Condensation solution required in loft

[mitch...]

Hi all and Happy New Year,

I'm trying to find the best solution to a condensation problem in a loft in a 1980's new built block of flats (top floor flat with loft access).

The condensation forms on the inside of the roofing felt then drips down onto stuff stored up there. The loft is well insulated just above the ceiling. I don't want to increase ventilation due to water pipes being up there (as I don't want frost damage).

Maybe some sort of screen fitted over the rafters or some kind of insulation between the rafters? What would people suggest as a cheap and easy to fit solution? It does not have to look good.

Thanks

S

[Martin Brown]

The simplest solution would be to improve the ventilation of the loft space and/or the impermeability of the vapour barrier between the occupied space underneath and the loft.

You can't really prevent the roofing felt from being cold so you have to get the humidity lower somehow.

Prevent moisture getting in or allow more fresh air to displace it.

The condensation you describe is capable of allowing serious rot to develop in roof timbers as well as cosmetic damage to stuff it drips on. Advice on Usenet may be worth less than you have paid for it...

[Dave Liquorice]

+1

or combination of both.

Are the holes where cables penetrate the ceiling for light fittings vapour proof or just a roughly cut hole with loads of space around the cable? Particulary for fittings in rooms with high humidty, kitchen, bath/shower rooms. Are any ceiling down lighters open at the back? They ought to be fire proof anyway to slow the spread of fire from the room to loft space.

[Andrew]

Do the top floor flat(s) have downlighters ?. These, especially if fitted in bathrooms where an electric shower is in use will allow moisture vapour into the loft. Properties built in the 1980's may not have had enough (or any) loft ventilation and if the top floor flat owners have added more insulation in there they have made the loft colder so increasing the chance of condensation.

Where in the country is this building ?. Does it stay in shadow a lot at this time of year (so the solar gain does not warm up the loft) ?.

Have any other flat owners done something silly like fit a new kitchen and terminate the cooker hood inside the cavity wall ?.

You imply that the water tanks are in the loft. Do they have tightly fitting lids ?. If not, get them retrofitted. A slab of 4 inch celotex is better than nothing. If they are well insulated make sure there no insulation under the tanks and 'building regs' pipe insulation (with 19mm walls) should protect the pipes. Pay attention to bends.

Apart from that, just make sure any stuff you store up there is inside sealed plastic bags for protection.

[mitch...]

Yes, non-fireproof downlighters in kitchen and bathroom (as was at purchase last year) but now fitted with LEDs. Didn't realise these may be contributing to moisture ingress. I'll also have a look at sealing wire holes in ceiling for normal light fittings (non-downlighter type).

Thanks

[Brian Gaff (Sofa]

I still think though that in order to get condensation, there has to be a temp difference somewhere. So maybe beefing up the loft insulation over the pipes also might keep it down. Anyone tried stretching that substance called papronet across the void. It seems to stop drips in greenhouses quite well while stopping the drips its porous enough though allow air through.I will age though. Goes brittle.

Brian

[The Natural Philosopher]

But not it would seem, equipped with a vapour barrier.

What you probably would find easiest is to board over the joists with something impermeable, Foil backed plasterboard - extra celotex insulation - or even just tack a plastic sheet over the insulation.

HOWEVER that does run a risk of transferring the problem to that vapour barrier - the real fix is to have the impermeable layer INSIDE the insulation. In short replace your ceilings with foil backed plasterboard...or if the attic is rockwool between joists, lift that and paste foil down (or lay a thick plastic sheet) on the plasterboard and replace the insulation

Insulated pipes with any water movement through them should be OK

You are tackling the symptoms, not the cause. The cause is a combination of too little ventilation and too little vapour barrier.

On reflection the simplest cure is to lift the insulation, lay down a plastic sheet over the joists, and replace the insulation.

Make sure that any hatches are airtight and have a vapour barrier as well

[mitch...]

I'm now looking at these easy-fit devices to modestly increase natural ventilation on the back of the roofing felt, has anyone used these?

S

[Andrew]

You can achieve the same with chunks of expanded polystyrene pushed into the overlap midway betwen joists. Much cheaper, but doesn't solve the underlying problem of reducing the amount of moisture vapour getting into the loft.

Fire hoods on downlighters are a start, but even they are not a vapour barrier so you might want to make up your own downlighter covers out of something fermacell or other class O board material and cover them with turkey foil too.

[mitch...]

Final report: The damp loft is now dry thanks to those Amazon spacers (to push out a gap in the roofing felt). Amazing how such a passive device added with a bit of wind works so well. Anyway I would recommend them or equivalent without hesitation. Now on to the next problem...