flat roof Cold roof help please

[Mark Fenemer]

Hello, I would like some info/advice regarding a cold concrete flat roof. Sorry for the length of explanation.... 

But first a little background info...... 

The house we purchased was gutted, renovated and then sold on to us by a local building company... All certs for all upgrades and works were checked and passed.... 

Our problem.... 1

We moved in Sept 2107 all is fine until the first frost off this winter late 2018..when we started to notice a water stain in the rear utility ceiling (formerly downstairs rear loo and storeroom). These rooms were incorporated in to the house via the kitchen to form a utlity/cloakroom and extra kitchen space..... 

The roof itself is around 8 inches of concrete topped off with felt.. This was refelted just before we moved in... And the rooms below batterned, insulated, boarded, skimmed and redecorated. Oh and rads fitted. 

Since this watermark has appeared and got worse over the last month or two.. I cut a peice of board out, to be greated with a soaking wet inner roof, droplets everywhere....

Our problem 2....

The house is a semi formerly a council house, we have great neighbours who are council tennents... They have the same sort of rooms as us but still the original loo and store (no heat sourse).. And attached to ours

So taking advise online of fitting a VCL then a thick insulation then watertight layer is not really an option without affecting the council properties roof.. 

 

So I am left with trying to sort the problem from inside.. Although there is much advice regarding how this is now not advisable.and insulating from outside is they way to go... Is there anyway I can fix this from within... I am happy to take down the internal ceiling and start again. But what would be the recommended materials etc etc.. To ensure no water ingress... You can see by the images that there is no VCL inside and the insulation is very thin and not sealed in anyway. Just placed in the roof void not a good job at all.. ... Please please any advice would be gratefully received.... Mark

[Mr Punter]

Remove the ceiling and insulation.  If possible, stick 75mm sheets of Celotex to the concrete underside with low expansion foam.  Foam any gaps and seal joins with alu tape.  Cover with clear polythene vapour barrier with all laps sealed, held in place with 38mm x 50mm battens as service void, fixed through to the concrete with insulation fixings.  Finish with foil backed plasterboard.

[Jeremy Harris]

Welcome.

 

I'm concerned that this work was apparently signed off by a building control body fairly recently, as it's definitely sub-standard and the company that undertook the work must have known full well that they were creating a major defect by doing the job in the way they have.  Sadly I doubt that you have any realistic prospect of getting recourse from them, even though the work is clearly not to an acceptable standard.

 

I agree 100% with the above solution, it seems about the only sensible way to fix this.  Stopping water vapour from inside being able to reach the cold concrete and condense is really the only way to fix this.  Stopping the concrete getting cold by insulating the outer face would be ideal, but is obviously not an option.
 

The only thing I'd add to the fix described above by @Mr Punter would be to try and make sure you lap the VCL down the upper part of the walls and seal it tight to the walls if you can, just to try and make sure water vapour can't find it's way up to the cold concrete at all.  You probably need to have a think about any ceiling lights you may have in there and how best to fit any recessed lights you may have.  You can get very low profile LED lights that are barely any thicker than a sheet of plasterboard now, that aren't that expensive and work very well when there is only a narrow space behind the plasterboard.  They also run cool, so don't present any significant over-heating risk, unlike something like halogen downlighters, that wouldn't be a good choice, IMHO.

 

Be prepared for drilling the fixings into the concrete to be bit of a tough job.  If, as I suspect, that concrete roof is around 50 or more years old, then it will probably be close to peak hardness by now, and be hard work to drill.  However I'm sure it will be worth it, both to stop the condensation problem and markedly improve the comfort level.

[Andyoxon]

I also have a cold concrete  roof that I can not insulate on top of due to a neighbours adjoining property having a window at my roof level. So I’m trying to work out the best way to insulate from inside without loosing too much head room. I wondered if spray foam was an option or perhaps using     Aerogel insulation attached to timber battens https://enviroform-insulation.com/product/envirostud/

 

Building control haven’t been much help as best practice is to insulate from the outside which isn’t possible so I’m seeing if I can achieve good insulation levels with too much thickness, I know traditionally you need a vented void but I would like to avoid this as it uses up head room I was hoping there is away to round this. Building control suggested  a spray foam could be a good option but wasn’t used to seeing large concrete flat foot in residential buildings.

 

any suggestions welcome.

 

Andy

 

[Mr Punter]

Could you do 50mm Xtratherm / Celotex with joins foil taped, held in place by 25 x 50 battens then 15mm plasterboard?  You could even staple a VCL to the battens before you board.

[Temp]

That aerogel has a thermal conductivity of 0.015 W/mk compared to 0.023 for PIR boards or PU spray foam insulation (lower number is better).

 

So 65mm of aerogel space blanket is about the same as 100mm or PIR/PU foam...

 

100mm *15/23 = 65mm

 

 

 

 

 

[scottishjohn]

I like closed cell foam -but maybe simpler to use pir backed plaster board and stick it up

other wise you going to need to attach some sort of hangers to attach your PB to 

maybe .

areo gel is very expensive 

spray foam will also make it water proof and airtight 

so simplest is pir boards if no moisture problems check out insulation value of a 75mm foam backed plaster board 

Edited August 14, 2020 by scottishjohn

[Andyoxon]

Thanks for the replies all helpful, yes I know the areo gel is very expensive that’s why I was only looking at the thin stuff 10mm connected to batterns which I thought would produce a cold barrier from the surface of the concrete ceiling then I could attach an additional thin layer 25mm possibly 50mm of  celotex. Condensation is apparently a big problem with a concrete cold roofs we haven’t had a problem with this as we have owned the property for 13 years and at present the only thing on the underside of the concrete is a thin battery and foil backed plaster no insulation at all, but as we are doing extensive renovations and extensions I wanted to do the best job possible without spending rediculous amounts. 

 

If I use spray foam I will still need to put up some battens  to attach the plasterboard to what is the best way to attach these to a solid concrete roof to avoid cold bridging?

 

 

[Russell griffiths]

Nudura do an insulation board that has a batten built into the sheet, so imagine a 75mm sheet of insulation with a 35-40mm batten buried into the insulation, you would glue and screw this to your ceiling. 

 

I was  very impressed with it, but didn’t use it as I needed over 300m and it was getting ridiculously expensive, if it’s a smaller area it might not be to bad I think it was about £45 a sheet. 

 

 

[Andyoxon]

Thanks will look into it the area is around 65square metres