Cost of Moy Metac -v- Rockwool

creedp

Hi

Builder wants to put rockwool/glasswool between 175mm joists of a sloping 45 deg roof and he claims this wont sag overtime. Im concerned it will and instead wan to use moy metac which I understand is more durable and stable for such a roof.

Can I ask what the price premium is for metac over rockwool/fibreglass and is it worth it for a 45 deg sloping rafter

Thanks

Southern Comfort

Hi Creedp. I presume you mean rafters, rather than joists, if you're talking about the sloping roof. I recently used Metac in my attic ceiling. The ceiling was already plastered and we ended up taking off the tiles on the outside and replacing the felt with a breathable membrane. The rafters are 100mm deep so the guy doing it put in 100mm of Metac. He wanted to use Rockwool but I insisted we use Metac. The u value of Rockwool if 0.44 but for Metac it's 0.34. Big difference. It's excellent stuff. Semi rigid and the company say it won't sag, age, etc.

It's dear enough at €60 per roll but it's 7m long and 1200mm wide. Rockwool is only 4.8m long.

The Metac, being semi rigid, friction fits between the rafters and won't shift of slide over time. If I were you, presuming you have a breathable membrane on the roof, I'd use 180mm of the Metac in your 175mm rafters. You're not supposed to compress insulation of this type but a 5mm compression will make little or no difference. If you have nothing, like a ceiling, on the underside of the rafters, it'll make no difference, at all. 180mm has a u value of 0.19.

http://idevelop.ie/iproducts/prd/436/blanket_insulation/metac_roll/

http://www.idevelop.ie/iproducts/productinfo.php?Product=191&prodgroup=72&display=

I hope that posting these links isn't against Forum Rules. Good luck.

creedp

Southern Comfort wrote: »

Hi Creedp. I presume you mean rafters, rather than joists, if you're talking about the sloping roof. I recently used Metac in my attic ceiling. The ceiling was already plastered and we ended up taking off the tiles on the outside and replacing the felt with a breathable membrane. The rafters are 100mm deep so the guy doing it put in 100mm of Metac. He wanted to use Rockwool but I insisted we use Metac. The u value of Rockwool if 0.44 but for Metac it's 0.34. Big difference. It's excellent stuff. Semi rigid and the company say it won't sag, age, etc.

It's dear enough at €60 per roll but it's 7m long and 1200mm wide. Rockwool is only 4.8m long.

The Metac, being semi rigid, friction fits between the rafters and won't shift of slide over time. If I were you, presuming you have a breathable membrane on the roof, I'd use 180mm of the Metac in your 175mm rafters. You're not supposed to compress insulation of this type but a 5mm compression will make little or no difference. If you have nothing, like a ceiling, on the underside of the rafters, it'll make no difference, at all. 180mm has a u value of 0.19.

http://idevelop.ie/iproducts/prd/436/blanket_insulation/metac_roll/

http://www.idevelop.ie/iproducts/productinfo.php?Product=191&prodgroup=72&display=

I hope that posting these links isn't against Forum Rules. Good luck.

Thanks for the information and advice. You are right it is rafters! I will be going with moy metac spacesaver as I don't want to use the rigid boards or loft roll which might settle and its too late to consider something like cellouse ... if I was building again!!