Insulation requirements for loft conversion

[Scruffle]

Hello. I have an old cottage and we are converting the loft to useable "storage" space (aka non-building regs compliant extra bedroom). We have done the staircase, beams, joists, flooring and Velux.

Next step is studding, insulation and plasterboard. The rafters could take 50mm Kingspan and leave some sort of air gap, but battens probably needed to "thicken" the rafters.

The builder (who is experienced) is happy to use 50mm although he makes the point that it is not building regs compliant. However, I have used 50mm in the roof of the lean-to kitchen and I'm not too impressed with the insulation level.

Please could someone advise me. Is 50mm generally enough and to achieve better ventilation, should the board go on top of the rafters, rather than fitted in between?
If so, what would be the construction technique, including battens and plasterboard? There is limited space so I would prefer not to install too much insulation which encroaches in the living space.

Anyone heard of "superquilt"? - someone recommended this.

Any suggestions on whose prices are keenest on insulation products would be gratefully received.

Thanks very much.
Alistair

 

[freddiemercurystwin]

You'll need around 130mm insulation for the roof,

If there's existing felt under the tiles, you'll need to leave your 50mm air gap above.

If there's no felt or its a breathable membrane beneath the tiles you don't neeed an air gap at all.

A thinner solution could be to use Triso Super multi layer insulation or similar (like superquilt as you mentioned) which is a lot thinner, 30mm or something, but you need to check this is approved by your local authority before you buy it as some do and some don't accept it.

 

[noseall]

As per Freddy, current standards require say 100mm between rafters with 25mm under-draw. This would be in conjunction with a ventilated 50mm air gap above the insulation.

 

[Scruffle]

Hi and thanks for your responses. The objective here is not to make a Building Regs-compliant room. It is just to titivate a loft space to make it more useable than it is. Building regs require 130mm in order to stop heat loss through the roof. However, I already have more than sufficient insulation under the new loft floor, so heat loss through the roof will only be the small amount of heat generated in the loft space. Will the room be useably warm with 50mm in the rafters?

Thanks

Al

 

[The-brickie]

Why not do 50mm between and 25mm under? Or use the super quilt under.

 

[freddiemercurystwin]

Whats the point in doing evrything bar the insulation req'd by the regs and then not getting approval?

 

[alittlerespect]

For refurbishment you do not need to meet the more stringent requirements of a new build in so far as U values are concerned - an 80mm PIR board fitted between the rafters will more than satisfy the requirements - press fit the insulation between the rafters. If you need to thicken out the rafters choose either 2x1 or 2x2 and screw fix to the rafters (nailing may cause too much disturbance to the roof tiles!) I would also suggest that you fix a vapour barrier over the rafters before plasterboarding. Ideally you need to leave around a 50mm cavity between the insulation and the roof. Once you have the plasterboard fitted you can cut off any excess vapour barrier material. You will also need to follow the manufacturers instructions when fitting the vapour barrier - job done!

Regards

 

[legs-akimbo]

I dont think 80mm between rafter will comply, will do the u value calculation tommorrow.
As for multi foil type insulation, they are not certified and do not perform to the exagerated standards that are claimed in hot box laboratory environments let alone in the real world.

 

[^woody^]

As for multi foil type insulation, they are not certified and do not perform to the exagerated standards

Does TRADA know about this .... after all they did the tests