How do you know if a flat roof is a warm roof?

[ralkicks]

Hi,

How do you know if a flat roof is a warm roof?

We've recently moved into a new house which has a bay window with a flat roof. The plasterboard ceiling was bowing in places so I have taken it down. There was no insulation above the plasterboard, and I can see the tongue & groove boarding sitting on the joists.

How can I be certain there is insulation above that boarding? There are a few nails from above poking through the boarding, which I assume are from the roof covering, suggesting any insulation (if present) is not deep.


Second question:
If I do have a warm roof, can I fill the void between the joists with more insulation as you would with a cold roof? If the boarding above is warm, condensation wont be able to collect on it, will it?

Thanks in advance.

 

[tony1851]

When you say 'new' house, do you mean new to you, or 'new' as in just built?

If the bay roof has t&g boarding, it is unlikely to be a newly-built house.
If that's the case, it is unlikely there will be any insulation above the boarding.

If you insulate from below, use something like Kingspan pushed up between the joists, and cover the lot with a decent, well-sealed vapour barrier before fitting the plasterboard.

You may still get a little condensation, but there's not much you can do with bay-window roofs as they are generally not vented.

 

[ralkicks]

Hi,

Thanks for the reply.

Yes sorry, the house is new to us but was built in 1930s, however it has been extended at some point in the past and this bay window is part of the extension.

The previous owner said the extension was there when they moved in, some 20 years ago, but he said he did have the flat roof re-done - I didn't think to ask at the time if he insulated it.

What makes you think it isn't insulated? I thought warm roofs were the norm these days?

 

[freddiemercurystwin]

Doubtful there is any insulation in my opinion too.

A few years back roofers generally wouldn't have even considered insulating replacement roofs and many householders were none the wiser. Though the tide is changing now. Some diligent measuring might establish the overall thickness which may or may not suggest some insulation, depending on the thickness though even with accurate determination of the overall thickness is not conclusive.

 

[ralkicks]

Hmmm its beginning to sound like there isn't any up there - that room must get seriously cold in winter!

Is there any benefit in completely filling the voids between the joists with insulation? ie not leaving the recommended 50mm 'ventilation gap' as there is no way to ventilate it anyway... This way condensation would not be able to gather on the timber boarding above, would it?

And then as was suggested above, fit a vapour barrier. Would this run straight across below the joists? Would this get damaged when I fix the plasterboards to the joists? Or would it be better to use a plasterboard whish includes a vapour barrier?

Thanks

 

[freddiemercurystwin]

Pack it with insulation, use a foil backed plasterboard underneath.

As mentioned by tony old fruit its not ideal but there's nothing else you can do apart from redoing the whole roof.

Its only what you would do if you were converting a flat roofed garage and leaving the roof finish as is. And Building Control accept that all the time.

 

[tony1851]

This way condensation would not be able to gather on the timber boarding above, would it?

That's the problem - it would. Warm moist air would eventually find its way up to the underside of the boarding, which will be freezing cold because of all that insulation underneath. It will then condense as droplets on the boarding.

Best you can do is fix a v.b. before the plasterboard and - as suggested by the fount of all construction knowledge above - use a foil-backed board for additional defence.

(There is always a problem with bay roofs and IIRC, Building Regs don't specifically ask for venting of them due to the practical difficulties).

 

[ralkicks]

Thanks for all the advice.

I will pack it full with rockwool, install a vapour barrier beneath the joists and use a foil backed plasterboard.

Hope it lasts!

 

[freddiemercurystwin]

Be aware that even when originally tightly packed, that Rockwool can sag, leaving a nice little air pocket.

 

[ralkicks]

Would you suggest to use something more rigid than rockwool in that case?

As for the vapour barrier, what is the benefit of using one as well as just using foil backed plasterboard? And by using one, do I risk locking any moisture that gets in there in above the barrier?

 

[freddiemercurystwin]

Before answering your vapour barrier bit how deep are the joists? Or how deep will the void be?

 

[ralkicks]

From memory, around 150mm deep.

 

[freddiemercurystwin]

well whilst i would not normally promote its use I would suggest polystyrene would be OK to use here. As you have a good sized void and should fill it then that would seem to be good contender.

You could go to the expense of Kingspan or Celotex but seeing as the voids quite big that would be overkill IMO.

Seal any gaps up with expanding foam. For the best vapour barrier, use a polythene sheet. With the best will in the world VB's always get punctured no matter what you use. Though IMO foil backed plasterboard would be adequate.

Don't worry about trapping any moisture up there as you fit it all, you are thinking too much.

 

[ralkicks]

Just had a quick look at B&Q website.

2.4mx1.2mx100mm thick Jablite insulation is £25 - I'd probably need 2 or even 3 of these, and even then they wont fully fill the 150mm deep void.

It's a bit expensive compared to rockwool.

Anyone know of any cheaper sources?

 

[Alastairreid]

Anyone know of any cheaper sources?

http://www.gumtree.com/for-sale/england/insulation+board