Attic insulation, 30 cm rafter spacing.

GreySquirrel

Attic insulation, 30 cm rafter spacing - centre to centre. What would be best way to insulate with this narrow spacing. Could I use space blanket streatched over rafters?

RKQ

No! Always insulate between rafters, then lay a second layer perpendicular to the joists.
Buy a 100mm thick or 150mm thick roll of insulation - thickness depends on thickness or height of your joists. Most rolls come pre-cut. An un-cut roll would suit you but isn't a problem if you can't get it.

Firber glass or rockwool Insulation comes in rolls, wrapped in plastic - Cut the roll with a hand saw - while its still wrapped in plastic. Cut it to your required width / spacing.

Remove plastic and unroll between joists! Try to achieve an overall depth of 300mm. ( 100mm between + 200mm perpendicular OR 150mm between + 150mm (or 200mm) perpendicular.

Never insulate underneath your water tank - insulate sides and lid of tank only!

SteM

There's 10cms of insulation in the attic of the house that we just moved into up to the top of the joist). We were hoping to use the attic for storage but if I we lay 20cms perpendicular to what's there already then we wouldn't we able to put down t&g flooring on part of the attic.

Can anyone suggest a way around this problem?

GreySquirrel

RKQ,

Thanks for quick response. I've read about an air gap being needed on old roofs?

Is it bare fibre glass you are suggesting, the plastic being what it comes in?

RKQ

SteM wrote: »

There's 10cms of insulation in the attic of the house that we just moved into up to the top of the joist). We were hoping to use the attic for storage but if I we lay 20cms perpendicular to what's there already then we wouldn't we able to put down t&g flooring on part of the attic.

Can anyone suggest a way around this problem?

50mm insulation board, then ply on top. Special on at DIY store at moment.

RKQ

GreySquirrel wrote: »

RKQ,

Thanks for quick response. I've read about an air gap being needed on old roofs?

Is it bare fibre glass you are suggesting, the plastic being what it comes in?

50mm air gap required between roof felt and insulation, fitted between roof rafters of a pitched roof - ie dormer sloping ceiling.

No air gap required in horizontal floor joists (in a pitched roof - standard semi-detached house). Standard bare fibre glass is fine. I'm referring to cutting it to size when its in its wrapper - 1200mm wide roll. Much easier and quicker to cut to size required, when its in a roll.

Carry cut rolls into attic, remove plastic wrapper and unroll between joists.

SteM

RKQ wrote: »

50mm insulation board, then ply on top. Special on at DIY store at moment.

So pull up the 100mm wool that's there at the moment and put down 50mm insulation board instead?

RKQ

SteM wrote: »

So pull up the 100mm wool that's there at the moment and put down 50mm insulation board instead?

No!
Always insulate between joists, so maintain existing insulation. The 50mm board is a suggestion if you want to floor on top of it for storage.

Approx overall thickness of 300mm fiber glass or equivalent would be best, if possible. Depends on joist depth and roof construction etc

Do you need to floor whole attic?
If not use board over floored area and fiber glass over rest - in low height areas. This is only one of many possible solutions.

GreySquirrel

RKQ wrote: »

50mm air gap required between roof felt and insulation, fitted between roof rafters of a pitched roof - ie dormer sloping ceiling.

No air gap required in horizontal floor joists (in a pitched roof - standard semi-detached house). Standard bare fibre glass is fine. I'm referring to cutting it to size when its in its wrapper - 1200mm wide roll. Much easier and quicker to cut to size required, when its in a roll.

Carry cut rolls into attic, remove plastic wrapper and unroll between joists.

Is it rigid enough to stay in place by itself? or how do I fix iti n place?

RKQ

GreySquirrel wrote: »

Is it rigid enough to stay in place by itself? or how do I fix iti n place?

Cut it so its tight fit between the horizontal joist. Its laid flat so where can it go?

GreySquirrel

it will be going between roof rafters

RKQ

RKQ wrote: »

50mm air gap required between roof felt and insulation, fitted between roof rafters of a pitched roof - ie dormer sloping ceiling.

Look at Rafterloc, kingspan, xtratherm etc
http://www.insulation.kingspan.com/ireland/pdf/futureproof_solutions_guide.pdf

Maintain 50mm air gap. Fibre glass help back 50mm from felt by string or ply will work. Staple string in a zig zag pattern. Might need to increase depth of Rafters, with battens screwed to rafters. Plastic Vapour barrier stapled over insulation will keep it in place. Cut insulation so it is tightly fitted. Tape overlapping joints in vapour barrier.

Kristopherus

Dont want to hijack this thread, but the ceiling joists in my gaff are 30cm aoart. Is there any supplier of insulation products that have a 30cm or thereabouts. I'm looking at something like the Space Blanket from www. thinksulation.co.uk . Available here from Homebase and B & Q. The big advantage with it is that it is clean to lay, but you would probably ruin it by cutting it. House is a typical early 70s 3-bed bungalow, with no insulation, except for 25 odd years of all types of rubbish.

Lex Luthor

The Knauf space blankets you can get in B&Q/Homebase are 37cm width.

Not sure if they would be rigid enough to stay in the 30cm rafters, but you could always buy one to try.

You could always get yourself a few lengths of 2"x1" and nail them to the rafters and fit the insultion between the battens and roof space. That way the insulation would have a better chance of staying in.


I put down a load of the Knauf Insulation yesterday (150mm thick on top of 150mm already) and can already feel the difference in the house upstairs.

RKQ

I'd agree with Lex. Kristopherus buy one roll and see how it fits - 70mm isn't hard to squeeze. Take advantage of the special offers in at least 3 DIY stores.

Otherwise buy the 1140mm wide precut roll, and trim 70mm off each, while still in wrapper (as explained above)

RKQ

Here's a copy of page 11 - Document F of the Building Regulations.

Attachment not found.

It will help you see where the 50mm air gap must be maintained, to ventilate a pitched or flat roof.
Regards RKQ