Cold roof or warm roof

[jamoritch]

we’re currently looking at replacing the flat roof on our kitchen. It’s the only flat roof on the property. However we’re stuck between a cold roof or a warm roof. Currently our kitchen has no insulation and due to it being a solid wall would have to be done internally or externally which we’re currently not looking into. One person who came to quote advised as we’ve got no insulation in our kitchen we won’t see the benefit of a warm roof really and spending the extra money for it to be a warm roof. However a builder who lives next door thinks cold roofs may become abit obsolete due to having to be a certain energy efficient standard. So I don’t really know what to do! Any opinions would be greatly appreciated. Thanks

[Big Jimbo]

Have a look at the cost difference between the Two. You will get some benefit from a warm roof. If you think that you might stay for several years, and you can afford it, go for the warm option. If you are planning on moving on soon, go for the cheapest option.

[Iceverge]

What's the roof pitch? Any pics?

 

Warm roof tends to be safer for rot/decay unless you can really guarantee the workmanship. 

[Dave Jones]

warm roof better, harder to build correctly. Issues if not detailed correctly. Need more height 200mm

 

Cold roof, easier to build, just leaks warm air if not done correctly, doesnt need extra height.

[ProDave]

I am a warm roof champion.  A good compromise is hybrid, some insulation over the top before the deck and covering, and insulation between the rafters as well.

 

Upgrade the roof insulation as much as you can now while you are doing it, the walls can be done later e.g next time you re fit the kitchen.

[Temp]

On 12/05/2023 at 06:47, jamoritch said:

One person who came to quote advised as we’ve got no insulation in our kitchen we won’t see the benefit of a warm roof really and spending the extra money for it to be a warm roof. However a builder who lives next door thinks cold roofs may become abit obsolete due to having to be a certain energy efficient standard.

 

Your builder next door is correct.

 

Cold roofs need ventilation between the waterproof layer and the insulation. This can be difficult to achieve if the rafters run perpendicular to the wall of the house (which they normally do. You should really fit a special vents all along where the roof meets the wall of the house under the flashing. Many roofers don't bother to do this.

 

The only real disadvantage of a warm roof construction is that the overall thickness of the roof is normally greater. You can mitigate this by putting some insulation between the rafters as well as above but not too much. The normal rule of thumb is no more than 1/3rd between and 2/3rds above, but you should really get a condensation risk analysis done to check the exact make up.

 

National Federation of Roofing Contractors..

 

https://www.nfrc.co.uk/docs/default-source/form-protected-documents/homeowners/householders-guide-to-flat-roofing-2015.pdf

 

Quote

In Britain as a whole, the WARM ROOF is strongly recommended.

 

LABC Guide..

https://www.tendringdc.gov.uk/sites/default/files/documents/planning/building control/Flat roof guide for website PDF V2.pdf

 

[Dave Jones]

1 minute ago, Temp said:

 

Your builder next door is correct.

 

Cold roofs need ventilation between the waterproof layer and the insulation. This can be difficult to achieve if the rafters run perpendicular to the wall of the house (which they normally do. You should really fit a special vents all along where the roof meets the wall of the house under the flashing. Many roofers don't bother to do this.

 

The only real disadvantage of a warm roof construction is that the overall thickness of the roof is normally greater. You can mitigate this by putting some insulation between the rafters as well as above but not too much. The normal rule of thumb is no more than 1/3rd between and 2/3rds above, but you should really get a condensation risk analysis done to check the exact make up.

 

National Federation of Roofing Contractors..

 

https://www.nfrc.co.uk/docs/default-source/form-protected-documents/homeowners/householders-guide-to-flat-roofing-2015.pdf

 

 

LABC Guide..

https://www.tendringdc.gov.uk/sites/default/files/documents/planning/building control/Flat roof guide for website PDF V2.pdf

 

 

be careful of using a 'hybrid' method with insulation below. very much harder so seal the roof structure to make it airtight.

 

 

 

[Temp]

Google found a condensation risk analysis for a warm flat roof using 120mm PIR above and 100mm Mineral wool between the rafters. Hope they don't mind me posting a llink and extracts here..

 

https://bmdevelopments.net/wp-content/uploads/2016/10/Condensation-Risk-Analysis-Flat-Roof-120mm-PIR-100mm-Wool-17Sep2020.pdf

 

 

 

[Temp]

2 minutes ago, Dave Jones said:

 

be careful of using a 'hybrid' method with insulation below. very much harder so seal the roof structure to make it airtight.

 

 

 

 

+1

 

Best put it all above the rafters if you can.

 

 

[Temp]

These are the sort of vents that should really be used on a cold roof construction. Note the OSB deck doesn't go right up to the wall. There is a gap for air flow.