Loft condensation

[b0rn2sk8]

TLDR Lap vents are the way to go as an initial step.

@Nemiyen modern roofs are not built to be sealed at all, quite the opposite actually, they are all fully ventilated (or should be).

We used to build houses with attic spaces with zero ventilation and non-breathable membranes. Ventilation often needs to be retrofitted as loft insulation is increased as this cools the loft space and the condensation risk increases.

The way they are built now is the complete opposite. They all have either cross ventilation via soffit or ridge vents or vent tiles cut into the roof itself. They are also have fully breathable membranes to slow water vapour to pass through.

If you are getting condensation in a roof, you have one of 2 issues:
Water getting in
Water vapour failing to get out

It’s usually the latter, the former usually has more obvious signs of water ingress somewhere.

If there is low wind as the temperature drops, condensation is all too common and increasing airflow will revolve this.

Increasing ventilation via lap vents (or similar) are effective, easy and cheap.

You also need to check warm moist air from your house isn’t leaking into the loft. Check the loft hatch seal and any extractor fans which go through the loft are assembled correctly and are not leaking air or water.

 

[Nigel Kingdon]

Thanks for that, most informative. I moved into a new build 3 years ago and this was one of the snags we raised. I've since changed the loft ladder for a better wooden one (with a good seal etc), but I'll still get some vents as this time of year I still get some condensation.

 

[SoundsGood]

Thought I'd give this a go too. I don't have any visible condensation but do have black mold growing on some of the felt and consistently high humidity readings. I installed 6 along the worst of it at 20:25 tonight but opening the loft hatch spiked the humidity and it's yet to come back down, hoping to see some improvement overnight.

Well the vents dont appear to have made much difference to my loft, the 2 yellow lines are when I installed the first and second set of 6 lap vents.

 

[robj20]

TLDR Lap vents are the way to go as an initial step.

@Nemiyen modern roofs are not built to be sealed at all, quite the opposite actually, they are all fully ventilated (or should be).

We used to build houses with attic spaces with zero ventilation and non-breathable membranes. Ventilation often needs to be retrofitted as loft insulation is increased as this cools the loft space and the condensation risk increases.

The way they are built now is the complete opposite. They all have either cross ventilation via soffit or ridge vents or vent tiles cut into the roof itself. They are also have fully breathable membranes to slow water vapour to pass through.

If you are getting condensation in a roof, you have one of 2 issues:
Water getting in
Water vapour failing to get out

It’s usually the latter, the former usually has more obvious signs of water ingress somewhere.

If there is low wind as the temperature drops, condensation is all too common and increasing airflow will revolve this.

Increasing ventilation via lap vents (or similar) are effective, easy and cheap.

You also need to check warm moist air from your house isn’t leaking into the loft. Check the loft hatch seal and any extractor fans which go through the loft are assembled correctly and are not leaking air or water.

I first had the issues after removing all the fluffy insulation and replacing it with foil backed PIR.
Loft hatch isn't the best it's on my list of things to do, just needs some insulation applying to it and some gasket around the edges.

 

[b0rn2sk8]

Well the vents dont appear to have made much difference to my loft, the 2 yellow lines are when I installed the first and second set of 6 lap vents.

Given the outside humidity is also very high, this is expected.

What you are trying to avoid is large differences in humidity and temperature between the loft and outside as the temperature dips in the evening and spikes in the morning and that’s what the additional ventilation helps prevent.

 

[danlightbulb]

Just been up my loft to get the boxes down for xmas decorations.

All of the plastic storage boxes have quite a lot of water on their lids. There's no obvious signs of leaks, its very cold up there (see breath), but there are signs of condensation forming on the bitumen felt underlay which must be continually dripping down onto the floor and boxes. The floor is OSB, so presumably absorbing it but the plastic boxes it just sits there.

My lap vents should be coming today.

 

[b0rn2sk8]

Today’s wind will help clear that out if you get them in.

I found the most effective thing I did to clear it was to put a fan up there to help circulate the air which will clear the build up moisture.

Once I got the vents in, it never returned.

 

[Vestas]

I first had the issues after removing all the fluffy insulation and replacing it with foil backed PIR.

In general if you're doing that as a retrofit then you want to fit a vapour barrier on the plasterboard between trusses as well. Rockwool "absorbs" a fair bit of water vapour which gets evaporated later, PIR doesn't & ceilings leak a fair bit of air at edges and fittings.

The products may be similar in insulation performance but not in install terms.

 

[robj20]

In general if you're doing that as a retrofit then you want to fit a vapour barrier on the plasterboard between trusses as well. Rockwool "absorbs" a fair bit of water vapour which gets evaporated later, PIR doesn't & ceilings leak a fair bit of air at edges and fittings.

The products may be similar in insulation performance but not in install terms.

Not had any issues so far after a fair few years, except it keeps the house warmer and I have more usable space in the loft

 

[Vestas]

Not had any issues so far after a fair few years, except it keeps the house warmer and I have more usable space in the loft

I did the same in a previous house. Found out later you're supposed to fit a vapour barrier for PIR in lofts. Didn't seem to cause any problems as I taped the top side of the PIR but I have no idea what the section underneath was like...

 

[robj20]

I did the same in a previous house. Found out later you're supposed to fit a vapour barrier for PIR in lofts. Didn't seem to cause any problems as I taped the top side of the PIR but I have no idea what the section underneath was like...

Id imagine it matters most of you have a big difference between the humidity in the time before, but even this time of year I have a fair few windows partially open so a very dry house, right now between 36% and 39% inside. Loft is nice and drafty.
All it would do in my case is result in less moisture ending up in the loft so you don't want any cold spots on the inside facing side for it to condense on.

 

[Vestas]

Id imagine it matters most of you have a big difference between the humidity in the time before, but even this time of year I have a fair few windows partially open so a very dry house, right now between 36% and 39% inside. Loft is nice and drafty.
All it would do in my case is result in less moisture ending up in the loft so you don't want any cold spots on the inside facing side for it to condense on.

Its probably based on a new-build, where wood/etc has a lot more moisture which will evaporate.

It was our builder who was doing renovations at the time who mentioned it. He said it was just ignorance really as the "vapour barrier" is a roll of sticky-back plastic cut to size for truss gaps & cost about £50 for a normal house. It doesn't add much to labour costs either so he couldn't see why people didn't do it anyway.

I pled ignorance at the time

 

[b0rn2sk8]

I did the same in a previous house. Found out later you're supposed to fit a vapour barrier for PIR in lofts. Didn't seem to cause any problems as I taped the top side of the PIR but I have no idea what the section underneath was like...

Taping the topside is the last thing you want to do.

Not taping it means you don’t benefit from their air tightness aspect of that type of insulation. That’s fine, it just doesn’t perform as well as it could.

The tape creates a vapour barrier and taping the top traps the moisture into the top of the insulation and risks serious issues in the long term.

The trapped moisture has nowhere to go other than into your joists and eventually your joists will start rotting away.

It’s not a matter of if, it’s a matter of when. It may take a year, it may take 20 years but it will happen eventually if there is nowhere for the moisture to go.