Insulating a Concrete Flat Roof - Can We Make a Cold Deck Work?

20 Jul 2021
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United Kingdom
Hi everyone, just signed up to ask a question that's been on my mind for a long time:

We live in a house which was built to house workers during the war effort. They were very functional, but unless modified have appalling thermal performance (think metal frame single glazing, concrete flat roof, vents in all the walls to prevent condensation due to said concrete roof, but just let in all the freezing air). The concrete roof was put on to repel incendiary bombs, but they're not a big problem these days...

Most of the houses have been updated, so ours now has double glazing and cavity wall insulation, but the big problem is the roof. In this weather, because its concrete, not only is it really hot to begin with during the day, but it stores up all the heat from the sun and then re-radiates it like a storage heater at night. It's really the last thing you need when trying to sleep!!

So obviously we would like to fit some insulation. What some people have had done is the installation of insulation above the roof, to create a warm deck(?), but the problem with that is that it looks a bit unsightly, and one day we'd like to have a pitched roof installed, so don't want to throw away money as it's not all that cheap to do this. Also, because of the way the houses are constructed, I suspect thermal bridging may be an issue with this implementation where the concrete roof section meets the walls (but in this instance it may be a case of a little knowledge being a bad thing).

We'd have a pitched roof put on now, but you need to do it at the same time as your neighbour because of the way the adjacent houses fit together (where we adjoin our neighbour, the front room is ours, but the back room is theirs). Because we need their buy in it's not something you can plan for very easily.

The other (cosmetically and practically preferable) alternative is to fit the insulation internally, but I understand that this results in a cold deck and is undesirable in our climate. Is there any way to mitigate the condensation issues? For example, could installing insulation right up against the deck help? It might not be as effective, but any improvement would be a good thing. Rightly or wrongly I feel that this would not allow condensation to happen as the cold air couldn't actually penetrate to the concrete as there'd just be insulation right up against it.

The other thing I thought might work would be to put some insulation up right against the ceiling, then an air gap, then some more insulation, to try to reduce the temperature gradient where moist air will hit the coolest surface?

The other possible enhancement would be to open up the vents which are just below the ceiling between the insulation and the cold deck, but I'm not sure if that would allow adequate air flow to prevent condensation.

Any comments would be appreciated! Thanks.
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The last thing you want to do is put the insulation right up against the underside of the deck. Water vapour will find its way through joints in the ceiling (eg light fittings) and joints between the boards themselves. Once the vapour reaches the cold side of the insulation, you'll get condensation and brown staining, particularly near the roof/wall junction.

Depending on what ceiling height you've got, you might need to consider a false ceiling, with insulation just above the plasterboard, and an air gap between the top of the insulation and underside of the deck. This void could then be vented by careful positioning of air bricks at the top of the external walls, to ensure there was air flow front-to-back. (There are figures in the Approved Document for ventilation which stipulate how much vent openings you need in mm² per meter run).
Thanks Tony

That's really helpful and backs up my suspicions. The only problem is that we'd have to air brick internal walls I suppose to give the air the best chance of circulating beneath the entire deck. If only our neighbour was as been to put a pitched roof on top as we are...
In theory you could insulate internally, but you would need to have insulation tight against the concrete with no gaps at all AND a complete vapour barrier preventing moist air reaching the cold concrete roof.
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Thanks Notch, this is what I was originally thinking. We might need to do something like this on our landing, even if we can implement ventilation, because it is enclosed by other rooms on all four sides.
Thanks Notch, this is what I was originally thinking. We might need to do something like this on our landing, even if we can implement ventilation, because it is enclosed by other rooms on all four sides.

If you fit Celetex tight to the concrete roof, you can foil tape all the joints and use expanding foam to fill any gaps, you stand a chance.

basically if any warm air from inside the house can reach the underside of the concrete it will cool and cause condensation.

if the underside of the roof is pretty flat you might be able to do it well enough.

I suppose you could use foam adhesive and some plasterboard support clamps to hold in place whilst it sets.

if you stick each piece to the roof, use adhesive between each board, around all edges etc. Then once done, you could foil tape all the joints and abutments.

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