Attic Insulation options

covey09

Hi,
Doing a new build (bungalow) and we are just getting ready for slabbing the roof, before we close up i want to slip up the obs boards for flooring the attic for storage. So my BER calls for 2 layers of 150mm of mineral wool laid in opposite directions to achieve the value of 0.15 W/m2K. My BER Engineer has said if i want to remove one of the layers of mineral wool that i would need to do a 82.5mm Xtratherm Thin R insulated board(70mm insulation + 12.5 mm plasterboard) on the underside to make up for the 150mm. They are very costly and im looking for other options. If i raise the floor i will have to bend down for every more, and i want the full space for storage. Just looking to see what my options are if any
Thanks

sydthebeat

32.5mm insulated plasterboard on ceiling
150 quilt between joists
50 PIR on Joists under flooring.

Ergo you only loose 50mm of headroom

The_Bullman

I'm in a similar dilemma.

Is there a website(or table/chart) for comparing different insulation types and thicknesses that you could recommend.

Calahonda52

covey09 wrote: »

Hi,
Doing a new build (bungalow) and we are just getting ready for slabbing the roof, before we close up i want to slip up the obs boards for flooring the attic for storage. So my BER calls for 2 layers of 150mm of mineral wool laid in opposite directions to achieve the value of 0.15 W/m2K. My BER Engineer has said if i want to remove one of the layers of mineral wool that i would need to do a 82.5mm Xtratherm Thin R insulated board(70mm insulation + 12.5 mm plasterboard) on the underside to make up for the 150mm. They are very costly and im looking for other options. If i raise the floor i will have to bend down for every more, and i want the full space for storage. Just looking to see what my options are if any
Thanks

What depth are the ceiling joists?
What material is under the slates/tiles?

covey09

The Joists are 220mm the roof is felted and slated

BryanF

Is it cross (double) battened outside the breathable felt?

Calahonda52

covey09 wrote: »

The Joists are 220mm the roof is felted and slated

so the ceiling joists are 220 and your BER-head wants you to lay 150 between and 150 across.
This is a careless design, as the across layer will undulate across the joists like the bog roads around Ferbane, leaving an air passage underneath for wind from the ventilated eaves to pass under the second layer and largely negate the value of all the insulation.
The across layer will only go as far as the intersection of ceiling joist and roof rafter, leaving maybe 10 inches of the ceiling/wall/wallplate with only one layer.

As I said, careless.

Is the felt breathable to address BryanF's question

Lumen

Loose fill cellulose?

Calahonda52

Lumen wrote: »

Loose fill cellulose?

depending on the full eave/soffit/facia/roof detail, cellulose will work only with properly fitted ventilation baffles

covey09

Calahonda52 wrote: »

so the ceiling joists are 220 and your BER-head wants you to lay 150 between and 150 across.
This is a careless design, as the across layer will undulate across the joists like the bog roads around Ferbane, leaving an air passage underneath for wind from the ventilated eaves to pass under the second layer and largely negate the value of all the insulation.
The across layer will only go as far as the intersection of ceiling joist and roof rafter, leaving maybe 10 inches of the ceiling/wall/wallplate with only one layer.

As I said, careless.

Is the felt breathable to address BryanF's question

Thanks your very helpful.

Vote4Napoleon

I went with 8" metac between 9" joists and then slabbed with 50mm pir