Converting flat roof from cold to warm roof

[askman]

One of the many little gems I've come across when renovating my house is that my flat roof is a cold roof - the insulation sits directly upon the plasterboard with a gap above between the insulation and actual roof.

I'm contemplating converting this to a warm roof as I may as well do it now whilst all the other work is going on and am trying to decide upon the best way to do it.

Would it work to fit new insulation tight to the underside of the roof instead of removing the felt and going over the top? My concern here is that I'd still be limited to going between the joists which doesn't seem ideal.

I could theoretically run the vapour barrier under the insulation and under the joists to provide an uninterrupted vapour line, but would have to screw through this to fix the plasterboard which seems like it will defeat the whole point of a vapour seal. Would this make a difference?

 

[jonnyben10]

Fitting insulation only between/under the joists will still behave like a cold roof, because the timber remains a thermal bridge and you don’t fully move the dew point outward. That’s where condensation problems usually start.

Your idea of adding a continuous vapour barrier sounds good in theory, but in practice every screw through the plasterboard slightly compromises the seal. You can reduce this risk with a properly taped VCL and a service void, but it still relies heavily on perfect installation.

If possible, the more reliable solution is a true warm roof build-up, where insulation is continuous above the structure. This avoids thermal bridging and greatly reduces condensation risk compared to internal-only insulation. you can use: https://jalft.com/collections/washers-caps-sheet-fixings

In short, internal insulation can work if detailed very carefully, but a proper warm roof is the safer and more durable long-term fix. know more on jalft.com

 

[phatboy]

Fitting insulation only between/under the joists will still behave like a cold roof, because the timber remains a thermal bridge and you don’t fully move the dew point outward. That’s where condensation problems usually start.

Your idea of adding a continuous vapour barrier sounds good in theory, but in practice every screw through the plasterboard slightly compromises the seal. You can reduce this risk with a properly taped VCL and a service void, but it still relies heavily on perfect installation.

If possible, the more reliable solution is a true warm roof build-up, where insulation is continuous above the structure. This avoids thermal bridging and greatly reduces condensation risk compared to internal-only insulation. you can use: https://jalft.com/collections/washers-caps-sheet-fixings

In short, internal insulation can work if detailed very carefully, but a proper warm roof is the safer and more durable long-term fix. know more on jalft.com

What a lovely AI generated reply, of zero value