insulateing solid brick walls

[veloman]

Hi guys, I have removed all the plaster from 2 ground floor rooms ready for the outside walls to be insulated ,ie 25mm battens,celotex T break 50mm board,then 12.5 mm plaster board over,my question is, do I put any thing in the gap between the wall and the celotex,the plasterer says he puts 30mm of fibre glass in this space to remove any air gap,(under instruction from his gaffer),and the damp man said to leave it as an air gap,the damp man also suggested foil backed boards for the internal walls,as a belt and braces approach, thanks for your indulgance

 

[Deluks]

Damp man is correct, now pass him a towel.

 

[veloman]

I just new that was coming.........thanks for that

 

[ajrobb]

All is not lost. Celotex has low emissivity coatings, so a ventilated air gap will have some extra insulation - possibly an R value of 0.46, which is about as good as 20 mm of glass wool and probably just as good as squeezing 30 mm of glass wool down to 25 mm.

 

[veloman]

thanks for that AJ,so the celotex T break is ok for the job,we are using the T break board, as we have 2675mm ceilings and can use a 2700 p/board to cover so we will have no horizontal joints in the face,thanks again

 

[ajrobb]

thanks for that AJ,so the celotex T break is ok for the job,we are using the T break board, as we have 2675mm ceilings and can use a 2700 p/board to cover so we will have no horizontal joints in the face,thanks again

To be honest, I'm not sure about 'T break', I am assuming it is like CW3000.

According to Proctor Group's "Reflective Technology" brochure, they claim that a sealed 19 mm service void behind the plasterboard will add another 0.644 to the total R value, which is equivalent to an extra 15 mm of Celotex. However, this is a 'brochure figure' for their own reflective foil, Reflectatherm, and not for Celotex. It's worth a punt though for the cost of 19 mm room space and some wooden strips - plus you get a handy service void.

 

[veloman]

I had a look at the procter stuff ,but to be honest the figures went over the top of my head , I thought the T break was the stuff to use,so could some one confirm that it is ok for the purpose of wall insulation,I dont want to buy 40 sheets then find out its not quite the stuff for the job,thanks

 

[ajrobb]

I had a look at the proctor stuff, but to be honest the figures went over the top of my head. I thought the T break was the stuff to use, so could someone confirm that it is OK for the purpose of wall insulation. I don't want to buy 40 sheets then find out its not quite the stuff for the job, thanks

A quick look at the Celotex site shows that T break (TB3000) is meant for treating thermal bridging. It is only available up to 45 mm. It might not be strong enough to mount across battens. Look at GA3000 instead - it has a reinforced core. All my previous comments apply to GA3000 too.

Thin TB might be ideal for insulating reveals to prevent condensation.

The point with the Proctor figures is just that you can get the equivalent of an extra 15 mm 'free' Celotex by installing a 19 mm service cavity between the insulation and the plasterboard.

 

[ajrobb]

Looking at Celotex's Solid Wall Internal Insulation application brochure, you choose TB3000 or GA3000 based solely on thickness. Up to 45 mm thick you use TB3000. 50 mm and thicker you use GA3000.

 

[veloman]

AJ, I only said T Break 50mm because there is a firm advertiseing it on the net at 50mm and it was a very good price,I shall get more details before I order it

 

[ajrobb]

AJ, I only said T Break 50mm because there is a firm advertiseing it on the net at 50mm and it was a very good price,I shall get more details before I order it

Maybe I have it wrong. I guess T break is TB3000 but Celotex say this is currently only available up to 45 mm. However, there are plenty of search hits for "Celotex TB3050", so I guess it must be old stock - nothing wrong with that if the price is right.

What do you think of the 'service void' suggestion?

 

[veloman]

the way we were going to do it, ie 25mm laths on the wall celotex over and then board over the top gives a void of 25mm ish behind the celotex,or did you mean something else,sry if I've missed your point.

 

[ajrobb]

the way we were going to do it, ie 25mm laths on the wall celotex over and then board over the top gives a void of 25mm ish behind the celotex,or did you mean something else,sry if I've missed your point.

Using the R values from Proctor's "Reflective Technology" brochure and assuming the cavity values apply to Celotex.

1. With a cavity on both sides of the Celotex, you get a U value below 0.3 (Scottish regs):

Untitled

• ajrobb
• 4 Mar 2010
• 1

2. Without a service void, you get a U value above 0.3 but still below 0.35 (British regs):
View media item 20265

 

[ajrobb]

The Celotex application brochure for "Internal solid masonry wall applications" specifies that battens should be put between the Celotex and the lining surface (plasterboard) - so you get a service void. Also, by following the manufacturer's application brochure, you keep the building inspector happy.

I don't think Celotex by itself would be firm enough to stop the plasterboard joints moving/cracking. Alternatively, you might use 2 layers of 12.5 mm plasterboard directly against the Celotex with joints staggered by 600 mm. (Battens would be cheaper.)

Celotex also recommend the joints between and around the Celotex boards are taped to complete the VPL.

 

[veloman]

I have just read the celotex pdf again and i stand to be corrected,but the way I read it you put the celotex against the brickwork and the battens on top of the celotex then apply your plaster board to the battens so your air gap is between the plaster board and the celotex,ie air gap on the warm side of your insulation board