Warm Flat Roof?

[Steve111222]

Hi, hope I have posted in this in the right section of the forum..

i'm getting a loft conversion built - the builder has put 25mm of insulation above the joists and 120mm inbetween the joists in the flat roof. he's said this makes it a warm roof therefore it doesnt need any ventilation..

is this correct? do i need to be worried about condensation etc? Usually the insulation on top is 100mm or so from what i understood.

attached a basic drawing of what he's done

 

[pilsbury]

No expert, but that sounds like a hybrid warm/cold roof...... not sure about ventilation requirements for cold roof applying here. If I were to guess I’d say being both types of roof it would need both types of moisture surpression.... VCL for warm part, ventilation for cold part. But just a guess.

 

[freddiemercurystwin]

What do the drawings say?

 

[Steve111222]

The drawings said a warm flat roof with 120mm insulation above the joists. the builders put 25mm at the top because they said that will provide additional height in the loft. they had already done then told me afterwards. The drawings were then changed to show a cold deck roof, but they haven't allowed space for ventilation.

 

[Buck_68]

That’s a hybrid roof and will need ventilation as the insulation below the roof structure is greater than that above. 25mm is nothing really. Most should spec 100-150 these days.

 

[noseall]

Its a cold roof and will require venting.

 

[Notch7]

I presume its got 120mm celetex type insulation in between?

If so it needs to have the joints taped above to join all, the celetex foil together forming a vapour barrier.

The 120mm celetex should have all gaps filled with foam gun.

If thats done, then I would be surprised if thermal bridging at the joists will be enough to allow interstitial condensation to form.

I reckon it will fine. I would advise using foil backed plasterboard to minimise any vapour getting through.

I would say the problem is that the insulation manufacturers dont test for hybrid roof options and they wont approve any as they dont want any risk at all of claims, so hybrid roofs arent therefore cibsidered acceptable building practice.

How many timber window frames suffer condensation on the timber? Then imagine adding a layer of 25mm insulation to the outside. Even less chance of condensation. I know thats not a true example as a warm roof isnt open space so any moisture would build up, but it still paints the picture.

 

[ban-all-sheds]

What are Building Control expecting to find?

 

[^woody^]

is this correct?

Yes

 

[Steve111222]

I presume its got 120mm celetex type insulation in between?

If so it needs to have the joints taped above to join all, the celetex foil together forming a vapour barrier.

The 120mm celetex should have all gaps filled with foam gun.

If thats done, then I would be surprised if thermal bridging at the joists will be enough to allow interstitial condensation to form.

I reckon it will fine. I would advise using foil backed plasterboard to minimise any vapour getting through.

I would say the problem is that the insulation manufacturers dont test for hybrid roof options and they wont approve any as they dont want any risk at all of claims, so hybrid roofs arent therefore cibsidered acceptable building practice.

How many timber window frames suffer condensation on the timber? Then imagine adding a layer of 25mm insulation to the outside. Even less chance of condensation. I know thats not a true example as a warm roof isnt open space so any moisture would build up, but it still paints the picture.

Thanks for the reply, and sorry in delay replying!

That's a bit re-assuring. I think it is just the 100mm celetex they have used though.

Not sure if they have filled gaps with a foam gun, i will ask the question. Noted on the foil-back. I did reach out to Celetex and yes they said they don't approve usage in between joists which got me concerned.

 

[Steve111222]

What are Building Control expecting to find?

Building Control came in and they were expecting normal warm flat roof as per the plans (i.e. 100 / 120mm on top of the joists)

 

[^woody^]

Building Control came in and they were expecting normal warm flat roof as per the plans (i.e. 100 / 120mm on top of the joists)

That is fine, but there is nothing wrong with something unexpected as long as its acceptable.