Loft insulation with 70mm deep rafters/joists

[mcgotts]

Evening all, and Happy New year ! I've got a bit of a dilemma with my loft insulation and would welcome suggestions.

My House is a late 70's build and the loft members are only 70mm deep. At present there's some blown insulation in there, but it's practically non-existent where there has been traffic.

What I am keen to do is lay insulation so that I end up with a warm loft and can set up a railway in there (lack of space elsewhere!). I had boarded some of the loft a few years back with some 100mm thick mineral wool squeezed under rafters with boarding over, but aside from the fact that I'm not sure I can still get 100mm roll insulation it's clearly not doing it much good being compressed.

The other issue is insulating under the rafters. There are only 70mm deep, so allowing the 50mm gap means I've got 20mm, therefore any sheet insulation would have to go over the rafters unless 20mm polystyrene is enough!

Would it be good enough therefore to put a thinner layer of insulation at ceiling level and another at rafter level, or should I be looking to beef up the ceiling insulation to current standards and put some token insulation under the rafters. Would the latter make the loft usable as the heat from the house would be very reduced?

Any suggestions gratefully received. Oh - I'm on a tight budget too, as if it wasn't confusing enough! Thanks in advance

 

[noseall]

The proper way to do it is to add thickening timbers to the existing rafters, thus affording you the correct insulation depth.

What you need to remember is that, unless you have breathable membrane or traditional sarking felt +ventillation grilles, then you will have inadequate means to ventilate the moist air.

Regardless of the above, there is no cheap or easy way to correctly retro-fit vaulted insulation. Current regs ask for 25-50mm air gap, 100mm celotex or kingspan between and 40mm across/under-draw.

Polystyrene/fibreglass of the equivalent thickness is not up to the task.

 

[mcgotts]

Hi, thanks for the reply

The proper way to do it is to add thickening timbers to the existing rafters, thus affording you the correct insulation depth..

Alternatively, could I insulate under the rafters?

What you need to remember is that, unless you have breathable membrane or traditional sarking felt +ventillation grilles, then you will have inadequate means to ventilate the moist air.

Currently it's sarking felt with no vents whatsoever. I am considering installing eaves vents and as there is no ridge board this can therefore vent eave to eave.

Regardless of the above, there is no cheap or easy way to correctly retro-fit vaulted insulation. Current regs ask for 25-50mm air gap, 100mm celotex or kingspan between and 40mm across/under-draw.

Polystyrene/fibreglass of the equivalent thickness is not up to the task.

Are your figures above meaning that I need to put 100mm plus the 50mm gap under the roof? this will result in a severe loss of headspace! If I fully insulate the ceiling (ie put in counterbattens and extra insulation) and then put in a nominal layer under rafter to minimise draughts and try and raise the temperature in there a little would it be marginally habitable, or would I be better off just concentrating on under-rafter insulation rather than trying to do a little of both, as I originally considered?

Thanks for your comments noseall, much appreciated.

 

[Bahco]

You should raise the floor up a few inches and try for 11 ins of insulation. Ideally you should have roof vents too to ventilate the loft.

I would forget the railway in the loft until the summer months!

 

[Chukka63]

if you create a warm roof i dont see the need to worry about the insulation under your floor as any heat loss from downstairs will aid the heating of the upstairs so its swings and roundabouts.

i would just board below the rafters for the sake of saving 20mm.even if you used 100mm you would still have better insulation than 90% of the houses in the country to date.

if you go about this in the correct and current Building regs spec then thats a different matter to the DIY options above.

This advise isnt how i work in my professional capacity but realise a lot of people do stuff in there own homes that are not to a professional standard but it does the job the home owner is seeking.
thats the legal stuff covered,lol.

 

[sympology]

You could at something like this.

http://www.space-insulation.com/board.html

 

[JamesBlonde]

That Rafter Roll looks good, but has anyone seen it anywhere?

For the flooring, might also be worth considering the loft boards with a big wodge of insulation glued to the bottom that B&Q have been doing?

 

[Chukka63]

im now doing a loft conversion with 70mm rafters,im adding a 25x50 batten and then 50mm celotex between the rafters and 70mm celotex under the rafters.

 

[Phyl1]

I've got the same kind of project going but there is no roof felt, just the rafters then the tiles. Can I still go ahead and use the celotex then plasterboard if I leave a gap between the celotex and the tiles? Ideally I would like to have the tiles lifted on the outside and a lining put in but I can't afford this just now. I need to get on with the insulation. I would appreciate any advice.

 

[freddiemercurystwin]

I would appreciate any advice.

Start a new thread then, I can't be arsed to read a 3 year old thread just to find out what project you've got going on.