vapour check in warm roof?

7 Jan 2013
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United Kingdom
Hi there
I am trying to make a decision on how best to upgrade the shallow mono-pitch roof on my bungalow. I'm hoping for abit of advice if possible!

The existing roof (100 metre sq) across the whole property is a cold deck construction, with rockwool insulation packed tightly between joists. The cold deck is not ventilated.

Essentially, I am hoping that it will be possible to build a new 'warm roof' construction over the existing roof (leaving the mineral felt, and existing insulated 'cold deck' in place). The roof covering would be zinc.

What is keeping me awake at night is the concern about condensation. The current cold deck is insulated but not ventilated. There is no vapour check. I'm hoping that with new insulation added over the top, the existing felt will act as a sufficient vapour check (albeit above the existing rockwool & ply)?

I'd really value any input on this. Thanks in advance for your help!
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Ok point taken! I'm renovating the whole house and I'm pretty amateur at DIY so i've worked myself up into a cold sweat over a few things in recent months... but yes, the aim with the roof is literally to avoid any 'cold sweats' in the construction!

I do hope when I'm old that i'll remember this time merely as fun & character building rather than anything more depressing!
The vapour control layer is the most critical point in any warm roof construction. It is often overlooked by architects, builders and even roofers!
Although its important what is used its more important how its installed. We install some of the most sophisticated and expensive roof systems on the market and offer guarantees for hundreds of years so I'm sure the methods we use stand up to the most stringent of tests.
Above the rafters we would install a marine grade plywood sheet with a minimum thickness of 18mm.
We would then prime the plywood and install bituthene, lapping joints and sealing against abutments and penetrations.
On top of this we would install between 100mm and 350mm insulation. Again sealing all joints, abutments and penetrations.
On top of this would be a separation layer, usually a breather membrane (tyvek or klober).
Then we would install zinc, fixing down through the insulation into the 18mm ply with stainless steel fixings through stainless steel clips with a plastic plug to stop thermal transfer.
Each stage is important and cannot be skipped, rushed or corners cut. A specialist job and typically around £200 a square metre.
Hope this answers some questions.
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I've had this argument many times, who is responsible for the vcl?
Builders say roofers, roofers say builders, etc............

Most roofers I speak to don't even know what it is!
Thanks for the input so far. I'm not really able to install a vapour check on the warm side. As above, any vcl would be above the insulated, non-ventilated cold deck & felt, but also below new insulation board & Metal covering.

I think the best I can do is foil backed plasterboard stuck on to existing ceiling board, but it's still sure to be an imperfect seal across every room in the bungalow. Is this a bad idea? Will more insulation and a Metal covering be worse or better than the current construction?

I've attached a couple of photos - of cold deck & exterior. View media item 63879 View media item 63880
It will be better than the current construction. You just have to decide how much insulation above the deck you're going to install?
Thanks for the input so far HMK
A metal roofer has advised 100mm of Kingspan type 26. Does that sound like enough? We already have 100-150mm rockwool in the existing structure.

So, it will be warmer than the current, but will there be less possibility of condensation? I'm assuming there would be less chance of condensation since the dew point will be removed to the top of the new insulation board, and our existing felt will act as a vapour check and prevent moisture from getting that far?

If I'm right on the above, when the warm roof is fitted I guess it means we'll have some fairly luke warm moisture present within the existing cold deck part of the roof, but, at least it won't be condensing up there!?

I'm hoping that by heating and ventilation of the house, this won't be a huge problem??

Any help on this appreciated
Sometimes I wonder if this condensation problem is overstated.
@OP; look at your present set-up; you have a basic plasterboard ceiling with 100 - 150mm of the itchy stuff between the joists.
Over the years, vapour from your hose will have been making its way up through the ceiling, through the wool, and up to the deck. Has it caused you any problems? Apparently not.
What you are proposing will make interstitial condensation even less likely, yet you still worry.
Presumably you have fans in the kitchen and bathroom, don't dry clothes on the radiators, and don't have a heated swimming pool in your lounge.
It's as much to do with lifestye as details of construction.
Go with the warm roof, and use foil-backed plasterboard if you must, and for goodness sake get some sleep!
If you're using the current felt roof as a vapour control I'd be very careful because its needs to be self-healing..... The metal man is about to put 300 holes through it to fix his zinc down! Remember this is supposed to be an air tight seal.
He has mentioned that expensive fixings will need to be used - I will go back to him and check on that.
But, if that problem can be overcome, does the existing felt do an effective vapour Control job?
Thanks for the dose of realism Tony. As I say I am pretty wet behind the ears with this renovation stuff, and for all it's benefits the internet has plenty of horror stories about the various pitfalls.
The other thing is just being very sacred of spending alot of money and causing bigger prproblems than I started with!
but, thank you for settling my mind abit

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