Flat roof insulation

seneca

Hello,
I've moved into a house with an old flat roof extension which has a bathroom in it. I've stripped out most of the room and removed the plasterboard ceiling. It has exposed the underside of the roof which did not look well insulated. There were a few rolls of old fibreglass type quilt and I removed those too so I have a completely empty space. I am about get quotes for new plastering of the bathroom and thought I would get new insulation put up there before it. Could someone please advise me on what the best form of insulation is, how it should be installed, where ventilation space should be and anything else I could do to improve the existing roof. I can't afford to have the thing rebuilt, but need a new bathroom in it. The top joist beam is 3" deep and the larger cross joist is 6". The sides of the roof have wood panels covering them and air flows through the gaps around it. I have some pictures but don't know how to post them? What are these kinds of roof called: warm or cold decked? Are any extra precautions needed, as it is a bathroom that will be more humid.

travellerboy

Since your budget doesn't extend to installing a hot roof you have to do all you can to avoid condensation on the inner roof structure.
You need to ensure a minimum of 50mm air space between any insulation and the roof and ensure that this space is well ventilated.

Best installation is solid styrofoam which has a higher U value. You need to cut to fit between the joists minimising any gaps for moist air to pass. When you re-install new plaster boards ensure they are foil backed, this again is a barrier to moisture.

Being a bathroom it is essential to install fan ventilation linked to the light switch with a time delay to remove as much moist air as possible.
You will still get condensation in the roof space above the ventilation but will be kept to a minimum and extend the life of the roof boarding.

Furts

Adding to the above there should be a vapour barrier - say 500 guage polythene,

phil24_7

Agree with both above. I would go as far as to say put 100mm of PIR/PUR insulation between the joists with 25mm below to minimise cold bridging on the joists. Then install the vapour barrier before putting moisture board on the ceiling and plastering.

Any penetrations through the ceiling need to have careful consideration given to them to minimise cold bridging, air or moisture penetration.

If you have the headroom, some roofing batten under the vapour barrier, with all penetrations by nails/screws sealed (run sealant along the back edge of the batten before holding them in place to be nails or screwed) will provide a channel for any electrical cables.

Of course the best solution would be to rip off the deck, install new OSB board on top of the current joists, followed by 120mm insulation and then fibreglass, but this will certainly cost a bit more than insulating below.

It may be worth considering though if the roof is ageing or it isn't that big.

I have a the above spec'd warm roof on my garage (along with a top layer of OSB, and insulated door and 60mm in the cavity walls) and it is warmer that any part of my house (Victorian house with 110mm external insulation) and I have never heated it!

phil24_7

I will also add that the vapour barrier should be sealed properly around all of the edges.

If your walls aren't insulated (and you aren't able to easily insulate them), you could do carry out internal insulation of all the external walls at the same time as you redo the bathroom, you could then continue the vapour barrier right down to the floor and seal it there.

seneca

Thanks everyone for the help and advice. It looks not too difficult. I'm a reasonably competent DIYer, and so am considering doing it all and then just leaving the plasterboards for a professional to skim. I'm looking to draw up a list of things I'll need, so would appreciate help on a few points

1) 120mm thick insulation board leaves around 30mm on the 150mm lower joist. Should I leave a gap before the plasterboard, as some recommend, or use 150mm thick insulation and have the plasterboard touching it.

http://www.wickes.co.uk/Celotex-XR4000-Insulation-Board-1200X2400X120mm/p/113799

2) Which plasterboard should I get: the vapor panel with foil backing?

http://www.wickes.co.uk/Knauf-Vapour-Panel-Square-Edge-2400x1200x12-5mm/p/220230

OR

The moisture panel with no vapourshield foil?

http://www.wickes.co.uk/Knauf-Moisture-Panel-Tapered-Edge-2400x1200x12-5mm/p/190540




If foil backed vapour panel or the non foil backed moisture panel, do I still need a polythene vapour barrier between the board and the joist?

http://www.wickes.co.uk/Wickes-Green-Polythene-Vapour-Barrier-2-5-x-20m/p/153230

phil24_7

You need a gap above the insulation to stop the joists rotting. The cross joists at the very top will not help airflow very much so allow 50mm-ish at the top of the bottom joists for this purpose.

This will give you the 100mm insulation between the joists and double up with 20-25mm insulation below the joists.

Tape all the joints of the lower insulation carefully and seal (something like acrylic acoustic sealant that remains flexible) and tape around the perimeter.

You wont necessarily need to vapour barrier once this is done but I feel it would be worth it in a bathroom setting for a belt and braces approach.

I would then cross batten with roofing battens, nailed or screwed to each of the joists to provide a small service void, making sure all of the penetrations are sealed against vapour passage (using the sealant on the upper side of the batten).

Then moisture board the ceiling and get it plastered (I wouldn't bother with vapour check board as it is never as good as either of the other vapour barriers I have recommended and these easier to achieve, plus moisture board will outlast most other boards in a bathroom where there is the possibility of moisture getting into the service void). Any penetrations for wiring need to be done carefully so as you don't pierce the vapour barriers.

seneca

Ok. Thanks. I'm going to go with the 120mm between the joists. Then 25mm below joists, vaporshield sheet, moisture board and then skim. Sound ok? (simplified form , but then I'm simple!)

phil24_7

You really don't need 120mm between the joists and it will reduce the airflow above the insulation. 100mm will be fine. This is needed to stop the joists and deck rotting.

Don't forget the gap for electrics between the moisture board and the vapour barrier!