consisting of a polyurethanO/T Thermal Insulation

[Man On The Clapham Omnibus]

Kerry's thread about replacing roof felt showed how many people on here know a lot about buildings so I shall try to glean a bit of advice from the Massive.

My son lives in a top floor flat in a 1938 built block in SW London. The roof is thick concrete with an asphalt weather proof surface and the ceilings of the rooms are simply plastered direct on to the underside of the concrete. The is no air-gap between the room and the outside world and the only 'insulation' is about a foot of concrete. It gets cold in winter because of the sheer thermal mass of the roof and unless the flat is heated 24hrs a day (unrealistic for a single man living alone and working long hours) the ceilings remain cold. Condensation forms on the ceilings and mould growth is inevitable in high humidity areas such as the bathroom. In an attempt to improve the situation I have installed a false ceiling in the hallway (easiest because it is less than a metre wide and dead straight) with insulation above. What I need is someone who actually knows what should be done rather than my educated (more or less) guesses. The ceiling I have fitted is a Clipso stretch ceiling click which comprises a strong fabric coated with polyurethane stretched across the room just below the existing ceiling. Above that I have fixed (by adhesive to the concrete ceiling surface and wall battens) a 50mm 'Celotex' foam board with aluminium foil surfaces, the edges and joins of which are taped with waterproof tape to try to maintain a vapour proof barrier. My fear was that water vapour might penetrate above the board and mould would build up on the concrete. So, to sum up, I have a 300mm asphalt coated concrete roof, under which and in intimate contact with, is a 500mm foil coated PU board with taped joins. About 50mm below the underside of the Celotex is the Clipso decorative ceiling. What I need is knowledgable views as to whether I have done anything that might cause problems in future, and if there might be a better way of improving the thermal insulation of the flat. I resisted use of battens and plasterboard in favour of the Clipso route due to the physical difficulties of handling and fitting plasterboard single handed, and the plastering expertise need to make a good finish. Also, the ceiling is, I think, reinforced concrete and difficult to fasten battens to.

Thanks to anyone who can offer guidance.

[Man On The Clapham Omnibus]

Oops! The thread title seems to have picked up text from the post...

[KerryS]

Not getting quite the response that I did are you, Clive  

[Man On The Clapham Omnibus]

No Kerry, it's a bit of a non-standard problem. I have tried various sources including the Energy Saving Trust or somesuch, and they singularly failed to grasp the problem. I can only hope that what I have done is going to work, because I have just taken delivery of a Clipso ceiling for another room in the place. Wandsworth Council make a lot of noise about energy saving but they won't stump up any grants unless over a certain proportion of the occupants are on benefits. The roof could be insulated outside but trying to get co-operation (i.e. dosh) from the other flat owners is like trying to nail jelly to the wall.

Good luck with the roof.

[Mark (smokey mow)]

Not getting quite the response that I did are you, Clive  

Sorry I've been and work and had to do some research on it .  I'll post properly later once I've had a feed and been watered.  

[Man On The Clapham Omnibus]

I look forward to that Mark. Your contribution to Kerry's post, among those of others, prompted me to put my query up for thoughts. Thanks, in advance, for your help.

[Mark (smokey mow)]

The short answer……  

What you’ve done so far in the hall is ok.  My only suggestion when it comes to doing the other rooms is to use 100mm Celotex, you’ll appreciate the extra insulation and the additional cost when compared to 50mm is minimal.

The longer more technical answer…..  

Condensation has been forming on the inside of the roof because the inside surface of the concrete is at a different temperature to that of the room.  Now if we insulate sufficiently the inside of the roof as you’ve done (or the outside) our ceiling is then at or around the same temperature as the room and hence condensation should no longer form on this surface.  We then need to consider if there is a risk of the condensation forming on one of the surfaces elsewhere within the roof construction. With the insulation on the underside of the concrete deck as you’ve done there is potentially no airspace between the two and thus there is no risk of moisture forming here, but if we introduce an air gap between the two then potentially the air in this void will be warmer than the outside air but colder than the room air and so there is a potential condensation risk.  Referring back to Kerry’s post this is the reason  why a roof with the insulation at ceiling level needs to be ventilated to cool and remove the damp air but a roof where the insulation is in physical contact with the roof deck is unventilated.

Hopefully the document here may graphically explain better.  The table and chart on page two show the construction you’ve got so far in the hall and plots the changes in temperature through the roof fabric and the potential points where there is a risk of condensation forming.  The graph shows the two lines of vapour pressure and saturated pressure neither cross or converge and hence there should be no risk of condensation within the roof.  If the lines were to cross this would indicate the point in the construction where there is an issue.  The chart in first column plots the change in temperature between the inside and outside surface of the room and the temperature on each of the surfaces of the materials.  

Adding extra insulation as I’ve suggested probably wont have a significant effect on these figures but what it will ensure is less of the heat you put into the room will ultimately be lost back out through the roof again and thus should save a bit on the heating bills and put less demand on the heating system.  The document here would indicate the roof construction over the hall has a U-value of 0.38W/m2K that is to say that for every degree in temperature between the inside and outside 0.38watts of heat energy would be lost per square metre through the roof.  If we were to increase the thickness of the insulation to 100mm then this figure would be closer to 0.16W/m2K (less than half) and so a lot less heat would be wasted.

Simply put in a summary…

1- when fitting the insulation ensure there is no gap or airspace between the top of the insulation and the underside of the roof deck and

2- make sure all edges of all insulation boards are tightly butted together and  joints taped to minimise and cold spots and to act as a vapour barrier.  

Best of luck!

Mark

[Man On The Clapham Omnibus]

Mark, that is a most helpful reply - many thanks for that and for the excellent calculation sheets! I am relieved to learn that what I have done so far is not potentially problematic. The solution used in the hall was 'wobbled' towards; I was going to use 2" polystyrene block stuck to the ceiling with 'No More Nails' and a polyethylene film membrane fixed (by a means feasible but as yet unknown ) from wall to wall beneath it to act as a vapour barrier. However, my son and I were in Wickes in Wimbledon looking for the materials when he spotted the Celotex on the rack - more expensive than EPS but that's how sons are with fathers' money. There is a blocked off skylight (covered with a timber lid and finished with Artex in the hallway and that was always the first to grow mould so vapour exclusion seemed to be paramount in whatever we did). Because the hallway is narrower than the width of the Celotex it was easy to hold it up with battens around the walls - already in place to hold the Clipso profiles - and to belt-an-braces the fixing with 'No More Nails'.

In the next room the width exceeds the Celotex board dimension so another fixing method is needed. The manufacturers of Celotex (and Quinntherm for that matter) say the use of acrylic adhesive like NMN alone is inadequate and that I must use at least one batten parallel with the major axis of the board and on the centre line, screwed through to the substrate (or superstrate is it really is) as a secondary fixing. I have to make allowances for the ceiling light fitting anyway so this is not a problem. They recommend aluminium foil tape to seal the joins and silicone sealant for the edges, but what I did before is fine if the tape was vapour proof.

My reason for seeking advice from knowledgable people like your self, Mark, was to see if there was any 'official' remedy for internal insulation application in such cases. My solution might work but if there's a simpler one that I, as a DiYer, can use, so much the better. By the way, I would use 100mm board but there are features in the rooms that preclude lowering the ceiling by more than 80mm - high-level doors of built in wardrobes, etc. Anyway, in the 'wobbled towards' solution I used for the hall, it was what was on Wickes' shelf that was used by default. Maybe for his sitting room, where there are no such constraints, I shall use 100mm.

Once again, thank you for the really helpful information. Is there really no subject that the WSCC boardroom cannot supply?