insulating a room, part 2

[Fred]

Hi,

I made a start at redecorating a bedroom over the Easter bank holiday and you may remember I asked about insulating the exterior wall with celotex. I'm wondering if I have misunderstood what I am supposed to do.

I was hoping to cover the celotex with plasterboard and for this I would need a frame with studs at 40 or 60 cm. I was thinking that I would cut the celotex into smaller oblongs and fit them into the rectangles between the studs and noggins. However, it occurs to me that doing it this way there will be an area of the wall that is covered in wood rather than celotex, and if I don't cut the sheet accurately, there may be small gaps between the wood and celotex.

I am wondering whether you meant for me to fix the celotex to the wall as one big sheet and then put the frame for the plasterboard in front of this. The advantage of this method is better insulation but the disadvantage is that it will eat up more of the room and it is a box room to begin with, so I would like to lose as little as possible. If the celotex went inside the frame, I would use less space.

What is the best way to proceed?

TIA

[The Natural Philosopher]

Correct. That's what is known as a 'cold bridge' that conducts heat past insulation via structural members.

In reality the gaps are not important. The air is the insulator, and the celotex is just a way of trapping air so that convection doesn't act to use MOVING air as a a heat loss path.

You should actually be using foil tape over the battens/studs and the celotex to hold it all together and totally seal it from a water vapour and microdraught point of view anyway.

You may use expanding foam to full any large vertical gaps. Horizontal are a bit less important. Stopping convcetion is the name of the game.

The advantage of this method is better insulation but the

As you are. However the smaller the studs, the less the cold bridging,

[sm_jamieson]

When I did my bathroom, I had no studs, and fixed cement particle board to the wall using frame fixings through the celotex. However, I'm not sure I would have fixed plasterboard that way since it is a lot weaker and needs a higher number of finer fixings. Simon.

[Bruce]

I'm sure others will disagree, but I think the best way to proceed is to use Celotex PL3000 which is Celotex insulation bonded to plasterboard.

The product page is here:

is also a .pdf datasheet accessible from that page but only if you have registered with Celotex. That's annoying, but it is well worth the small effort to register IMHO. Others will disagree.

The suggested method of fixing Celotex PL3000 is dot and dab so this helps to minimise the reduction in room dimensions. The plasterboard has tapered edges so getting a smooth finish should be less of a problem than it might be.

[The Natural Philosopher]

Its a good product, but does it come in e.g. 50mm thickness?

Also, its a good product BUT there is NOTHING substantial left to mount interior fittings to.

[Andrew Gabriel]

I did the back of a fireplace which was only a half-brick wall, in order to reuse the fireplace as a display area or somewhere for the HiFi, etc. Not sure this would work on a large scale, but what I did was prop/jam a piece of celotex/kingspan in place, hold the plasterboard in front of it, and hammer long nails right through the plasterboard to mark the celotex/kingspan. Then removed the plasterboard, drilled through the celotex/kingspan nail holes into the wall and plugged. Finally, put the plasterboard back in place, and screw through the nail holes and celotex/kingspan and into the wall plugs using long plasterboard screws. Finally skim the plasterboard.

I already had the celotex/kingspan left over from something else. Might have considered the combined plasterboard otherwise, but it's not cheap.

[Bruce]

Yes, it comes in the following range of thicknesses:

PL3025 25 + 12.5 PL3040 40 + 12.5 PL3055 55 + 12.5 PL3065 65 + 12.5

In each case the first figure is the thickness of the Celotex insulation in mm, then the 12.5 mm is the plasterboard.

You can drill through into the brick wall.

I'm considering using it in my Victorian terraced house.

[John Rumm]

Another rough and ready solution, is to dot and dab expanding foam on the back the celotex and stick it to the wall, and then repeat with the plasterboard.

They make a special low expansivity foam for the purpose, but the ordinary stuff also works as long as you restrain it in place while it cures.

[stuart noble]

In a small room I'd use the 25mm. Even that thickness makes a huge difference. If it's an old house with an inch of plaster everywhere, I'd hack that off first.

[js.b1]

...

...

Method-1

- Battens + Celotex + PB over the top

- Cold Bridging occurs across battens

Method-2

- Shallow Battens + Celotex + Celotex over top + PB over top of that

- No Cold Bridging Occurs because battens have Celotex over them

Method-3

- Dot-n-Dab Celotex+PB "composite" to cavity wall

- No insulation maker advises dot-n-dab with solid-wall

- Reason #1 is damp causing the dot-n-dab adhesion to fail

- Reason #2 is condensation in the interstitial gap around dot-n-dab

You need foil tape over Celotex joints. You need scrim tape over PB joints.

Tommy Walsh on Channel 38 recently did a ?garage? and bitumen the walls (tanking seal), then batten, the celotex, then PB. I can't recall if he put Celotex over the battens to avoid cold bridging, it comes down to calculations since you assume 5-7% loss re battens and add more Celotex to meet BR.

Obviously for certain rooms you can not achieve BR figures, that is normally only an idiot with a loony BCO - some are well gone, but thankfully most are not.

[js.b1]

Typo... ovens beeping :-)

Obviously for certain rooms you can not achieve current building regulation U-values, but if presented by a loony BCO the phrase is "reasonable". Some BCO are well gone, but thankfully most are not and actually helpful subject to over-reaching demands, limited budgets, limited manpower & insufficient hours. For example it is reasonable to remove an architrave in order to go from 16mm to say 37.5mm wallspace (25mm PIR & 12.5mm PB), but not reasonable to remove or limit an existing doorway.

For decades people have insulated without BCO involvement, it would be impractical to pay the not inconsiderable fee for every single room in the house and some people do go on to insulate internally even with cavity wall insulation when redecorating if browning/plaster is blown severely (50-120yr old house). Realise certificates are legally worthless, a house may have a certificate for X Y Z, but they imply no legal redress against the council whatsoever if works do not comply - no matter how outrageous the non-compliance.

One thing not mentioned - window reveals. If you insulated much more than 10mm beyond original plaster thickness you will need new deeper window-cills. The side & top window reveals should really be insulated - typically

12.5-25mm polystrene is used which can be grip-filled in position and will take a plaster skim once PVAd adequately. Marmox is superior for this application in that it is very thin, completely waterproof (unlike expanded polystyrene) and takes plaster directly - but costs far more.

Remember to tape the doors if the original plaster is browning - some of the dust is almost smoke-like and will convect throughout the house for days, depositing a film after you think you have cleaned up :-)

12.5mm insulation is only just noticeable, but 25mm is very noticeable

- the cold draught off a wall is gone and it heats much more quickly even with body heat (for small rooms). I recall 20mm Marmox (equivalent to 13.5mm Celotex) is 45% heat loss reduction on a cavity wall, which is quite astonishing and very noticeable.

[Fred]

I didn't know that, thanks, I will do. Any reason why it has to be foil tape? Won't any waterproof tape do?

What are the smallest studs I can get away with? I was thinking 2"x2"

Am I right to think they need to be at 600mm spacings? What about noggins: would you fit those every 600mm too or are they positioned more for shelf/tv/etc mounting in the future?

TIA

[Fred]

I thought 50mm was the recommended thickness. I'll see what I can get. I think that "plain" celotex is much more readily available and cheaper than the plasterboard/celotex variety.

Hacked off in anticipation already; that's was my bank holiday Monday.

[Fred]

Yes, I had already thought of that.

I thought polystyrene wasn't liked because of its inflammability, or is it ok when buried under plaster? Why can't you grip-fill an off cut of celotex into place instead?

How solid is the polystyrene once skimmed? Will it take a few knocks or will the plaster break and the polystyrene dent?

It was (browning).

The gap around the door was sealed for that reason. I've learned from my mistakes in the past;)

[js.b1]

You can get F/R and non-F/R forms of Polystyrene, PIR foam etc. The layer of plaster is material however.

You can - but watch thickness because you need to plaster over it :-)

You have to PVA it so as to ensure the plaster bonds well. Then a few millimetres of one-coat plaster are surprisingly tough. Try ripping a cable buried in oval with 6mm one-coat plaster coverage, it's hard work.

[JimK]

"Is it possible to apply the Kingspan Kooltherm K17 Insulated Dry- lining Board onto an adhesive dab system to the inside face of the solid 9 inch brick wall? Yes. Solid brick walls can be penetrated by rain, depending on exposure and type and condition of the bricks and mortar. It is with this in mind that we would not recommend the use an adhesive dab system but instead that Kingspan Kooltherm K18 Insulated Dry-lining Boards are applied to either a timber batten or metal furring system in order to create a cavity between the boards and the brickwork skin."

"a cavity" which presumably would be your interstitial gap you reckon is a prob?

...in any case the boards are supposed to be fixed (after dabs set) by a couple of frame fixers into the wall at say 6 or 7 foot high - so if it all starts burning you stand a chance of getting out before they start falling off the walls whilst burning - apparently.....so, they would be mechanically held up *even if* all the dabs failed...

Cheers JimK

[stuart noble]

A trade off between heat lost and space lost I suppose. I doubt you'd notice the difference in comfort levels between 25mm and 50mm.

Still coughing then?

[Bruce]

That's what I would go for. It's a small house, with a narrow downstairs room (2 knocked into 1) and we couldn't afford to lose more than 75mm (25 + 25 +[2 x 12.5]) across the width. We are mid-terrace and both long walls are party walls. I'd like to use something thicker on the front and back walls, which are exterior walls.

Next door neighbour did that. He used a professional plastering company and they knocked off several pieces of our plaster. Not good.

[js.b1]

Yes, the cavity will stop moisture tracking across...

Yes, if the dew point moves out of the wall into the air gap. It is most likely with rendered single brick, where water loading can be considerable in exposed conditions.

Treated timber battens as opposed to untreated are usually used for this reason.

[chris]

Dear Fred

See the link

I have cut and pasted

4) Walls either external or internal

Internal is favourite as Victorian semis are normally nice brick outside

Be prepared to lose 2.5" of every external wall if you do this I did it and it has been brilliant in performance I ignored the advice to put on battens as I wanted the space I took off all architraves, skirtings, picture rails etc. and lifted floor boards at edges using plasterboard adhesive and a serrated trowel prepared the wall and the 2" Kingspan TP 10 PU foam sheet of 8 x 4 and fixed it to the wall direct using 5 No EXTERNAL insulation plastic and stainless steel fixings designed for the outside insulation fixing one at four corners and one in centre together with the glue applied vertically on one surface and horizontally on the other (like a tile). Did the same but staggering the plastic washer s/s pins at other centres for 3/8" plasterboard (also staggered joints) scrimmed and skimmed the lot with finish Put back all the joinery timbers with 5" long fixings or plaster board fixings depending on need for load put back floor boards Bob's your uncle