what type of insulation??

moan 77

Hi All,

If you had to choose an insulation for a roof to meet or beat the current regs, what type would you choose ?. Take money is not the problem but chossing a quality product for a roof with 175 mm rafters is.

All or any advise would be great.

Southern Comfort

moan 77 wrote: »

Hi All,

If you had to choose an insulation for a roof to meet or beat the current regs, what type would you choose ?. Take money is not the problem but chossing a quality product for a roof with 175 mm rafters is.

All or any advise would be great.

Hi Moan. Rafterloc is a great product. It's designed to compress between rafters. With 175mm of rafter depth, you could use 150mm of it if you used a breathable membrane. If you check here, http://www.xtratherm.com/products/thinrrafterlocirl.php you'll see you'd get a seriously low U value if you used it and put insulated plasterboard on the ceiling.

If you don't wish to go the rigid insulation route, you wouldn't go far wrong with 150mm of Moy Metac. http://www.moyisover.ie/upload/medias/uk/metac%20new.pdf;jsessionid=291958594B44A7A10A20B839BE24CC46 It has a U value of 0.23 and you'd lower it further with the insulated plasterboard also. Metac is superior to Rockwool, etc.

Good luck!

beyondpassive

You're insulating on the slope, if you havent put on battens or slates yet, I'd recommend softboard such as 22mm Gutex or thicker as a windtight sarking board. Line the inside with 50mm PIR insulation board after you've filled the joists with Metac. A layer of OSB then for vapour control and airtightness and then a services cavity where appropriate so cables, downlights and socket boxes dont compromise the vapour barrier (I prefer to use natural materials like cellulose and wood fibre, but if you're not worried about VOC's and allergens in bedrooms, then use the polymer derived products, like PIR and PUR boards.) Pay attention to knee wall doors and attic trapdoors, these just pis out warm air.

Because a house is like a hot air ballon and the air is bouyant and at pressure, you need a good lid on your house. You heat your house from the top down, so a good roof thermal spec is critical in terms of thermal insulation and airtightness. Merely lobbin a bit of fibreglass roll between the knee wall studs and between rafters is a false economy. heating such a house is like filling a bath with the plug out, sure you'll get the level you desire but the container wont be able to maintain that level for any length.

Perversely, its actually summer when you need the thermal inertia and airtighness most, when the temperature under the slates can get up to 50 degrees and cause discomfort in the 'room in the roof'. More critically the high temp in the roof buildup cooks the timbers and insulation and leads to bad air quality and ionisation of the air. Typical sick building syndrome.

Dont mind me, I'm a bit of a nerd on building comfort, prob better to rely on the Irish builder mantra, 'a shur 'till do grand'.

sinnerboy

beyondpassive wrote: »

Gutex or thicker as a windtight sarking board. Line the inside with 50mm PIR insulation board after you've filled the joists with Metac. A layer of OSB then for vapour control and airtightness and then a services cavity where appropriate so cables, downlights and socket boxes dont compromise the vapour barrier (I prefer to use natural materials like cellulose and wood fibre, but if you're not worried about VOC's and allergens in bedrooms, then use the polymer derived products, like PIR and PUR boards.)

BP - agree with above but do you not think it would be better to

1. omit the PIR
2. place OSB ( taped ) for VC+AT layer directly below rafters
3. upsize the services cavity to 100mm - with Rockwool or better yet Cellulose batt infill

Reason - easier to avoid any stiff boards and consequent risk of poor cutting and jointing . And in the case of PIR - avoid off gassing and loss of lambda over time .

Oh - I would "chicken out" and use a breather membrane over the top layer Gutex too

moan 77

Thanks for all the info, not sure if i understand it all but at least i have questions to ask now. One more thing what would you think about the quilted foil insulation good or bad, would it better or worse than what has been recommended already.

The felt on roof will be a breathable type, also i will hope to have led down-lighters in the bedrooms upstairs as this is a domer type roof, will i have to allow space for the down-lights between the insulation and room ceiling and should this space be insulated.

My builder is old school and will stuff some stuff into the void and slab it up, this not what i want so i want to direct what will happen next.

One last thing can the quilted foil insulation be used as an air tight membrane, as i would like to have an air tight house.

Once again all or any advise will be a great help.

beyondpassive

sinnerboy wrote: »

BP - agree with above but do you not think it would be better to

1. omit the PIR
2. place OSB ( taped ) for VC+AT layer directly below rafters
3. upsize the services cavity to 100mm - with Rockwool or better yet Cellulose batt infill

Reason - easier to avoid any stiff boards and consequent risk of poor cutting and jointing . And in the case of PIR - avoid off gassing and loss of lambda over time .

Oh - I would "chicken out" and use a breather membrane over the top layer Gutex too

Of course it would be better, but while you and I strive for best practice in construction, the legacy of 'the free for all years' means that for many somewhere approaching minimum standards is sufficient. In this particular case, I'm assuming the home builder wants to place themselves mid way on this range between minimum and best practice.

I agree entirely with your points 1-3 and equally with the breather. I always specify a textile breather as sarking felt over the softboard.