Advice on insulating ceiling of extension.....

[zoolanda]

Hi All

Some advice would be appreciated on the following. I recently plasterboarded my extension ceiling with the foil back plasterboard under the reccomendation of the buildings inspector. The problem is that on completion of the task and admiring my handywork I realised I hadn't insulated! 'Doh' was said several times.

What do you think my options are, obviously I want to avoid pilling it all down.

It is a ground floor extension, flat felt roof, all walls are to be injection insulated and the wooden floor has celotex type stuff underneath.

Thanks.

[Chris Bacon]

In this situation, I think that you'll have to pull it down to insulate properly. Sorry! Why did the Buildings Busybody recommend foil backed PB?

[sm_jamieson]

You say you are insulating the wall with injection. Will the BCO let you use that for the ceiling ? Otherwise, you'll just have to take down some of the ceiling or roof to get the celotex in. You didn't say its been skimmed yet. At least that's something ! Simon.

[zoolanda]

Youre right, it hasn't been skimmed yet :) What about if I take down the boards at one end and shove rockwool up there and push it down with some rods....? Would this cause any problems?

I guess the one other option I have is to leave it, but does anyone think this would make the room too cold and would the inspector notice? I know this is a cop out and at the end of my options...

[sm_jamieson]

Surely the inspector will not sign off the work if the insulation cannot be proved. Our building services were very fussy about the insulation in my soon-to-be-started extension plans. Note, celotex has a much higher thermal resistance than rockwool. Also, without insulation, you will freeze and/or be skint in the winter ! Simon.

[Chris Bacon]

There's some way of getting Google to include context (useful) in your posting. See other recent posts on the same topic, I don't know as I don't use Google to post.

Anyway, the problem is that there's *supposed* to be a space between the insulation and the underside of the flat roof deck. If you shove a load of rockwool up there, there won't be a space. You might be able to push sized pieces of cavity wall batts along, with some sort of tongs type arrangement, but IIWY I'd remove the boards - they're not expensive, or time consuming to put up - you'll probably lose that much time trying to retro-fit, anyway.

You can always heat it up with something, but that's not really a wonderfully good idea.

If it's all fitted and he does not ask you to expose the insulation to look at it, I doubt it!

I should just re-do it...

[Hugo Nebula]

On 13 Apr 2006 07:18:48 -0700, a particular chimpanzee named [e-mail] randomly hit the keyboard and produced:

Yes. You need ventilation above the insulation. Filling with Rockwool will block this, and, unless your joists are over 200mm thick, the Rockwool won't be thick enough.

Yes.

Unless he can see into the void (ie, through a ceiling rose or downlighter), then no. He might ask you if you've insulated and what with. It would be then up to your conscience whether you're willing to lie.

If you haven't skimmed it yet, your best option is to bite the bullet and unscrew the boards (unless you nailed the boards!).

[Hugo Nebula]

On Thu, 13 Apr 2006 15:10:55 +0100, a particular chimpanzee named Chris Bacon randomly hit the keyboard and produced:

To prevent the passage of moisture through the plasterboard. Foil-backing acts as a vapour barrier.

[Chris Bacon]

But where's the need in this application?

In any case I've never heard of "normal" moisture passing through plasterboard being a problem.

[zoolanda]

Everyone - thanks for the respones. The boards are on their way down as we speak, I did nail them so 'double doh'. !!

One more thing you can help me on. I know Celotex is pricey and in any case the drawings specify 1200mm thick celotex, and I can't find any reference to that. Is it a typo? As the roof is made of 8x4 joists so fitting 1200mm slabs would leave very little ventelation space.

I have a local B&Q, Jewsons, Builders Centre and Wickes. Can anyone recommend a suitable insulation slab brand from any of these that won't break the bank? (I have already been to B&G and they don't have Kingspan)

Best S.G

[zoolanda]

"the drawings specify 1200mm thick celotex,"....I'm an arse, it was

120mm not 1200mm Celotex.............. so alternative's welcomed.

S.G

[zoolanda]

Me again - what about a warm roof type thing? Would this be of comparable cost and of comparable efficiency? The boards are still up (i know I said they were coming down but I am pausing for the night). Advice welcomed.

[Mathew Newton]

For Celotex/Kingspan I can highly recommend

- they sell 'seconds' which in my experience have been practically as good as 'firsts' - and very cheap too.

I don't know where you're based but they're in Powys - however I believe they may also do nationwide delivery from the back of a lorry.

Mathew

[Mathew Newton]

For Celotex/Kingspan I can highly recommend

- they sell 'seconds' which in my experience have been practically as good as 'firsts' - and very cheap too.

I don't know where you're based but they're in Powys - however I believe they may also do nationwide delivery from the back of a lorry.

Mathew

[Chris Bacon]

'Kinell.

8x4?? You sho'?

Are you a bit pie-eyed, S.G.? I are slightly. If you've got all that room, then Jablite is cheap and will do the job well enough.

[Chris Bacon]

Ooops. Part of the fun of not reading the whole thread first.

120mm of Celotex? Wow! You could do "enough", cheaper.

[John Rumm]

1.2m would indeed would be a rather well insulated if small room!

Polyisocyanurate Foam or PIR or Eurothane Foam are all names for the same sort of board. Typically it is foil covered on both sides and has a very high insulation value for a small thickness. Other well known brands of similar stuff include Kingspan and Ecotherm.

Look for a local supplier of seconds if at all possible. Failing that try one of the online suppliers like :

prices charged by builders merchants for this stuff is silly (one quoted me nearly 30 quid for a 50mm thick sheet), got it from a local supplier for less than half that in the end.

[Hugo Nebula]

On Thu, 13 Apr 2006 18:39:48 +0100, a particular chimpanzee named Chris Bacon randomly hit the keyboard and produced:

BS 5250, S8.4.3.2.2, "A cold deck roof is shown in Figure 22. This type of roof should be avoided because interstitial condensation is likely and its effect on the structure and insulation can be severe... The vapour control layer should have a resistance of at least

250MN·s/g, and should have sealed laps to preserve the integrity over the whole roof. Gaps in the ceiling should be minimized and service openings should be avoided; if they cannot be avoided it is essential that they are sealed".

A foil-backed plasterboard is the very least that is needed, and strictly speaking is not nearly enough.

Then you've never lived. Or at least never lived outside of Arizona, or had anything to do with buildings.

[Chris Bacon]

[snip]

So if "strictly speaking" it "is not nearly enough" (your words) then why is duplex plasterboard allowed, and why aren't the insidious effects of *not* having a vapour barrier, or only foil-backed plasterboard, which *you* say is "not nearly enough" not seen extremely frequently? Answer: it is a non-problem.

That is a particularly ignorant statement. I have never seen or heard of heard of a problem caused by vapour passing through plasterboard in the UK.

[zoolanda]

Thanks again all. I have been lucky in that my brother in law acquired me 7 sheets of Celotex from a building site he works on today. Some are cut, most are slightly imperfect but all are usable so I only need to actually purchase one sheet from Jewsons to cover the entire area.

**here is the question** Can I add 50mm of wickes polystyrene sheet above this? The only reason I say that is because I have the 50mm polystyrene sheets lying around and will not use them otherwise. I will still maintain an airflow above and below the insulation. I know the polystyrene sheets cannot come into contact with wiring. Is this a fire hazzard?

Thanks.