Using anti condensation paint on platsic surface

[Owen74]

To control condensation you have to consider the complete system. Even if you prevent condensation in one spot, if the air is saturated it will condense somewhere else.

Agreed. It's the whole thing, including ventilation and insulation.

But imagine if you had a perfectly insulated/ventilated loft with no condensation, complete system was good, then one day you stuck a single-glazed thin window in the middle of the roof. Then imagine ice, snow, rain, wind, winter air, all making the window cold from the outside.

Would you get internal condensation on that glass during wet/cold weather? Yes - because the glass would be so much colder than everything else in the loft, even if the loft is say 5 degrees C, the surface of the window would be maybe 1 degree C.

That's basically my situation, albeit with a sheet of thin plastic instead of a window. It gets cold because of the outside conditions, and loft air condenses on it, even despite ventilation and insulation. Somebody on here said a surface will attract condensation if it drops below 14 degrees C, and that's what's happening, and will always happen, no matter how well insulated and ventilated the loft is.

 

[mrrusty]

@Owen74 - Exactly. And yes, the dew point of ambient room air is in the order of 12-14 degrees depending on the original humidity.

 

[robinbanks]

Very thin multifoils are definitely in snake oil territory in my book; 3 millimetres is incredibly thin - even the best known insulator ( wouldn't be much cop at that thickness

If the builders had put felt under the roof valley this wouldn't have been a problem.

Felt is not innately insulating. If they'd left a gap then perhaps the felt would have carried away any dripping condensate but it wouldn't have formed anything like an effective insulation layer inhibiting its formation in the first place

Another idea is to stuff loft insulation over the top

When you say "on top" do you mean "of the gutter" ? (In my book that would be outside - by "top" do you mean "the underside of the gutter"?)

Yes, lining 100mm of loft insulation and holding it in place with some multifoil should reduce the dehumidifier effect

 

[Owen74]

When you say "on top" do you mean "of the gutter" ? (In my book that would be outside - by "top" do you mean "the underside of the gutter"?)

Yes, lining 100mm of loft insulation and holding it in place with some multifoil should reduce the dehumidifier effect

Thanks for the reply.

I meant put some insulation on top of the superfoil. On the underside of the roof valley.

I'll try this - or maybe an 'insulation sandwich' - loft insulation between two superfoil layers.

 

[robinbanks]

Multifoils get their "impressive" performance claims in very carefully designed situations where there is an air gap on both sides so the reflective surface can work to return radiant heat and also, being a poor emitter, work to create a low radiant heat emissive surface.
If a multifoil is in direct, conductive contact with a material, with no air gap, then they don't really work any better than normal wool because reflection/emission needs that air gap

As such I wouldn't bother with a "superfoil under the gutter" layer

 

[aveatry]

I understand your problem. When I had an extension the valley's were first covered with breathable membrain then made up of 2 lengths of 1 x10 or whatever in a "V" and then the fibreglass valley on to that so from the loft the plastic / fibreglass vallys are insulated by the wood and membrane.
But you you have is you can actually touch the underside of the plastic valley.
what about sticking PIR 25mm directly onto the underside of the vally with foam PIR adheasive.
Or this may be the only senairo where that spray insulation foam - (the one that gets refused mortgages ) is used but only sprayed on the underside of the valley tray.