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