Insulating ceiling (flat roof)

[Gwynee]

I am seeking advice on how to insulate the flat roof above my flat. I live on the top floor of a 3 storey block and the flat has always seemed cold in winter and expensive to heat. Yesterday I opened up a small hole in the ceiling to have a look at what is up there; the answer is nothing. No insulation, just the plasterboard ceiling with timber joists approx. 290mm deep and an air void between. Above the joists I can see the underside of what looks like wood wool board, which I assume is the roof deck.

I had the flat redecorated when I moved in a couple of years ago so I’m reluctant to have all the ceilings taken down to install insulation boards or fibreglass, but maybe it’s the best option. Is there a better way anyone can suggest? Would blown insulation work?

Also what about condensation? Would it be a problem due to creating a cold roof?

I thought about applying for a Green Homes Grant but from what I have heard it might be more trouble than it’s worth. I have a good friend who is a general builder/jack of all trades so would probably get him to do whatever work is required, unless it needs a specialist.

[staffordian]

I don't know how practical it is, but if you have high enough ceilings, I wonder if it might be possible to fasten insulated plasterboard onto the existing ceiling?

A downside as I see it might be cold spots where there are existing internal walls which could lead to condensation.

It's not something I've ever seen, but it's something I've idly considered for our bungalow which has such a shallow pitch in the tiled roof that it's extremely difficult to get any meaningful thickness of insulation up there.

[AsleepInYorkshire]

I am seeking advice on how to insulate the flat roof above my flat. I live on the top floor of a 3 storey block and the flat has always seemed cold in winter and expensive to heat. Yesterday I opened up a small hole in the ceiling to have a look at what is up there; the answer is nothing. No insulation, just the plasterboard ceiling with timber joists approx. 290mm deep and an air void between. Above the joists I can see the underside of what looks like wood wool board, which I assume is the roof deck.

I had the flat redecorated when I moved in a couple of years ago so I’m reluctant to have all the ceilings taken down to install insulation boards or fibreglass, but maybe it’s the best option. Is there a better way anyone can suggest? Would blown insulation work?

Also what about condensation? Would it be a problem due to creating a cold roof?

I thought about applying for a Green Homes Grant but from what I have heard it might be more trouble than it’s worth. I have a good friend who is a general builder/jack of all trades so would probably get him to do whatever work is required, unless it needs a specialist.

In order of cost and ability to insulate starting with the cheapest/worst insulation properties

1. Glass fibre/quilt
2. EPS (polystyrene)
3. PIR

None of which can be placed within the ceiling void without removing the plasterboards. I've no experience of blown fibres or the like in existing roof spaces. Replacing with insulated plasterboards is optional but in reality they cost more than separate plasterboard and insulation.

Condensation could be a problem as you may move the dew point of the roof structure. This could be overcome, I think, using a foil backed plasterboard. It's usual to vent above the insulation with air circulating from the outside. This may add another problem to the works and costs.

If you have high ceilings you may prefer to leave them in place and insulate underneath them, using the airspace above to create ventilation. Noting that the critical issue in all of this is not to overlook ventilation or cut corners with it. If there is not sufficient airflow to the void it will have to be added. If you use PIR insulation it will provide the highest thermal value per mm thickness. If you only have a roof void of 290mm above the existing boards you will not be able to fill that with quilt insulation. Although the most expensive PIR may be your best option if you pursue the work.

AiY

[sg31]

What have other top floor tenants done about the problem? Has it worked?

Modern flats rarely have high ceilings but If you think you can get away with yours being lowered without too much detriment I would go for some form of board insulation like 'Kingspan' ,the thicker the better but bear in mind the ceiling height. Check insulation values and cost. There are some very highly insulating materials that are very thin but they were eye wateringly expensive. From memory I think I first heard about them on this board.

I would leave the existing ceiling in place, it's a messy job removing it and you have to consider the fireproofing. Adding something below the existing ceiling avoids that problem.

I can't comment in detail on the condensation problem without knowing the buildings construction. I would expect that if the ceiling attracts condensation now then making the ceiling warmer will move the condensation elsewhere. If you don't have a condensation problem now you shouldn't have one later. Moisture always condenses on the coldest surfaces.

[Gwynee]

Thanks for the replies and advice.

The ceiling height is 2.35 metres (about 7ft 8ins). I suppose I could afford to lose a bit of height but I'd prefer not to. The recommended thickness for PIR loft insulation seems to be 150mm and that is certainly too much to lose, but could be accommodated in the roof void and still have 140mm air space for ventilation.

An added complication is that my block is the last one in the development to have its roof re-felted and when that gets done it will be insulated externally, but it could be several years before it happens. I'm not sure I can put up with it that long as it is!

Perhaps one option would be to install a reduced thickness of PIR board (50mm, say) as an interim measure to make it more habitable for the next few years until the roof gets done. Losing that much from the ceiling height wouldn't be too bad.

[AsleepInYorkshire]

Thanks for the replies and advice.

The ceiling height is 2.35 metres (about 7ft 8ins). I suppose I could afford to lose a bit of height but I'd prefer not to. The recommended thickness for PIR loft insulation seems to be 150mm and that is certainly too much to lose, but could be accommodated in the roof void and still have 140mm air space for ventilation.

An added complication is that my block is the last one in the development to have its roof re-felted and when that gets done it will be insulated externally, but it could be several years before it happens. I'm not sure I can put up with it that long as it is!

Perhaps one option would be to install a reduced thickness of PIR board (50mm, say) as an interim measure to make it more habitable for the next few years until the roof gets done. Losing that much from the ceiling height wouldn't be too bad.

A finished ceiling height of 2.35m isn't something you would want to go below. So you are left with the options of insulating the void or above the existing flat roof. As long as you can achieve a decent ventilation flow above the PIR board then that appears to be your only option. However, if the flat roof is to be re-felted and insulation added at that point you may find it cheaper and more convenient to spend more money on increasing the heat in your flat. PIR insulation costs and re-plastering costs will not pay for themselves on heat savings in less than a few years. Plus there's the hassle of it all. Until you've see the dust made when doing this kind of work it's difficult to understand how much there is.

AiY

[Grumpsimus]

Thanks for the replies and advice.

The ceiling height is 2.35 metres (about 7ft 8ins). I suppose I could afford to lose a bit of height but I'd prefer not to. The recommended thickness for PIR loft insulation seems to be 150mm and that is certainly too much to lose, but could be accommodated in the roof void and still have 140mm air space for ventilation.

An added complication is that my block is the last one in the development to have its roof re-felted and when that gets done it will be insulated externally, but it could be several years before it happens. I'm not sure I can put up with it that long as it is!

Perhaps one option would be to install a reduced thickness of PIR board (50mm, say) as an interim measure to make it more habitable for the next few years until the roof gets done. Losing that much from the ceiling height wouldn't be too bad.

External insulation under the roof covering would almost certainly be the best and cheapest solution.

Looking at an internal solution, have you ever come across Aerogel which is a super insulator and regarded as the best insulation material in the world. It is ideal where space is at a premium. It is possible to 10mm of Aerogel attached to a 9mm carrier board, so 19mm total thickness. The catch is that it is quite expensive.

[sg31]

External insulation under the roof covering would almost certainly be the best and cheapest solution.

Looking at an internal solution, have you ever come across Aerogel which is a super insulator and regarded as the best insulation material in the world. It is ideal where space is at a premium. It is possible to 10mm of Aerogel attached to a 9mm carrier board, so 19mm total thickness. The catch is that it is quite expensive.

Aerogel is probably the one I was thinking of when I said some insulation was eyewateringly expensive. Having just checked the price my pulse is still racing.

I think the information that the OP has provided that the insulation will be done in a few years anyway will mean Aerogel will be far from cost effective.

Turn up the thermostat and bugger the expense.

[kempiejon]

Get a £200 ceiling fan to push the heat down?
https://www.creoven.co.uk/the-ceiling-fan-during-winter

[mysoulisyou]

Any planning gurus or people who have gone down this route please? We're hoping to convert our loft. As we will need to add a dormer to the principle elevation (for stairs) I understand it will not be allowed under permitted development and will require full planning.

I guess I'm a little confused by the term "roof space". Can anyone explain?

[DrFfybes]

AIUI you still need planning permission to build beyond the roof line (ie a dormer) at the front of your house, but not the rear or sides. Some councils frown upon this - our neighbour had permission for 'chicket' dormers refused despite being in a bungalow set back and down from the road, which is also a private road, and no objections from any of their neighbours. You wouln't even have seen the dorners from the road!

Can you rejig the floor below so the stairs run to the back of the house?

Paul

[jackdaww]

i have used 9mm tongue and groove pine cladding extensively on walls (solid) and ceilings .

you could contrive some air space with battens .

even two layers would fit into a couple of inches .

once in place , its easy to paint , and easy to attach things to if wanted.