Unventilated cold flat roof - insulation

[Axel]

There's a lot written on this! I have a top floor bedroom with a roof above it that is in two sections - one side is pitched with felt and tiles, the other is 'flat' with boards and felt. The tiled side appears to be ventilated, the 'flat' side is unventilated and cannot be ventilated.

I have removed the layer of plasterboard beneath the joists. I will replace this but am contemplating heat insulation.

Already I can see droplets of condensation on the bottom of the felt in the tiny gaps between the boards.

There is no insulation between the boards and the felt.

I am on a low budget with this one.

What is the best economical solution?

I am considering just putting 2 layers of plasterboard back up to marginally improve the insulation as I am concerned about the condensation issues with an insulation layer. Maybe do insulation at a later stage if and when I replace the 'flat' roof from above?

Please advise, bearing in mind the low budget!!

 

[pinenot]

I would fit some 100 mm rockwool between the joists from the underside, leave a gap between the underside of the boards and the insulation as a vent space, you can put some plasterboard cuttings, sprung between the joists (cut 3" strips of p/board slightly longer than the space between joists, bend slightly pushing them in under the insulation to support it) this will provide sufficient temp support until the ceilings up. The moisture drops seen will be condensation forming on the cold underside of the roof deck, so can be ignored, try fitting the rockwool to one channel only leave it overnight and check it for moisture in the morning. If it's dry compared to the rest of the channels then it's condensation and can be ignored...pinenot

 

[Axel]

Thanks Pinenot... only problem is - the 'flat roof' part of it is not ventilated so any moisture reaching the cavity above the rockwall will have nowhere to go but condense on the cold boards and then make the rockwall soggy!

 

[pinenot]

Believe me there will be enough air movement to prevent condensation causing any problem, Bear in mind that by placing insulation in such close proximity the condensation should be eradicated or at least reduced to a level that wouldn't matter, that's why I suggested the trial, if it's not there after putting up some of the insulation overnight it won't be there ~ period...pinenot

 

[freddiemercurystwin]

Rockwool is inappropriate for this situation, it needs a rigid insulation that will not sag and leave a void.

 

[pinenot]

Rockwool is inappropriate for this situation, it needs a rigid insulation that will not sag and leave a void.

Sorry to burst your bubble but the plasterboard will fully support the mineral wool in the same way it's been doing so in lofts for years.

 

[Axel]

Hmm...It's not a loft, it's an unventilated cavity between plasterboard and the boards that the felt is laid onto, which will attract condensation if there is any gap under the boards at the top of the cavity. This condensation would soak into the rockwool leading to an unsatisfactory situation?

 

[freddiemercurystwin]

Rockwool is inappropriate for this situation, it needs a rigid insulation that will not sag and leave a void.

Sorry to burst your bubble but the plasterboard will fully support the mineral wool in the same way it's been doing so in lofts for years.

No of course rockwool sags and why would you bother with 100mm rockwool, it'll do hardly anything anyway.

 

[noseall]

Have you been at the cooking sherry again Pinenut?

Unless your flat roof joists are over a foot deep forget using mineral of fibreglass insulation.
You need rigid foam board insulation i.e. celotex or kingspan.

Some guys sanction the method of fully filling the flat roof void thus removing any air gaps into which water vapor could migrate. Use foil sandwich insulation and foil tape all joints. Some b.c.o's don't like this method, but i guess that if you are fairly robust in your gap filling and tape sealing then it will work.

 

[Hpd]

Surely the condensation point is the underside of the board and not actually the felt? How big are these 'tiny gaps' for you to be able to actually see the felt? I'm suspecting a leaky roof to be honest, and the boards are likely ply so any water ingress will find a join to show up long before it rots through.

Definitely agree with Kingspan or equivalent over rock wool. Obviously there's no soffit to at vents to?

 

[Axel]

Ok, here's a question: the roof area is about 3m x 4m - is it worth spending £200 on insulation - how many years saving on gas will it take for this to pay off in London climate??

 

[pinenot]

Ah! that's a different question

Probably best to create a warm roof structure by putting loose laid waterproof insulation on the top of the roof weighted down with paving slabs. Polystyrene's pretty cheap about £50 for your roof size, and if you use 50mm thick you could stagger the joints over two layers to reduce any cold bridging View media item 55962, your local freegle/freecycle for the slabs (cost off uplift) It's called warm or inverted roof design - look it up. You'll also protect your flat roof to-boot. pinenot

 

[Axel]

Yes indeed...If I do that in the future I'll probably put felt on top as well!