I’d appreciate some advice please!

I’ve been slowly converting our attic over the last 6 motnhs or so. Started by putting new C24 joists in between the old ones, as the old were only 80mm deep. I stuffed a load of the old rockwool between the old and the new joists, basically filled up all the voids, and have just about finished fitting new floorboards throughout. These don’t go all the way to the corners as I’m planning a ~400mm tall stud wall around the outside, rather than trying to fit flooring all the way to the edges.

The problem I have is during the cold weather, I’m getting a LOT of condensation on the underside of the membrane, sometimes all over, but especially around the edges, i.e. not above the floorboards. There are no soffit vents – presumably because it’s supposed to be a breathable membrane. It’s probably 12-15 years old though – do membranes become less breathable? I’ve bought a tall ladder and a load of 70mm soffit vents so hopefully that will help – but there’s no ridge vents as far as I’m aware.

I’m a bit alarmed by how much water is accumulating, particularly because I’ve just bought £1k of celotex to insulate between the rafters. I’d initially planned to fully fill the rafters (75mm) and do a second layer under the rafters, taping seams to form the vapour barrier. I understand this is ok if there’s a breather membrane, but I’m assuming fully filling is a bad idea now. Even if I leave a 25mm gap, a lot of the spaces between the rafters don’t go up to the ridge, but hit the hip beam, so are closed off at the top and I don’t get much/any air flow in any case.

Does anyone have any suggestions? Is it possible that this is only happening because I’ve been working up there, introducing warm air, and once the whole job is finished and taped, it’ll be ok? The worry is that once I insulate the rafters there’s no way to see what’s going on behind!

Cheers,

You’re correct in that you cannot completely fill between the rafters, there needs to be an air gap between the underside of the membrane and the celotex to allow airflow. If there’s restrictions you’ll need to look at fitting ridge and/or tile vents.

The condensation you are seeing now is the moisture in the warm air in the loft condensing on the cold surface of the membrane. This won’t happen when you’ve fitted the celotex but you must have an air gap.

As I understand it, you’re making it a semi-warm roof, that is, midway between a cold roof with all the insulation on the ceiling below, and a warm roof with the insulation under the roof itself. I considered doing that (actually scrapped it because I have trussed roof and cutting the celotex around all the bracing would be a nightmare) but I don’t think it meets building regulations Approved Documents. You’re expected to have the required thickness in one place or the other, but you could probably make a case for complying with the regs themselves not the AD. What you can’t avoid under the regs is the need for ventilation; for a warm roof I think you need 50mm gap, a specific area of soffit vents and vents at the ridge and hips.

I’m assuming you’re not in Scotland (Building Regs are different) because you have a membrane rather than sarking?

My thoughts are

You need to split the space, an insulated box for the turbo

A ventilated space to get rid of condensation

Essentially the bit under the membrane needs to be well ventilated, the ceiling insulation is fine to keep warmth in the main body of the house but it should have free flowing air to get rid of the condensation

The insulated box will help by keeping the warm moist air away from the membrane

Ventilation should help. But if that’s an old stock Tyvek from around 15 years ago it may not be that breathable. There were issues with them after the millennium.

My advice would be fit soffit and ridge vents and monitor it before doing any more insulation. You need to make sure that there’s a venter air gap of at least 50mm under the membrane too.

Make sure excess moisture laden air is not migrating from below, bathrooms and kitchens should have extraction.
Before plasterboarding fit a vapour shield.

Before plasterboarding fit a vapour shield.

Or alternatively and I get that it’s not as *in* paint the plaster board in the bathroom

The problem would appear to be you have started using the space as habitable space before it’s complete. And complete means insulated, with appropriate vapour barrier, and appropriate ventilation of the cold side. Stop using the space until it is done. It’s either the fact you’ve been habiting the space, or I do wonder if you’ve had the hatch open often allowing warm damp air from the house into the uninsulated loft.

Change of use of a loft to habitable space requires building control. You might argue you don’t need it, but to be frank building control is there to prevent precisely this sort of issue that could damage the fabric of the building and leave you or a future owner a liability.

I am also more than a bit concerned about the state of some wiring in the photos. At the very least get some cable clips.

You are right, the air gap is not always necessary depending on construction. However it is safer/easier to allow an air gap and allow plenty of ventilation through it.

The membrane is not working because the loft is cold. You need the loft to be warm to drive the moisture out. Same as if you take off a waterproof jacket and the inside then gets clammy. Once you sort out the insulation it should be fine. In the meantime if you are using the space, it might be worth trying to reduce the humidity in the rest of the house – dehumidifier bathroom fan overrun etc.

Thanks for the comments. I probably should have pointed out the turbo’s not been used up there since the summer, and I’ve kept the hatch closed when working up there.