Celotex versus multifoil

[Cath452]

Hi, I have spent a while reading these forums and learnt so much already. I am aware that there are lots of discussions about the values (or not) of multi foil insulation, but I am hoping someone might be able to give me a bit of advice on the following question.

I have a cottage which was built with a loft conversion. This was in a pretty poor state so I spent a fair few hours pulling down 200 year old lath and plaster ceiling and am now left with the shell of the roof. Thankfully all in OK condition according to the engineer and BCO.

I do have a limitation on space for insulation as I want to keep some of the original beams visible. The BCO suggested I use the super 10 or Thinsulex (he said that whatever insulation I put in would be better than what was there before) however after reading the forums the value of these multifoils seems questionable.

My question is - which would be better:
1) Multifoil (after air gap) / batton/ plasterboard then plaster or
2) just 50mm of celotex/ kingspan with plasterboard attached and then plaster?

These two options would still allow the beams to be visible but obviously I want the best insulation.


Also a second question, if I go for the multi foil which is considered better
the triso super 10 or something like Thinsulex. (Thinsulex seems considerable cheaper than the triso stuff)

Many thanks in advance for any comments / suggestions on this.

 

[freddiemercurystwin]

The foil insulations are pointless. A few years ago they appeared to have some space saving advantage but then after some big hoohaws (ie telling porkies as to how well they worked and being sussed out) and a refusal by Building Control to accept them it lead to them requiring an airgap above and below, rendering them, as an insulation system, no thinner overall than conventional PIR's such as Celotex.

In short there is no advantage to using multifoils.

 

[freddiemercurystwin]

BTW it will be cheaper to buy separate insulation and foil backed plasterboard rather than insulated plasterboard. Though this will be more labour-some but then that is presumably not an issue if DIYing.

 

[Cath452]

Many thanks for your comments. So I think the outcome will be I will go for the 50mm of celotex. Have just been up to the loft and remeasured, it should work out just about fine. (Just hope the BCO says it will be OK instead of the multi foil).

Thanks for the top tip on plasterboard - I am getting a builder to put it in as I am having a dormer window put in at the same time and he is going to put the ridge vents in, so decided to get it all done at once. (There's so much else to do on the house, I'll keep myself busy!)

Many thanks for your advice.

 

[geraldthehamster]

Many thanks for your comments. So I think the outcome will be I will go for the 50mm of celotex. Have just been up to the loft and remeasured, it should work out just about fine. (Just hope the BCO says it will be OK instead of the multi foil).

Thanks for the top tip on plasterboard - I am getting a builder to put it in as I am having a dormer window put in at the same time and he is going to put the ridge vents in, so decided to get it all done at once. (There's so much else to do on the house, I'll keep myself busy!)

Many thanks for your advice.

Remember that if your roof is felted, and if this is not a breathable membrane, you need a 50mm vented air gap between it and the back of the insulation.

Cheers
Richard

 

[RJRock]

geraldthehamster if your roof is not felted, in my case slate onto baten, is the baten thickness ok before celotex between joists?

 

[geraldthehamster]

I believe so