Condensation control in newly built loft conversion

by Lee Ray
(Norwich norfolk)

I have my new floor down and I am now cutting the insulation between the sloping ceiling joists (rafters) and have found my existing felt is saturated.

Although there are no leaks as such, i know once i have battened over the cut insulation with full boards of insulation, taped and masti sealed at the edges, i'm wondering will this be enough to stop any penetrating water or would it be advised to use a dpm or vapour barrier as well?

Don't want to have moisture problems later on..!

Many thanks,


Comments for Condensation control in newly built loft conversion

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Dec 10, 2014
Same problem!
by: Andy

Hi, Iam just in the middle of my loft conversion and I've noticed a condensation build up between my felt and the kingspan.

I've used breathable felt... I've got 75mm roof rafters so I've put 50mm kingspan in between, leaving a 25mm air gap between felt and kingspan..

Iam going to be putting triso supper 10 over the top next with an air gap , but the condensation is concerning me...

I plan to foam and tape all the joints between the rafters and kingspan to make it air tight

Iam hoping once the triso 10 is added and sealed, no warm air from the loft will be able to meet to cold air from outside and form condensation?

Problem is once it's sealed etc, I won't know until damp starts to show!

One other thing I noticed is, the problem is only happening on the north side of the house.. And the south facing side has exactly the same materials used etc

Any thoughts guys?

Jun 21, 2011
Loft conversion condensation
by: Anonymous

(Assuming breathable felt has not been installed there is still old felt on the roof)
If you don't have vents near the eaves (soffit) and up near the ridge (tile or ridge vents) and a clear passage for air to circulate between the roofing felt and insulation the roof will sweat. This will cause loads of condensation on the windows that may go mouldy and in a worse case scenario wet/damp patches in the plaster.

If you have these condensation problems in a loft conversion get a roofing company to install some vents for you.

Nov 17, 2010
How to prevent condensation building up in loft conversions
by: Gary

If you are absolutely sure there are no leaks and have checked everything that could possibly let water in (ridge,windows etc) then I can only assume the condensation is being caused by a lack of ventilation.

When lofts are insulated with kingspan, celotex or another type of sheet insulation there should be a gap left between the roof felt and the insulation boards themselves. That, and soffit vents allow a clear passage for air to circulate from one side of the roof eaves over and down to the other.

If the building inspector will allow it, you can insulate over the top of the rafters again with 25/50mm celotex/kingspan.
Or, another method is to use rolls of thinsulex over the rafters and then counter batten the skeeling (rafters) so you have something to fix the plasterboard to.

You must use foil backed plasterboard for the ceiling and skeelings (angled ceiling) so any moisture that does penetrate can run down the boards and not into/through them.

Normal plasterboard can be used for the purlin walls though.

If you definately have enough ventilation between the rafters and insulation, and enough soffit vents there must be a leak either from the outside or maybe a pipe/header tank?

Hope this helps,


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