Using xtratherm to insulate attic floor?

Aeneas

I want to insulate and put in flooring in the attic of an extension (single storey, pitched slated roof) to my home. I have put in fibreglass to the top of the joists and now want to install a kind of sandwich of extratherm or kingspan polyiso across the joists and cover them with plywood or other composite flooring. The polyiso and flooring would be screwed to the joists. My worry is that foil-backed polyiso could create condensation on the underside with the risk of wetting the fibreglass. Is this a real risk? If so would a non foil-backed polyiso be preferable? Is such a product available?

Mellor

Your concerns are well founded, condensation might occur as air and vapour pass through mineral wool insulation quicker than rigid boards, this could cause a build up of vapour and possibably condensation. Non foilback polyiso is available, but it may not help much, polyiso alone is still more resistive to vapour than mineral wool. If the two are to be used together the polyiso (or any rigid board) should be to the warm side.
In your situation its best to use a mineral wool as the addition insulation. mineral wool between batons running across joists, flooring fixed to batons.

Aeneas

Thanks Mellor. I was hoping to avoid putting down what would have to be rather high batons to accommodate a substantial depth of fibre glass. But maybe I will have to go down that road. I read somewhere that if you slash or pierce the polyiso with a knife that this will create passageways through which vapour/condensation can pass. Or is there another boarded insulation material whose rate of dispersal would be similar to polyiso?

bibibobo

Aeneas wrote: »

Thanks Mellor. I was hoping to avoid putting down what would have to be rather high batons to accommodate a substantial depth of fibre glass. But maybe I will have to go down that road. I read somewhere that if you slash or pierce the polyiso with a knife that this will create passageways through which vapour/condensation can pass. Or is there another boarded insulation material whose rate of dispersal would be similar to polyiso?

you can also staple a plastic sheeting membrane to the side of the joists and resting on the plasterboard ceiling to prevent moisture penetration. you can then lay ordinary fibreglass. some fibreglass rolls have a plastic layer wrapping which would negate the need for an additional membrane. are you converting the room for regular use ? if so then why not just insulate the sloping part of roof?

Aeneas

Thanks bibibobo. The attic is used for storage and I don't plan to convert it. Seven years ago or so I installed the fibre glass insulation between the joists and now want to top this up before I put in some kind of flooring to make the storage and moving around easier. I reckon that taking up the existing fibreglass and putting in a polythene barrier would be a fairly tiresome job. (In this case where does the water vapour go when it hits the barrier?) My hope was that I could lay some kind of boarded insulation across the joists underneath the flooring. But my worry is condensation on the warm side of the insulation board, especially if it is foil backed (the room underneath is a living room). Mellor above has confirmed my worries on this score but I would be grateful if anyone knows whether slashing the foil and the board with a knife would create passages which would allow water vapour through?

bibibobo

Aeneas wrote: »

Thanks bibibobo. The attic is used for storage and I don't plan to convert it. Seven years ago or so I installed the fibre glass insulation between the joists and now want to top this up before I put in some kind of flooring to make the storage and moving around easier. I reckon that taking up the existing fibreglass and putting in a polythene barrier would be a fairly tiresome job. (In this case where does the water vapour go when it hits the barrier?) My hope was that I could lay some kind of boarded insulation across the joists underneath the flooring. But my worry is condensation on the warm side of the insulation board, especially if it is foil backed (the room underneath is a living room). Mellor above has confirmed my worries on this score but I would be grateful if anyone knows whether slashing the foil and the board with a knife would create passages which would allow water vapour through?

sometimes i feel that alot of this stuff is theory and needs to be balanced with practice.

the GALL

I would leave the floor as is.As for the roof use a foil back hytherm 100mm board which can be expencive maybe $4o an 8x4 sheet but the advantages are you dont get itchey as with fiber glass and it can be cut to a snug fit (make sure there is a gap behind it for airflow)keep it tight to the bottomof the rafter (the side which faces the room . if your not converting the room for living i wouldn't put in a foil backed board it will still be warm enough to protect your stuff .............oh yea make sure to put a vent in or you will have a problem with condensation youll pick one up $2 any hardware or you could go to any building suppliers and buy the home bond manual

Mellor

the GALL wrote: »

I would leave the floor as is.As for the roof use a foil back hytherm 100mm board which can be expencive maybe $4o an 8x4 sheet but the advantages are you dont get itchey as with fiber glass and it can be cut to a snug fit (make sure there is a gap behind it for airflow)keep it tight to the bottomof the rafter (the side which faces the room . if your not converting the room for living i wouldn't put in a foil backed board it will still be warm enough to protect your stuff .............oh yea make sure to put a vent in or you will have a problem with condensation youll pick one up $2 any hardware or you could go to any building suppliers and buy the home bond manual

The OP was looking to add insulation to his attic floor, not put insulation to the roof. Also, installing insulation to the underside of the rafters is pretty pointless. The attic is ventilated so air will bypass the rafter insulation, or if it is changed so the attic is not vented the current insulation is pointess.

Aeneas

Yes, the attic is ventilated through vents in the eaves so there is constant air movement between the insulation and the roof. The storage material in the attic does not need to be insulated so I don't need to go down the road of insulating the roof itself. My main interest is in insulating the floor to keep heat in the living room below.

championc

Hi Aeneas

I am wrestling with the same dilema as you had last year. I too want to put panels at right angles to the joists which are currently filled with fibreglass. I think I have a slight gap between the fibreglass and the top of the joists so therefore, the foil backed panels wouldn't quite touch the fibreglass.

What did you do and how has it worked out ?


C

GreeBo

I just did it by using 200mm batons perpendicular to the existing joists and then using 200mm of the recycled plastic bottles stuff (for sale in B&Q at half price every now and again)
I'm now going to floor with cheap pine T&Q as I reckon it will be easier to fit and stronger than the MDF stuff.