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Trying to avoid condensation


I’ve boarded my floor in my loft and used 170mm earth wool the reasoning for this was due to the fact I have an old cottages and I could t raise the floor higher or else I wouldn’t get through. It’s a crawl as it is.

I have some ecotherm boards and will be cutting this to fit in the rafters.

I am worried about condensation though. Someone mentioned using a vapour barrier but I can’t fit this due to half the pitches roof being suspended on a weight baring brick wall. Plus the pitch goes so far I couldn’t reach to do it.

Any ideas?

I have some black felt under the slates but that has seen better days.

Any help welcomed thank you

3 Answers from MyBuilder Insulation Installers

Best Answer

To help reduce the risk of condensation, good ventilation is key 🌬

Sounds as if you have insulated the floor to suit, and this should reduce heat loss from the living area below.

You mention it’s an old house but do not indicate how old!

As long as overhead roof joist are strong enough ?
...You May be better stapling Tyvek (a breathable membrane) to underside of joist.

To reach higher level joist simply use a suitable step ladder, ensuring that there is secure floorimg below.

This will not only allow the loft to remain ventilated but also help keep clean (dust free).

Not so sure I would recommend insulating between main roof joist, especially if a very old property!!!

Hope this helps

Warm regards



Answered 10th Nov 2019

Hello0 PJ
you mention avoid condensation well if you haven't got it the chances are you won't get it, at the moment you have a cold roof but you have insulated the floor in the loft so you're keeping the heat below the main roof so no heat contacting the cold roof which would potentially cause some condensation but not for certain.
If you are really concerned then insulate between the rafters with celotex/kingspan 20mm boards and this will create a warm roof as well
I have a 200 year old cottage and I have a cold roof and no problems.
Hope this helps


Answered 28th Nov 2019

You can get a couple of slate vent tiles , you remove a slate fit the vent slate and this allows ventilation in loft area, putting the breathable membrane on the inside is pointless, it is for outsid3 the joist , where the black tar based f1 felt is now, the loft probably has ventilation with it being old slate roof, if you feel a breeze or cold air when you go in there it probably will be ok, but wait until the insulation has been down a few days with the heating on then go in loft on cold ,wet day and see if there is any droplets on underside of black felt, if so then try a coup,e of slate vents, only cost around £15 each


Answered 6th Dec 2019

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