How common is condensation in loft with such low temperatures? Best way to solve.

[Jason213]

Hi All, after getting the xmas decs down the other week I noticed the loft has a huge amount of condensation on the felt. The house is a 2017 new build, and I installed loft zone flooring in the centre two years ago with adequate ventilation under the floor.

 

Slightly worried, I decided to pull back the insulation at the eves to ensure adequate ventilation (it wasn't bad in the first place). I did check the soffits, and they are the vented type with a grille along the entire length.

 

Is this common, or shouldn't there be any condensation at all?

 

The other thing I did at the start of the year, is connect the bathroom and ensuite fans to the roof vents (rather than going into the soffits - it was just left loose a blowing in there). This was because of a failed extractor, so installed inline fans which are brilliant at getting rid of steam from showers! However, on noticing the condensation I've disconnected one of the pipes, so it is sucking air out of the loft to try and dry it. Which is helping a bit, but still wet after a week.

 

I have asked around, and quite a few people I know (not on our estate) have wet lofts due to the very cold, and non-windy conditions. I'm really just after your opinion if this is a problem?

 

 

Secondly, I think I've decided to strip out some of the loose insulation and put rigid board insulation (80mm or 100mm) at the eves to stop cold-bridging to the rooms below (as we've had some mould at the edges in the bedrooms that are slept in). Is this a good idea? 

 

Thanks!

 

[Radian]

13 minutes ago, Jason213 said:

I have asked around, and quite a few people I know (not on our estate) have wet lofts due to the very cold, and non-windy conditions. I'm really just after your opinion if this is a problem?

 

You've hit the nail on the head. It's unusually cold and calm. Normally the through draught would be carrying away the vapour but not lately. And the exceptionally cold roof is just the perfect thing to condense the vapour back into water. I think it's OK for this to happen occasionally (it's inevitable) and is inconsequential in the long run. It will cycle a few times (condensing, evaporating, condensing... ) but the weather will change and the next windy day will restore the former equilibrium.

 

But be very careful when you're thinking about using rigid insulation. Unlike glass wool insulation, it isn't vapour permeable so it has to be guaranteed that moisture laden air from the house below can't accumulate anywhere that's not fully thermally insulated. Whatever you do here will lock-in the consequences and then, in the long run, may cause permanent damage.

 

I recently realised that the eaves in my loft are poorly detailed for insulation. The inner leaf that carries the wall plate and trusses is around 300mm higher than the outer leaf (which supports the soffit board) and the loft roll stops at the wall plate. Here it meets the plastic eaves protector sheets that also form the eaves ventilation. Everything looks good from in the loft until you pull back the roll and realise there's a void forming the soffit:

 

[nod]

Our daughter had a Similar problem I purchased a pack of wedges from SF That simply allow a better airflow und let the felt It worked a treat 

[Radian]

3 hours ago, nod said:

Our daughter had a Similar problem I purchased a pack of wedges from SF That simply allow a better airflow und let the felt It worked a treat 

 

Unfortunately that would cause a different problem in our loft. Cluster flies crawl in between the tiles and find their way in between the overlapping sheets of felt to over-winter in our loft. The last thing I would want to do is make life easier form them!

[Jason213]

Thanks for the replies.

 

I've attached a pic how the insulation was before I pulled it back. I don't really think it was blocking the vents at the eves anyway.

 

If I did use rigid insulation, should there be a plastic membrane underneath against the plasterboard? My thinking was it would give better insulation at the edges to stop cold bridging to the rooms below.

 

Also, I think the extractors connecting to the loft haven't helped, so I might connect them to the soffits again. I'm not sure how a 100mm pipe will fit into a cut in the soffits, as I'm sure it's not even 100mm wide. Or is leaving this loose in the soffits ok? 

 

Also to add, in the pic you can see where the wood is stained. I'm guessing this isn't the first time this has happened: 

Edited December 14, 2022 by Jason213

[Jason213]

4 hours ago, nod said:

Our daughter had a Similar problem I purchased a pack of wedges from SF That simply allow a better airflow und let the felt It worked a treat 

 

Like these?

https://www.ebay.co.uk/itm/191857131873?

 

I can't find them on screwfix, but they seem a great idea!

[Radian]

6 minutes ago, Jason213 said:

If I did use rigid insulation, should there be a plastic membrane underneath against the plasterboard? My thinking was it would give better insulation at the edges to stop cold bridging to the rooms below.

 

It sounds reasonable enough in theory, but in practice you're interrupting the natural ventilation that cold lofts rely on. Wrapping things that are open to vapour on one side can end up making big problems with condensation. You're only thinking of using rigid insulation to improve the insulation levels at the outer horizontal edges but is this really where the problem lies?

 

I put up the diagram showing the issue I have at  the eaves. How does your insulation regime ensure a continuous envelope that wraps the inner leaf?

I.e. how does your vertical cavity wall insulation (you do have that right?) transfer to your horizontal loft roll? Mine has a 300mm break!

[S2D2]

1 hour ago, Jason213 said:

 

Like these?

https://www.ebay.co.uk/itm/191857131873?

 

I can't find them on screwfix, but they seem a great idea!

I've used these to good effect. However I topped up the loft insulation and the problem has tipped back to condensation like yours, so I've just taken delivery of a load more to try and get it sorted again. I only put 15 in first time. 

 

15 hours ago, Jason213 said:

However, on noticing the condensation I've disconnected one of the pipes, so it is sucking air out of the loft to try and dry it. Which is helping a bit, but still wet after a week.

What has happened to the extractor vent that was connected to this pipe, is it now exiting directly to the loft?

[Radian]

9 minutes ago, S2D2 said:

What has happened to the extractor vent that was connected to this pipe, is it now exiting directly to the loft?

The way @Jason213 described them was that they were left dangling in the soffits! Apparently there were unused roof vents so he connected them to those. Then he temporarily used the loft mounted inline fan to evacuate the loft - I hope the bathroom ducts are now back to be vented out through the roof!

[Jason213]

Hi, yes they were left dangling and there were unused roof vents so connected to those. 

 

Our en-suite isn't being used at the moment, due to it being re-tiled, so we're only using the main bathroom.

 

In terms of the diagram above, the outer wall is the same height as the inner. I think I'll not put rigid board insulation in due to your advice.

[S2D2]

2 hours ago, Jason213 said:

Our en-suite isn't being used at the moment

Make sure the vents are taped over or something if the extractor has not been reconnected as even without a fan stack ventilation will be pushing hot, humid air up into the loft space.

 

Rather typically when I went up to fit the new felt lap vents almost all of the condensation has blown off and dried up. We have had a fair bit more wind the last 24 hours but I'll probably have to wait until next year to see if the balance has been tipped back. A good airflow coming through the new vents though so they must be helping.

[Jason213]

I thought I would update this thread.

 

So, after the cold spell the condensation on the felt did disappear. I'm not changing to rigid board insulation, maybe that was a mad plan! I have also installed 50 felt vents, which should help!

 

The other thing I didn't mention was mould on the ceilings in the rooms against the external walls (only those being slept in and not the gable ends). I believe this is because the vapour barrier behind the walls is folded to be above the ceiling. That happens to be where the least insulation is too!

 

Seeing the attached pics, you can see how wet it is under the green plastic (even with the warm winter weather). Is this just lazy builders doing that? Either way, I've cut back the plastic as per the second pic.