warm / cold flat roof system ?

5 Jan 2011
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United Kingdom
I'm new to forums so please be gentle.
We had a flat roof replaced about 4 years ago and the lovely builder used a COLD flat roof system. Because there is no ventilation around the roof (external) we have since been told a WARM system should have been used. The room is now black with mould and unusable. Can this be resolved by removing the cealing and insulating correctly or does the roof have to be replaced :confused:
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A true warm roof system can only be fitted from above as the insulation sits wholly above the roof joists apart from a 25mm additional layer pressed tight up against the deck layer.

However, you may be able to fully fill the void with insulation from below fix 70mm celotex across the joists then fit a vapour control layer, plasterboard and skim.

Our local authority does not sanction the latter method as there are air pockets etc.

You may also have other insulation and condensation issues regards the walls....... :?:
Rip the ceiling down, and fill the entire space between the joists with insulation (quilt type is easiest) ensuring no gaps or air pockets - especially up against the roof deck, and then fit the vapour check layer (polythene) and plasterboard, or use foil backed plasterboard.

Noseall's suggestion of 70mm insulation across the bottom is better, but it will reduce the headroom, so may not be practical. Otherwise just a 10mm or 25mm insulation layer to the underside will lose less headroom.

But be sure that the roof work is the actual cause of the mould problem, and not just a case of ventilation or use of the room - otherwise you will have the same problem in another four years
If your ceiling is mouldy it means that you will need to ventilate between the joists, The warm air from the room is rising and when it hits the cold roofing membrane it condensates and becomes trapped, hence the damp on the ceiling. Filling the roofspace will not solve the problem, i would suggest installing soffit vents at either end of the roof creating a through flow of air allowing the condensation to escape. The other option will be to use mushroom vents installed on top of the roof. By doing this you may be able to leave the ceiling and re decorate saving you a small fortune. I hope this helps
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No, the ceiling is mouldy because moist air is condensing on a cold surface, and not because the ceiling void is not ventilated

By insulating the ceiling void, this removes the cold surface, raises the dew point, and so reduces or prevents the likelihood of condensation. There is no need to ventilate the roof
This official document seems to back up my comments? Make your own mind up!

by Charles Stirling
NBS Technical Author

The ever increasing levels of thermal insulation required by thermal regulations can result in a higher risk of condensation forming on those elements not enclosed by this insulation, e.g. roofing underlays; cold roof decks and external cladding or tiling.

In flat roof construction thermal insulation can be placed above and below the structural roof deck or waterproofing layer.When placed below the structural deck; ventilation is required to the underside of the deck.It is universally recommended that cold deck flat roof construction should be avoided. Where the insulation is placed above the structural deck (warm deck), and either above or below the waterproofing; there is no need for additional ventilation.
I don't think noseall or ^woody^ need a lecture in the rights and wrongs in warm & cold roof design.

A lack of ventilation in the roof void will not cause condensation to occur beneath the plasterboard. Condensation is occurring because the underside of the plasterboard is cold and the warm air as it passes through it into the cold void the water condensates and changes from a gas into a liquid which is appearing as black mold. Ventilating the void in this situation will not improve this situation as the void will still be cold.

The sensible solution here (assuming the OP doesn't want to rip the roof apart completely to redo properly) is to fully fill the void with insulation. It is not ideal as the the roof timbers will still be cold bridging but is about the best the OP can hope for as a fix.
Its always good to think in basic principles when faced with a room condensation problem - ie condensation forms on a cold surface, so how do I remove that cold surface.

The concept of a vented roof is purely to prevent condensation and subsequent timber rot within the roof void, and has no bearing on condensation within the room below

So again the next basic thought process is how do I prevent or deal with moisture that may enter the structure. And so a solid roof structure with no cavities or air gaps, protected from vapour ingress from below is preferable
The question is whether there is sufficient insulation within the roof, if there is then your mould problems may be nothing to do with the roof, if not then they may well be ,as no amount of ventilation within the roof space will prevent warm air condensing against your ceiling.

Did you notice the mould start on the ceiling or the walls first.

BC used to be quite happy in the past for a warm roof to be constructed with insulation between the rafters, but now won't generally accept this even with a membrane to prevent moist air entering the roof space.

It is also possible that you could have acombination of both problems and that both the ceiling amd the roof void is mouldy
Firstly the preferred method is a warm deck. If you choose to have a cold deck with inter-joist insulation then you should install a vapour barrier at ceiling height to slow movement of water vapour from the living accommodation into the roof space. In addition there should be provision for ventilation over the top of the insulation (between it and the cold deck). This is all for a good reason.

The vapour barrier at ceiling level tends to LOWER the dew point within the rood void, by slowing water vapour movement. Despite this some water vapour will find its way into the roof space so ventilation under the cold deck is required.

Filling the cavity fully with insulation is not helpful as it blocks ventilation. If this is done problems will arise on the cool side of the insulation, where the RH becomes higher in the small spaces within and around the insulation and the joists. Timber will reach equilibrium with the RH and when this remains above 80% RH the moisture content of the timber will rise to a level that will support rot.

You should consider adding insulation to the roof from the outside and creating a 'warm roof' design and save yourself all the aggravation indoors!

As a minimum you will need to use a 50mm thickness of PIR roofing grade insulation (or ideally 100mm.) The material comes with one side already prepared with a base layer, the choice is then yours as to what you finish off with e.g. EPDM, liquid plastic, or 2 layers of built-up felt?

Theorectically the warm roof design gets rid of the condensation problems that are encountered with 'cold roof' designs.

Your problem is likely to be either insufficient ventilation or too smaller or no air gap between the insulation and the underside of the roof.


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