Decorating celotex

you most certainly could wallpaper it even if you used PVA to do the job.

If you are going to skim and paint the plasterboard Celotex can you not remove the existing ceiling, fit an appropriate thickness of Celotex between the joists and then put up a new plasterboard ceiling and skim and paint that?

What is the artex currently applied to? Presumably plasterboard or similar?

ISTM that you will need to take down the existing ceiling plasterboard to expose the flat roof joists. Then you can insert thicker Celotex/PIR foam boards into the gaps, and finish with a 25mm layer of insulation board that sits under the thicker insulation and the joists. New PB under that.

For example, when I converted my loft at the previous house, insulating the under tile area used 50mm PIR boards in the gaps between the rafters, then 25mm under that covering the gaps and the rafters, then

Tape the joins on the lowest layer of PIR board to make a good vapour barrier.

You can get insulation boards finished with a fibreglass mesh finish designed for use in external insulation applications. The mesh allows them to be directly rendered / plastered on.

It strikes me as a bit of a bodge, when it would not be that difficult to do the job "properly" and get a better result all round.

It's on plywood, thickness unknown. I don't know to what extent it's part of the roof structure, although thermal pictures do show the timbers behind. However the artex is 1970s and so possibly containing asbestos, hence I'm not keen on disturbing it. Although I should probably drill a hole and poke a camera up there to take a look to confirm what's up there.

Kooltherm K5: 30mm has R=1.4 so that's plausible...

I think cutting out the plywood might make it rather more work to do it 'properly', especially if it ends up disturbing the waterproofing...

Theo

Yes that has been done over so called flame retardant polystyrene where a ceiling curves next to the eaves in this house and its been up for many long years and it cured the icicles condensation and mould issues the original tiny gap to the tiles created. Brian

Which is what I would do too. There are special clips for holding the celotex in place and then an additional 1 inch layer could be used over the whole ceiling.

There is of course that stuff intended for bathrooms which is a slab of extruded poly with a fibreglass mesh reinforced surface on both sides for easy skimming. Not cheap though.

At my age I'd not worry too much about asbestos and Artex:-)

Step 1. Make a hole and see what is up there. Mobile phone camera might do.

Assuming it is not overstuffed with braces, consider cutting slots to allow the insertion of appropriate width Rockwool roll. You need to leave space for ventilation so perhaps 150mm for 195mm joists. I have done this to insulate an admittedly pitched roof. Roll can be cut to width using an old woodsaw sharpened to a knife edge (angle grinder:-) Somebody cleverer than I am may design a flexible device for carrying the Rockwool to the soffit etc.

Replace the cut out bits of ply and screw up your choice of insulated plasterboard.

If you use rockwool in a ventilated cavity you might as well not use it at all

It would be a very strange construction if it were structural. Typically a flat roof would have horizontal joists, with firing strips on top of them (tapered timbers to add a slight slope to the "flat" roof) and then a "deck" on top of those. The insulation would typically be applied under the joists, and the cavity in the roof ventilated. That would be a "cold deck" roof - since the roof timbers would be on the cold side.

Modern designs tend to be warm deck, where the insulation is placed on top of the firing strips and a thin deck on top of that (and roofing membrane/felt on top of that). So all the roof timbers are on the warm side.

What kind of area of flat roof are we talking about?

You can often remove boards without disturbing the coating too much. If it is painted, the most of the Artex is sealed in. So the only risk is where the boards join, as you would need to break the coating there.

The ceiling ought to be nowhere near the waterproofing - that would be on top of the deck on top of the joists.

As you say, a small test hole (damp the area where you drill - collect dust as you go) will tell you. If it's ply, and nailed you would be able to pry off complete lumps of it without much disturbance. Screwed you could probably pull off as well, or try to unscrew - but you would need to find the screw heads (metal scanner or magnet!)

It's a loft conversion, and I think those are often built from the top down: ie cut out the rafters, do the structural frame, put the flat roof on, and then build the stud walls. It depends whether the ceiling was built before or after the studwork. If before, it would mess with the stud walls to try to remove it.

I'm trying to avoid this becoming a 'project', because I already have enough of those. The simpler the better.

It's maybe 30-40m2 in total, although some of that is sloping.

I would hope so, but you never know... (I have no idea when the flat roof was last done, but I'd guess more recently than the 70s)

Theo

Its not uncommon to lay the floor before the stud walls, but less common to do the ceiling (often because that will be PB rather than ply, and so not structurally sound).

When I did one, I put the flat roof up first on struts poked up between the rafters, then chopped out the rafters, and put in the walls[1]. That was all before floors or ceilings... (I had some load bearing stud walls as well)

This page shows the process:

Yup understood. A bit of investigation ought to let you know what the options are.

Depending on materials used, it would be doing well if it has lasted since the 70s :-)

Yup it will be interesting to find out what is there...

OK Point taken. Ventilated doesn't necessarily mean a through draft. Nice if you could get fibre with a seal on both sides.

Rip the ply off and use PIR foam is always going to be better.

Moisture barrier?

Nonsense. The airflow over the cold side of the rockwool will lower the temperature but even with only 50 mm of rockwool you will prvent the wind chill effect of ventilated air in contact with the warm surface.

How do you think polar bears, reindeer, certain breeds of dog and sheep can happily live outside during winter ?.

Late breaking thought on the *flexible fitting device*. Plastic litter picker:-)

anyone who wears a woolly jumper outside without a *windproof* covering will know why I said what I said

works fine when the air is still but once its moving it will strip all the warm trapped air out of the rockwool PDQ

I hope to live another half century, so I worry about it slightly more :)

Yup.

Given the thermal images, I suspect there's already Rockwool up there. I mainly want to stop thermal bridging, and add something to reflect solar radiation. I'm generally happy with the existing roof structure.

Which was where we came in, given the height limitations...

Theo

You can get rockwool contained in metallised/foil "bags".

Not come across that. Presumably for standardised joist spacing. Might as well be glass fibre if there is no exposure.

windproof