Flat roof insulation

[patrocks]

Hi,

I have just had a flat roof recovered. New board, added a Velux and EPDM. When I was getting a price I also asked that it was well insulated. I was clear about this. When the price came back I again raised insulation and said I am happy with the price but can you upgrade the insulation being used, happy for the price to go up but as this roof should be in for awhile I want to do it right. An increase of £100 was agreed for better insulation.

To cut a long story short my roof isn't insulated. I called the installer to ask what insulation he had used and he honestly said "oh eck, I forgot". He asked if we had boarded and said he could put some celotex in.

Now my worries is that this makes it a cold roof and whilst I am not a roofer I believe this is not the way a roof should be constructed nowadays. I asked him about this and he said he had never heard of insulation being placed above the board.

Is this normal? I have just googled flat roof insulation and everything I read says the insulation should go above the boards.

Am I wrong to think a flat roofer should also know this?

How bad is it to insulate under, ie a cold roof.

What would people suggest the best way forward.

Thanks

Pat.

 

[jonez]

Insulating under a roof is fine, the main advantage of insulating over is that it works better because you can fit a thicker, continuous layer. If insulation goes under then you have to take care that moist air from the warm side can't get to the cold side or you get condensation. Typically that means using foil backed plasterboard for the ceiling or a polythene vapour check membrane.

Unless it's a tiny roof £100 wasn't going to buy much insulation anyway. Not sure what your roofer had in mind when you agreed the job.

 

[patrocks]

Thanks for the reply. The roof minus velux is 7.5 square m. 100mm celotex is under £15,per m at Wicked so £100 for extra insulation will cover it. Any re roofing is required to have insulation anyway. The 100 was to put thicker on.

I am not sure I agree the main advantage of a warm roof is that you can get more insulation in. All my reading is that it avoids the condensation risk and this is my concern.

The roof is above a kitchen. Moist and warm. The plasterboard will not be complete as it will have down lights.

Looks like I am stuck with it any way.

 

[Alastairreid]

The only option left to you will be rigid insulation between the joists with a 50mm vented gap between the deck and insulation, then you will need to insulate across the joists to stop cold bridging.

 

[patrocks]

Can you expand on the thermal bridging please.

I plan to put solid insulation between the joists with a 50mm air gap between the top of the insulation and the board. Seal the gaps between the PU board and the joists with mastic to try and stop moist air getting to the cold side.

Where do I insulate across the joist?


Thanks

 

[Alastairreid]

The underside of the joists would have insulation boards fitted across them to prevent a cold bridge between the outside and inside ceiling which can cause shadowing on the plasterboard.

http://www.basildon.gov.uk/CHttpHandler.ashx?id=1284&p=0