1970's Hybrid Flat Roof. Help Me!

[BlueTangClan]

We recently had our plasterboard ceilings taken down in our mid-terrace 1970's house.

I thought I would be able to add some insulation before replacing the ceilings, and after getting some advice from a family member who had a contact with a "Technical person" at an architects, was all set to place insulation between joists leaving at least 50mm gaps to the roof.

However, the more I looked into things (videos of sweating rooves) the more worried I became about doing this.

 

The roof has no ventilation and previously had no vapour barrier, has also had leaks previously, but with no long term issues arising from these.

Hopefully the attachments will help explain things (please excuse my diagrams).

 

My questions are;

Can I safely add insulation to this roof from inside?

if not, do i just put a vapour barrier or a VCL up and then plasterboard?

Do I need to get a Residential Surveyor in to tell me what to do?

 

Any help and advice with any of this is greatly appreciated and will hopefully help to ease my anxieties about this.

 

Thanks,

Jonathan

[ToughButterCup]

14 hours ago, BlueTangClan said:

...

The roof has no ventilation and previously had no vapour barrier, has also had leaks

...

Can I safely add insulation to this roof from inside?

...

 

No. Not without ventilation or vapour barrier of some sort.

Here's a link to a sensibly written explanation of the differences between warm and flat roofs.

 

The picture isn't quite a simple as portrayed above, though. As I understand it, there are several membrane types to chose from. I'll leave discussion of that to people who know what they are talking about.

Here's how we designed our bit of flat roof ( cold roof)

 

Hope that helps a bit.

Ian

 

[BlueTangClan]

Thanks for the information.

I think it looks like we'll end up reinstating the plasterboard with a vapour barrier and not adding anything else. And hope it looks after itself like it seems to have done for the past 50 years.

Next time we re-roof I'll look into the possibilities of making it a warm roof.

[tonyshouse]

The amount of insulation seems wildly insufficient- 300mm of quilt or 150mm sheet insulation minimum, more is better 

 

better to do it mow than next time 

[SteamyTea]

Putting in a vapour barrier will improve airtightness, so that should help thermal performance a bit.

 

As it is a flat roof, can you add external insulation, and improve the covering to stop leaks?

[Dave Jones]

a cold roof with zero ventilation. good luck breathing in all the mold spores that will inevitably grow.

[BlueTangClan]

10 hours ago, SteamyTea said:

Putting in a vapour barrier will improve airtightness, so that should help thermal performance a bit.

 

As it is a flat roof, can you add external insulation, and improve the covering to stop leaks?

 

In theory I think we could add insulation externally and cover again, but I'm guessing any roofer would prefer to just to re-roof with a warm roof and we weren't really looking at external options at the moment (maybe we will be now) and due to neighbouring roof levels I doubt it would be much more than 70mm PIR.

 

Now this is where I start doubting every decision I come to.

It has woefully inadequate insulation by todays standards. BUT has this lack of insulation combined with no vapour barrier been a benefit to the roof? ie reducing the difference in internal and external temperature and/or allowing it to breathe back into the house? 

 

Would installing a vapour between the plasterboard and rafters be a mistake?