Internal wall insulation question

We’re redoing our bathroom early next year.

2 of the walls are external facing and one is very long – we get a lot of issues with condensation on it. As we’ve space to burn, we’re planning to fit internal insulation in the form of 50mm celotex over the existing wall surface with plasterboard over the top.

(For reference – the long wall is external gritstone blocks, cavity (non-insulatable), internal brick, plaster; the short wall is standard double thickness bricks.)

The question is – should the Celotex go straight onto the existing plaster, or should we leave an airgap ventilated into floor/roof voids? I appreciate we’re going to get/move a dew point somewhere, But I don’t really understand where and what effect it will have (or if it will be a practical issue).

Thanks!

It’s notifiable to building control (if you care) as you’re changing the thermal value of >50% surface. I’m not sure 50mm is sufficient to bring up to the regs required. Iirc I needed 60mm + plasterboard for my building construction.

I can’t speak for a Celotex/plasterboard combination, but I’ve just done a bathroom refurb where I also had two external solid brick walls to insulate. Previously they’d just been plastered with tile on top.

I stripped them back to brick and fitted 20mm Marmox Multiboard dot’n’dab fashion (an approved installation method). Then tiled straight onto that which is beautifully easy as it’s a perfect surface. It’s made a noticeable difference to the room in this cold weather. Much less condensation and hopefully less mould, although all the painted surfaces are now done in anti-mould paint, and the fan in the window is new.

Marmox Multiboard

Still got the internal partition wall to finish this weekend… that’s being done with 10mm Marmox boards with 50mm insulation boards in the partition.

Showing off time, I think it’s turning out quite well for a DIY job.

You should obviously consider the ventilation needs of the bathroom separately, make sure you have enough extraction and gap under the door to draw from.

Then you can properly seal the new insulated surface with a vapour barrier of some kind to stop moisture penetrating and condensing behind on the blockwork (which will now be much colder because you’re not heating it from the inside).

We have same problem. Watching with interest.

You need a vapour barrier on the warm (ie room) side of the insulation. So under the plasterboard. Or just use foil backed plasterboard. No need for a cavity, ventilated or otherwise.

The Marmox boards I used are fully waterproof, and once joints and holes are taped with waterproof tape (as you can see in some of my photos) meet the requirements for a moisture barrier. They’re actually designed for wet rooms really.

If you don’t strip off the existing plaster it’s not notifiable work.

I’d use Celotex PL4065 board rather than a seperate board and plasterboard. Celotex technical details require 25x47mm tannelised battens

It’s also not notifiable if you’re not renovating more than 50% of the total area of walls, floors and ceilings combined.

bensales, I’d need to check but as far as I recall the trigger for the works being notifiable was the removal of more than 25% of any elements finish, eg, a room with four external walls has four wall elements and it is the removal of 25% of the finish of any wall element means that wall element needs to comply. When the renewal of a thermal element part of the approved docs came in the  you would be able to take the example and replace the finishes wall by wall and not need to comply with the approved docs (that’s what we advised people to do to get round the docs when I was a building control officer) This loophole was taken out when the approved document was revised

It’s changed, section 10 of Part L

Requirements for the renovation or replacement of thermal elements
23. (1) Where the renovation of an individual thermal element—
(a) constitutes a major renovation; or
(b) amounts to the renovation of more than 50% of the element’s surface area;

Diagram 11.2 defines the ‘element’ as the internal face of a single external wall…if the plaster is being removed.

thanks dooosuk