Best Attic Insulation for New Build

gooseygander · 09-11-2014 08:41PM #1

Hi all, just looking for opinions for best form of attic insulation for new build, rockwool, bio foam, icynene spray foam, ?????. We are currently spec for 400mm of rockwool but am thinking of going with spraying rafters. Our house is story and a half and has 7 dormer windows in total. Attic space is going to have a small enough head height and we are going for a good air tightness rating with mhrv system installed. All inputs welcomed. Slates are currently going on roof. Many thanks

Curious Geroge · 10-11-2014 12:45AM

gooseygander wrote: »

Hi all, just looking for opinions for best form of attic insulation for new build, rockwool, bio foam, icynene spray foam, ?????. We are currently spec for 400mm of rockwool but am thinking of going with spraying rafters. Our house is story and a half and has 7 dormer windows in total. Attic space is going to have a small enough head height and we are going for a good air tightness rating with mhrv system installed. All inputs welcomed. Slates are currently going on roof. Many thanks

depending on the depth of your rafters; 150mm metac, airtight membrane and then a 62mm insulated board.

BryanF · 10-11-2014 09:41AM

Have you doubled battened above breathable felt? If not you need a 50mm vent below the felt

gooseygander · 10-11-2014 01:03PM

BryanF wrote: »

Have you doubled battened above breathable felt? If not you need a 50mm vent below the felt[

Not double battened, in your opinion is spraying rafters a better job than double laying the rockwool on attic floor. our attic is never going to be big enough to convert or use save for a bit of storage. salesmen are all telling different accounts about what best option is and just looking for a bit of independent advice or opinions really from people who have gone down this road before me. I know there will be extra expense to spray but is it worth it in my case even when we wont be really using the attic space ??, again thanks in advance for any and all opinions.

BryanF · 10-11-2014 01:19PM

To sum up, you can't use the Attic space, so why consider insulating a space that will never be heated?
Put the mineral wool on the flat. Ensure adequate ventilation of the attic space above. But there are other factors here you need to discuss with your architect/builder/specialist subcontractors such as running Hrv ducts, electrics, water tank, air-tightness, access hatch, boarding out an attic area for storage etc etc.

May I ask:Is your architect not advising you? Why is s/he not specified the attics insulation? If that has not been specified what of the other items listed?

ferryman35 · 10-11-2014 02:58PM

what stage of construction are you at? Is this something you're planning ahead or are you ready for this stage now?

gooseygander · 10-11-2014 04:30PM

ferryman35 wrote: »

what stage of construction are you at? Is this something you're planning ahead or are you ready for this stage now?

I suppose I am just about ready for this stage now as slates are going on roof but attic is still all open with just joists and rafters, my arc has spec'd 400mil of rockwool laid flat, this may be the best option but I am not sure about that and looking for alternative options. my arc is the type that he does not like to be questioned and gets offended (I know, don't get me started on my problems with arc). many thanks

MicktheMan · 10-11-2014 07:55PM

gooseygander wrote: »

Our house is story and a half and has 7 dormer windows in total. Attic space is going to have a small enough head height and we are going for a good air tightness rating with mhrv system installed. All inputs welcomed. Slates are currently going on roof. Many thanks

How is the roof being airtightened? What is your air tightness target?

MicktheMan · 10-11-2014 07:57PM

gooseygander wrote: »

rockwool, bio foam, icynene spray foam, ?????.

Considered cellulose iso rockwool?

gooseygander · 10-11-2014 08:16PM

MicktheMan wrote: »

How is the roof being airtightened? What is your air tightness target?

Hi Mick in answer to your first question the air tightness plan of the roof will depend on the method we choose to insulate it. It is a block build with sand a cement wet plaster to all internal wall (no warmboards) so we will plaster down to the floors, the Brethan slab is wrapped and all windows and doors will be taped so I believe we need to decide on roof insulation method before we will see what is needed to get same airtight. I am no expert on same so I hope this makes some sense to you.

MicktheMan · 10-11-2014 08:54PM

gooseygander wrote: »

we need to decide on roof insulation method before we will see what is needed to get same airtight.

This detail ought to have been worked out prior to starting!
As said earlier, there is no point in including an unusable space within the thermal envelope.
Insulate on the flat with air barrier on the warm side.

PS make sure to tape the floor to wall junctions

Best Attic Insulation for New Build

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