Insulation - can I do this?

[TonyRPH]

The initial message was deleted from this topic on 20 May 2022 at 16:54

[dasbimmerowner]

I did just this in a dormer property I used to live in. The downside was that I lost about 3" of ceiling height making the roof seem lower due to the insulation/second layer of plasterboard. Previously I'd just had plaster board > void that vented into the loft area > chipboard with roofing felt above the bedrooms and it was baltic in winter months even with the heating cranked up. The work did have the desired effect though and made the rooms considerably warmer.

EDIT: No idea if it's good practice or not

[Lotobear]

Ideally the void should be ventilated but I've done this before without any ill effects.

you can reduce the risk of interstitial condensation by finishing the plaster with two coats British Gyspum dry wall sealer which has some vapour resistive properties.

I would use one of the British Gyspum thermal laminate products - quicker and easier and they (or some of their products) have an 'inbuilt' VCL

[wolfracesonic]

‘Can I do this?’ Yes you can! Fix your celotex, then fix a ‘vapour control barrier’ (plastic sheet) over it then plasterboard+skim. Screw fix and Toolstation sell the vapour barrier, it’s usually green, so you can’t see through it to see where your fixing Rule is vapour barrier to warm side of whatever insulation you have. Insulated plaster boards are available as Lotobear said, some have an integral vapour barrier, they’re a bit spendy though.

[Hornsey]

As far as I'm aware the foil on foil faced PIR insulation boards is classed as a vapour barrier. Just make sure you fully tape all the joints with the proper aluminium tape. The aim is to stop any warm air getting through to the cold surfaces where it will form condensation. Otherwise use the plastic vapour barrier and again tape the joints.

[Aluminati]

Hornsey said:

As far as I'm aware the foil on foil faced PIR insulation boards is classed as a vapour barrier. Just make sure you fully tape all the joints with the proper aluminium tape. The aim is to stop any warm air getting through to the cold surfaces where it will form condensation. Otherwise use the plastic vapour barrier and again tape the joints.

Correct. Although dependent on rafter height, i would fit a thinner insulation between, flush with rafter and foil tape over the rafter onto the foil of the insulation.

Should give some ceiling height back and don’t think cold bridging will be a major issue.

Could put an insulated plasterboard over the lot if it was ?

[sunbeam alpine]

Sorry if it's a really stupid question (I'm not a builder), but if you're having to put in a new ceiling below the new insulation, whu not just remove the existing ceiling and insulate the void?

I'm just going on your drawing with zero knowledge of building practices!

[eps]

sunbeam alpine said:

Sorry if it's a really stupid question (I'm not a builder), but if you're having to put in a new ceiling below the new insulation, whu not just remove the existing ceiling and insulate the void?

I'm just going on your drawing with zero knowledge of building practices!

void usually needs to be around 50mm

[sunbeam alpine]

eps said:

sunbeam alpine said:

Sorry if it's a really stupid question (I'm not a builder), but if you're having to put in a new ceiling below the new insulation, whu not just remove the existing ceiling and insulate the void?

I'm just going on your drawing with zero knowledge of building practices!

void usually needs to be around 50mm

The drawing suggests an existing void of 39cm - that would still allow for 30cm+ of insulation. Isn't that enough, or have I misread it? (39mm instead of 39cm)

[sunbeam alpine]

TonyRPH said:

sunbeam alpine said:

Sorry if it's a really stupid question (I'm not a builder), but if you're having to put in a new ceiling below the new insulation, whu not just remove the existing ceiling and insulate the void?

I'm just going on your drawing with zero knowledge of building practices!

It's not a stupid question at all. I'm not a builder either, however I'm just looking for the easiest, cleanest way to do it, and this seems to be just that.

Otherwise we'll end up dragging down 40+ years of muck / dust / who knows what into the room!!

I did exactly this about 10 years ago when I was renovating and extending my house.

It's amazing how much crap accumulates in that sort of space - but very fulfilling to pull it down. In your place I'd bite the bullet and remove the ceiling. It was actually quite fun!

P.S. We bought some quite expensive masks to do it.

[Oakey]

Besides which, what are you going to fix the celotex and new plasterboard to? The existing plasterboard ceiling? Will it hold the weight?

[eps]

sunbeam alpine said:

eps said:

sunbeam alpine said:

Sorry if it's a really stupid question (I'm not a builder), but if you're having to put in a new ceiling below the new insulation, whu not just remove the existing ceiling and insulate the void?

I'm just going on your drawing with zero knowledge of building practices!

void usually needs to be around 50mm

The drawing suggests an existing void of 39cm - that would still allow for 30cm+ of insulation. Isn't that enough, or have I misread it? (39mm instead of 39cm)

It's probably 'okay' but usually you would counter-batten off the rafters to make up the extra 11mm, but you may well lose too much headroom. Manufacturers recommendation - I would suspect that some moisture would build up over time if below 50mm due to interstitial issues. It's worth reading the Celotex page(s) on this to work out what is required.

[Jambo85]

I believe an alternative is to insulate from the top when your flat roof covering needs renewed?

[dhutch]

Unless you almost want the ceiling in that area lowered, I would definitely consider taking the old ceiling down first, assuming its plasterboard its no more messy than plastering the new one will be and you save the height. Move things and sheet over like you were decorating, sheet over the carpet in that area, won't be more than a car boots worth for a window.

I did my sloping skelling sections this way, rafters where only 3x2 so I did 50mm between, 50mm over the top, plasterboard, skim. As you have around a foot you could do 100mm between easy, maybe 25mm over the top to minimum 'cold bridging' effects, plasterboard and skim. Else if cost was an issue you could even do 200mm of glasswool/rockwool and then 25/50mm over the top.


Daniel

Edited by dhutch on Wednesday 5th February 11:10

[chadders74]

I'm not trade, just somebody who's done the smashing and removing, then paid for making good.

Do not under estimate the amount of crap pulling a ceiling down makes, I would have hated to see the mess in a decorated room. Also the damage from large bits of plasterboard that suddenly drop, it's surprisingly heavy. You might need a skip, you're then looking at the cost of celotex or whatever, cost to fit, then plasterboard and skim. One room cost us a few grand all done, but the moment the insulation went in you could feel the warm layer (you stepped down into the room).

Ask a roofer about going in from the top if the ceiling is OK - ours was water damaged so replacing worked out better. You never know what's cheaper until you ask.

There is also insulated plasterboard but as someone has mentioned, not sure of the extra weight hanging that on a ceiling.

Maybe thinking differently, can you get access from a different point (outside side in) and stuff insulation up that way?

[DozyGit]

1. Remove ceiling
2. Measure gaps, assuming 100mm timber, get 50mm celotex style insulation
3. Cut and fit into gaps with projecting screw at 51mm mark to maintain air gap (you can put a vent on soffits if there is none)
4. tape/foam gun everything in place.
5. If head height an issue use normal plasterboard, if not buy insulated plasterboard 25mm or so
https://www.wickes.co.uk/Knauf-XPS-Laminate-Plus-I...

and then screw on joists, you can add a vapour barrier (if you are not sure of the taping etc )

6. Tape joints and get it plastered.