Insulating lofts and 50mm airspace

[Antwerpman]

First of all hi, I am new to the forum and glad to find such a good resource for information.

Having read some of the threads and subsequent advice on insulating lofts has brought about a concern for me.

I have an old (1903) house with a HUGE loftspace - about 100m² - which is about 4 m high at the apex, all of which can be used as living space. It is fully heated and the original stairway goes up to the attic. My idea was to insulate between the roofing joists, then place a moisture barrier (plastic sheeting) followed by small (1") battons to which I would attach plaster board. Finally then it would all be sealed. At present the roof does not have underfelt, but I would hope to have it re-roofed in the near future too at which point this would be done

Whilst I am not subject to UK building regs (I am living in Belgium at the moment) I dont want to do something that would store up problems for the future.

Would such a plan cause proiblems with moisture??

 

[masona]

I think you're doing it the wrong way round,best to get the new roof done first as there will be a lot a movement on the roof rafter plus 100% watertight to prevent further damage.

 

[Antwerpman]

Hi, thanks for the reply,

I agree that it would be better to do it that way around, but I had already put up some insulation just to try and keep the heat in during the winter months. As I havent been here that long I havent had the chance to get quotes on costs to decide if I do it myself or not.

But assuming that I do get the roof felted and retiled, would I be causing a problem for myself if I insulated it in the way described??

thanks
A

 

[masona]

I can only tell you the regulation in the UK,

There must be at least 50mm airflow at the back for the timber to breathe including soffit vent and need building permission if you're using it as a room etc .The rest you're doing I can't see a problem.

Also see this for soffit vent.

 

[Antwerpman]

If I knew how to upload a picture of the roof I would do it... unfortunately I dont!!!

What is the consequence of not having the airgap??

The roof is such that when you are in the loftspace there is about 30-40cm of the external walls visible above the loft space floor. From this the roof is then pitched very steeply (I never measured it but maybe 70°) up to ceiling height. Above that the roog is then pitched much more gently to a single apex.

To call it a loft is probably a bit misleading as it is really a third floor which has never been fitted out, with what is then a 'conventional' loft space above it (except there is no ceiling seperating the two.

If I insulated in the way I intended, with rockwool between the roof spars with a foil backing, would I still need the airspace?

 

[masona]

If it not ventilated then you risk condensation which will lead the timber to rot.
Instead of using insulation in between rafters,why not use Polyfoam Linerboard with a built-in insulation on a plasterboard backing and fix it to the rafters which will allow for ventilation thru' the rafters.

See this to help you for difference ways of insulating the roof space.