Roof Insulation

Hi all,

We've just had the details through of what our architect is proposing for the wall insulation and roof insulation and we wanted to see what your thoughts are.

We have given him the brief that we want the house to be as low energy consuming as possible.

He has proposed:

Cloisons non porteuses à panneaux à parements de plâtre sur ossature galvanisée

a. Cloisons placostil 98 mm
Cloisons épaisseur 98 mm ,ossature 48 mm , laine de verre 45 mm , parement 2 faces 2 plaques de
plâtre épaisseur 13 mm , isolation acoustique R = 48 dB(A) , isolation thermique K = 0,70 W/m2 °C .

b. Cloisons épaisseur 72 mm
Cloisons épaisseur 72 mm, ossature 48 mm , laine de verre 45 mm , parement 2 faces 1 plaque de
plâtre épaisseur 13 mm, isolation acoustique R = 40 dB(A) , isolation thermique R = 0,71 W/m2 °C .

Doublage panneau plâtre sur ossature métallique.
Panneau : panneau constitué d’une plaque de plâtre vissée, la plaque de plâtre aua une épaisseur 13
mm avec armature en carton spécial et bords longitudinaux amincis pour permettre le traitement des
joints sur la face vue avec une laine de verre de 100 mm.

c. Isolation en plénum :
Par matelas de laine de verre avec pare vapeur épaisseur 24 cm déroulé sur le plafond rampant.

I know that fibre glass is the traditional method of insulation but I know there are lots of new technologies out there and I recall one SFN member recently talking about multifoil but I've read some differing views on this - though I do like the idea of not sleeping beneath fibreglass. For the roof space we will have our bedroom eventually up there so the thickness is an issue and the thinner but more efficient we could get the better. Can anyone recommend any options other than 24cm fibre glass?



Sorry Brian, I have yet to find any document that rates rockwool at twice the thermal rating of fibreglass unless Aussie rockwool is different they are about the same. Rockwool is better in a fire so that should be a consideration. For twice the thermal value you have to go to some type of foam and they are not good in fires either.

Just one of many many comparisons available from the web.

Hi Suz

As an Ex Aussie builder, I've gone for rock wool, as it has nearly twice the thermal rating of fibreglass for about 50cents per square meter more without the 'itch' factor, besides once it is installed and all sealed, there should be no issue with what is up there.



Let us know how you get on, I have a pile of research on the multifoils and finally the industry is actually agreeing that the simple bubble wrap etc is not equivalent to 200mm of glass fibre. We installed it because of budget on a job for a customer last year and it made a huge difference to being able to work in the roof space in summer where previously it was unbearable. It needed bulk insulation too as winter proved but it still made a difference.

There are some who say the foils tarnish and then all the reflectiveness is lost. I have had samples out doors in all weathers for nearly 3 years and none of them have tarnished.

Good point, yes keeping the heat out is really important so will look into reflective films too

No, not used that but I would still recommend a reflective film to keep most of the heat out in summer backed up by another possibly wood fibre which would then only need to be around half of the thickness were a reflective film not used.

Thanks everyone - on the back of this and with uncertainty over our wall insulation/heating/cooling requirement we had a thermal study carried out. Basically the technician was in agreement with you and recommended for roofing we should be using rigid board panels placed over the rafters (to remove thermal bridges and to keep height of the living space max) and putting some extra soft insulation within the rafters (without losing too much head height). He agreed the 25cm laine de verre was a very insulation and he also added that the roof insulation was vitally important for reducing our overheating in Summer as well as reducing heat loss in Winter. He is eco so he proposed woodwool like Pavatex. Anyone used this?

Thanks Terry, yes it's the boric acid treatment which animals and insects don't like apparently as it's used to make the cellulose fire resistant.

The spraying water to create a crust is novel, I have seen geotextile laid on top to do the same thing.

Draughts as you say you doubt there are any as it's 25cm thick was exactly my point to prevent draughts it must be laid that thick and apparently still requires the crust. Other insulation (non wooll types ) can be much thinner for the same insulation level which if you are installing to insulate yet still be able to see the wooden beams is important.

At the end of the day insulation is good and with the anouncement that electricity prices are like to double by 2023 we will all be doing it!

It's the additives that drive them away, I was told. Either way, we no longer have a problem. They spray water on the finished job which creates a crust which deals with the draught problem if indeed there is one. I have my doubts as it's 25 cm thick laid on a vapour barrier.

Terry, I would love to know why small furry animals wouldn't nest in cellulose insulation, they will nest in anything.

I went to a demostration by Dow insulation that featured their own rigid panels and wool (rock + glass) and blown in cellulose. The slightest draft can pass through wool or cellulose products unless contained in panels like SIP's panels (structural insulated panels) so that is why they have to be that much thicker to achieve the same level of heat retention.

It's an interesting subject and one I have spent many hours on

No way I'd use fibre glass in the roof space, Suzanne. There are a lot of better and more ecological solutions. We opted for "ouate de cellulose" (sorry, don't know the correct English term). It's basically recycled newsprint mixed with a fire-proofing and an anti-mould additives. Ours was blown in as a loose product which gets into every nook and cranny but I gather it also exists in sheet form which might suit you better if you plan to convert part of the attic. It has the added advantage of being disagreeable to four-legged furry creatures that used to nest in the fibre glass! It made a huge difference to the warmth of our house and the heating bill!

Rigid board is twice as effective as fibre glass for the thickness so allowing thinner boards to be used. Multifoil is no good on it's own, it works well at reflecting and makes sealing the room easy from drafts but needs backing up with bulk insulation be it glass fibre or rigid. Rigid boads with aluminium reflective layer would be a good option but are harder to achieve an air tight seal so a thin multifoil + something else is a way to go. Reticel is a partner comapny of Kingspan.

I am looking imto rigid insulating panels that I can lay on top of the boards in our roofspace to replace the fibre glass rolls which get trampled over the years and are a dust trap. I'll let you know how I get on...

Hi Suz,
Because I cant afford to remove my roof before insulating, I had to buy something a lot more expensive than the norm. Its called Aluthermo Optima and it has a built in vapour barrier and breather membrane. The insulation is equivalent to building regs, even though its only 40mm’s thick. They are a Belgian based company, but they delivered it to France for me. I did a lot of research on this over 3 years, because everyone told me I couldn’t insulate without taking off my roof. The problem with most foils is that you’ll need several layers to achieve the equivalent to building regs, and if you’re fitting under an existing roof, you’ll need a vapour barrier and breather membrane. This then becomes bulky and labour intensive. Lambert at Aluthermo was very helpful and will send you all the specs if you ask him. He made it really easy to understand and was always available to explain things to me, whenever I rang, which was fairly often before I reached my decision.