Celotex - is it a swear word?!

[CatherineB]

Hello folks

Many thanks for the responses to my sash window question - glad to say I've had a recommendation from new neighbours for a jolly good local joiner, which is what you guys said I needed, rather than a "sash specialist".

Now for my next dumb question...

We need to replace the slate roof on our 1904 house, and will need to put some decent insulation in at the same time. The main part of the house has bedrooms in the roof, so there is a small eaves space either side and quite a lot of sloping ceiling, which will obviously need a fairly thin but effective layer of insulation. The back section of the house has a sloping roof with unused loft beneath, above a flat bedroom ceiling.

I'm having the name Celotex bandied about quite a bit. With my complete ignorance on all building matters, both historic and current, can I ask for a quick straw poll as to whether installing this is a sin, or a jolly good idea?!

Many thanks!

 

[MdB]

Hi Catherine, welcome to the forum.

I've fitted a load of Celotex into my house as have other here I believe. It has its place but it also has areas where it shouldn't be used.

Pros:
Celotex performs roughly twice as well as many other types of insulation for the same thickness (i.e. rockwool, sheeps wool etc.) and is of reasonable cost. It also a solid sheet which can be easily cut and fitted. As it is solid, moisture can't condense in the middle of it causing loss of performance.

Cons:
It is made of plastic and it will create a non-breathable layer.

In your application, you are working on the roof. The danger here is having two non breathable layers (insulation and roofing felt) with wooden battens in between. If moisture gets in, there is a danger of the wood rotting which is why an air gap above the insulation is normally a good idea. However, as you are redoing the roof, you can have a breathable felt fitted (which I think is standard now) which removes this issue.

I would therefore say in my (not very expert opinion) that a layer of Celotex is a good idea. Make sure that the gaps between the Celotex boards are taped with the right sort of tape as this can make a very big difference to its performance.

 

[CatherineB]

Thanks MdB! There is currently no felt at all under the slates, so we will definitely be able to make sure whatever is put in is the right sort - I will write "breathable felt" on my list of knowledgeable phrases to exchange with builders :wink:

 

[christianb]

Hi Catherine,

Celotex, Kingspan, Ecotherm, Recticel... all much of a muchness. And they do perform very well.

Incidentally, try typing the following into a search engine of your choice "kingspan" "seconds" "co" (so I don't sound like I'm doing some kind of advertising) and save yourself a fortune.

 

[Penners]

dumb question...

Catherine - we don't have dumb questions on this forum. Some of them concern simpler matters than others, but they're all interesting and worthwhile.

I echo everything that MdB has said. Polyisocyanurate foam insulation batts (impress your builder with that phrase! :wink: ) like Celotex and Kingspan are valuable additions to the energy efficiency of a house - provided they're properly installed, of course. They're probably the most efficient form of insulation available, thickness for thickness.

So, provided adequate provision is made for ventilation, as appropriate the structure around the insulation, I wouldn't hesitate to use this form of insulation myself.

 

[CatherineB]

@Penners, I don't think I have room on the back of my hand to write that down, but thank you all for the tips!

 

[Penners]

Recticel

I've always thought that that's an unpleasantly proctological-sounding name.