Steicoflex under TLX Gold?

[leucothea]

Hi,

I'm hoping for some help re. insulation on a roof that is currently open!

The basic plan is TLX Gold draped over the rafters.

But I'm wondering about whether to add some Steicoflex between the rafters in the orange box. Is this sensible? And if so, would I have to make sure there is an air gap?

I plan to insulate the yellow box from the inside, with wood fibre board and maybe some Steicoflex between the rafters.

I'm unsure what to do in the red box... a very disordered space. My default will probably be some Steicoflex against the lath and plaster with Cosywool laid out along the bottom.

Any help would be appreciated!

 

[malcolm]

For others according to the internet TLX Gold is a foil insulation that is not vapour permeable and Steicoflex is a wood fibre insulation.

The trick to insulation is to make sure the insulation is ventilated on the cold side. Any warm air making it past the insulation will condense water on the cold side that will drip back down if it is not ventilated away. and it's tricky to maintain an airtight seal for 100 years or however long you want the roof to last.

For mine I put celotex between studs on the attic leaving a 50mm ventilated air gap below a breathable roof membrane. Logic is the moisture that will get past the gaps around the insulation will be ventilated away. There would be some risks in insulating below an impermeable membrane where the cold side is not ventilated - the wood fibre insulation risks getting soggy if it can't dry out.

Something like this works, but only so long as the roof is really well ventilated. Breathable membrane is not enough by itself - you would need good eaves ventilation for this to work.

 

[leucothea]

Thank you Malcolm, thats interesting. Though just to clarify—TLXGold is vapour permeable. But I guess having the wood fibre up close would reduce ventilation and make it more prone to condensation regardless.

 

[leucothea]

For the main eaves (red) the builders are recommending Cosywool between the joist to insulate the room below and more Cosywool attached to the lath and plaster.

 

[paulc]

Some specs from the datasheet:

Core R Value 0.85 m²K/W
Water vapour resistance 0.50 MNs/g
Emissivity 0.16

Not the world's best R value, so as a barrier to conducted heat, it doesn't perform very well. However, it does reflect back radiated energy. An air gap between the TLXGold and any rigid insulation is essential if you want to get anywhere close to the claimed u-value of 0.13 W/m²K.

 

[Toby Newell]

Numbers are numbers. Dry timber has very low heat conduction but very high emissivity so a reflective based insulator actually makes a lot of sense. This system works very well but you need to make sure it is sealed properly and that the cold side is well ventilated as Malcolm says.