loft condensation

[bren47]

A couple of years ago I had my roof replaced on my 1930s detached house. Grey Marley tiles were used with a breathable membrane. I suspect 2 types were used. 2 elevations have a membrane I can't name but it is grey on the upperside with occasional red dashes on the underside which is white.The other 2 elevations have membrane made by Klober. It has the makers name on the upperside. This side became quite wet with condensation during the winter. The eaves were enclosed and so I had soffitt vents fitted between each rafter and then had the pleasant task of pulling back insulation which was well and truly packed into the eaves and multiple old birds nests. Gradually the condensation has improved 95%. I have some queries which I would appreciate any guidance on:

1. Today, a cold day, I have no condensation. But yesterday there was some very very slight wetting of the membrane well down in the eave . Is this important?

2. At some point chimney stacks have been dropped and an extra joist has been fitted deep in the eave. Obviously this limits air flow in these particular areas. At the moment they are dry but is this a cause for concern?

3. The insualtion is very old and very thin. the entrance hatch is not insulated at all. I intend to re insulate in 4 or 6 inch insulation and board the loft including the hatrch. Will this reduce any chance of future condensation?

Thanks in advance for any advice.

 

[RobC316]

It could be that the joist is restricting the air flow in the roof space, this could be a problem as stagnant moist air could accumulate in the loft causing condensation. Obviously this condensation can lead to rot in the timbers so it may be worth having a closer look.

Also make sure that there is a sufficient gap between the insulation and the eaves to ensure adequate ventilation from the eaves into the loft.

The best method for reducing the risk of condensation is sufficient ventilation, either eaves to eaves or eaves to ridge. You could use a vapour barrier under the insulation, although this is unlikely to be 100% effective due to jointing and fixing of the barrier.

 

[bren47]

Thanks for your reply. I've had a good look round and the loft now seems to be condensation free. I take a regular look especially on cold days and it now seems to be consistently dry.

The timbers themselves are solid but I've found a small ( hand sized ) patch of mildew /mould where the condensation was worse before I fitted the vents. The insulation was packed so high into the eaves that I think wet insulation was sitting against the rafters in this spot.

I'm still somewhat surprised that breathable membranes seem to require no ventilation. Surely if the sofitts are closed and unventilated then it's expecting a lot to have sufficient circulation of air to avoid condensation entirely.

Again, many thanks for the advice.