Insulating old flat roof from inside?

murphaph

Would appreciate folks ideas on best way (balancing cost of course-this will be a rented out property but want to make it comfortable for the occupants) to insulate an old flat roof. The roof exterior was skinned with PVC coated aluminium cladding which has done a fine job at keeping the rain out (old roof failed and leaked badly-property vacant for many years, now being refurbished) but it of course being metal is a nightmare for condensation (I had a peek through the old bitumen roof and it's very wet after just a few days of people breathing in the place).

I'm obviously going to have to include a vapour barrier in the equation but what would you do? The situation is that the ceiling plaster slabs will all be removed to access the voids between joists which presently have absolutely no insulation. Given that, should I use fibreglass then polythene then re-slab (obviously joists form a cold bridge) or reslab with insulation backed plasterboard? There is quite a large flat roof area to do so cost is important. What about incorporating foil in the equation to reflect radiant heat also? Any ideas more than welcome.

RKQ

You could consider 170mm fibre glass tightly fitted between ceiling joists, leaving 50mm gap between roof covering and top of insulaton. ( I'm assuming joist are 225mm deep) 500 guage plastic vapour barrier, 150mm lapped joints taped.

Then maybe 30mm - 50mm thick high density foil backed kingspan, xtratherm or polycomp, with 12.5mm plasterboard bonded to insulation. Maintain a ceiling height of 2400mm.

Alternatively, friction fit 2 layers of 60mm kingspan between joists. Giving 120mm overall depth. Vapour barrier and 9.5mm plasterboard. Might be abit more expensive but would give a better result. 8 x 4 sheets can be cut with a circular saw for speed and accurate straight edge.

Might be worth installing small vents in top side of fasciaboard, to cross ventilate over insulation level.

murphaph

RKQ wrote: »

Alternatively, friction fit 2 layers of 60mm kingspan between joists. Giving 120mm overall depth. Vapour barrier and 9.5mm plasterboard.

In the above are you talking about not using insulation backed plasterboard and just rigid between the joists?

Was reading another thread there and a link to Xtratherm Rafterloc was given-looks just the job instead of fibreglass, seems it'd be easier to maintain a reliable air gap between it and underside of roof than using fibreglass.

What would you use to tape the vapour barrier joints RKQ? Ok to staple the vapour barrier to the joists?

I should add-this is a high vapour (Kitchen) area.

RKQ

I was talking above about sheets of foil backed insulation,(Kingspan or xtratherm) friction fitted between joist - rafterloc is a good example but ordinary 8 x 4 sheets about 60mm thick may be cheaper. Use two layers of board, staggering and taping the joints in the insulation board. They are very easy to friction fit to maintain air gap. Then ordinary 9.5mm plasterboard slabs to ceiling ( check overall u-value )

Or use fibre glass between joists and kingspan with attached slab under joists. ( check overall u-value ) Covering over the ceiling joists gives a better u-value, if you have the headroom - 2.4m recommended minimum.

Staple plastic to joists, tape over staples if you can. Certainly tape all joints in plastic! 150mm overlap to plastic sheeting.

Here's a copy of page 11 - Document F of the Building Regulations.

Roof ventilation Regulations.pdf

It will help you see where the 50mm air gap must be maintained, to ventilate a pitched or flat roof.
Regards RKQ

thomasdubarry

how much is a roll of fibreglass insulation?? house is only 4 years old and its cold the whole time with damp penetration all over house and ideas on what to do

RKQ

thomasdubarry wrote: »

how much is a roll of fibreglass insulation??

All major DIY stores are currently running special offers on attic insulation. Read this thread above, to achieve an overall thickness of 300mm.

As a guide, I got 2 rolls for the price of 1, only cost me €26 last Sunday.