Insulating flat roof (moved from Projects)

[Duncaroo]

Hi DIYnot,

Wonder if I could call on your expertise. I have 1973 house with flatroof with asphalt on tonge-in-groove construction. Under this is an 11" high cavity with no ventilation between rafters. There is 20mm polystyrene insulation resting on plasterboard with the obligatory artex finish. Internal walls do not intrude on cavity.

I know the above is not ideal and would fail any modern-day regulations but what can i do with this to stop me cooking in summer and shivering in winter?

My thought is to remove some of central part of the pasterboard and push fibreglass into each cavity, then board up the hole with new plasterboard.. I could then get a contractor to skim the result..
Any thoughts on this?

I've knocked a hole to take a photo up and down cavity:
Looking west:

Lookingwest

• Duncaroo
• 1 Jun 2016

Looking east:

Lookingeast

• Duncaroo
• 1 Jun 2016

 

[tomfe]

Make sure you keep 25mm ventilated void above what ever insulation you add. I would probably use rigid insulation 25mm smaller in depth of the joist and keep it flush with bottoms.
Depending on the condition of the roof (if it needs replacing soon) you could do a warm roof, over board with insulation and a new roof ontop.
What kind of lights do you have in the ceiling?

 

[Duncaroo]

Thanks for the reply, Tomfe.

The rooftop is a continuous stretch along the terrace of houses so adding to the top of this is complicated with party boundaries. I know that warm roof construction is recommended but for this reason I would prefer to pursue options to tackle this from underneath.

Ceiling lights are just pendants from the plasterboard. I will look to replace these at some point. As you can (just about) see, there are cables running to these through the void which is partly why i'd thought to opt for fibreglass rolls.

Can i ask why you suggest rigid installation?

Thanks again,

 

[tomfe]

Easier to to maintain the air gap, better insulation values.
At the moment you have a u-value of around 1 (25mm eps, unheated void, felt)
I am guessing they are 175mm joists, so if you are really luck you can get 150mm fiberglass in there, that gives a u-value of 0.35.
But I would be a little worried about blocking off your ventilation so next size down in 100mm which would give you a u-value of around 0.49.
100mm pir would give 0.39, 150mm 0.28 (this includes thermal bridging joist at 450mm centers and 175mm deep)

So now I have confused you with numbers, or maybe not, anything extra you add will make a massive difference!
You could do 100mm fiberglass and insulated plaster board.
Depends how much you want to spend and what the rest of the house is like.

 

[Duncaroo]

A little blinded by science here. I'm sure there is plenty of reading to be done in this area.

I think your last comment on cost and state of rest of house is important here. I will likely settle with fibre glass rolls only and try to keep the existing insulation in place where possible.

There is a 280mm void to partially fill.. Any improvement will likely make a huge difference from what's in place atm.

Thank you for your kind help.