Room in roof insulation redo

I’m trying to find a contractor serving Manchester to re-insulate our existing room in roof. It was meant to have 100mm but given how cold/hot it gets I was not surprised to find none there when I made some inspection holes.

I have spoken to a few local companies but a lot want to use spray foam or have said things which suggest they don’t have a clue about basics like leaving space for ventilation under roof/ensuring vapour barrier. I’m thinking something like kingspan/celotex between rafters, superquilt over that , and then sealed insulated board on top to bring it up to something like 300mm. Does anyone know anyone good for this sort of job around Manchester? Surely this is a good/easy job?

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Hi Ben,

Spray foam does have some advantages - and unventilated (aka warm roof) systems can work. However, it’s better to have a ventialted roof (cold roof), not to use petrochemicals, and use vapour open (flow through) materials/assemblies when possible.

Building code will determine the ventilation gap, as well as the soffit size/spacing and ridge vent details. It will depend on the length/pitch of your roof and climate zone etc. For your roof, I think it’ll be a very conservative 50mm. If your ceiling plane is air tight, and air leakage from within is not a source of moisture, 25mm would be fine, but code is slow to change. Worth checking with local officials though.

Therefore, 100mm is probably what’s left of your cavity. You can use any insulation since it’s ventilated. I personally would use a natural, carbon sequestering, vapour open fibre. Solid, but slightly compressible/spongy batts, that are easy to handle/cut to size and fit. Choose a contractor that has experience with one of these systems.

Your air control layer would be on the underside of your roof rafters. This is often done with a taped vapour permeable membrane. Detailing here is absolutely crucial, and should be tested to hunt down air leaks.

It is now good practice to use a service cavity to separate the plaster board and wiring from your air control layer. This can also be insulated. Often battens are 35mm, giving a total depth of 135mm of insulation. This is good for our climate.

A phased approach can get you to the passivhaus levels you’re aiming for. When the roof needs replacing, filling the ventilation gap with cellulose and 35-50mm of over rafter exterior insulation, and a new ventilation space/over-roof etc, will get it into the passivhaus range without any condensation risks (since 20-30% of the thermal resistance will be outboard of the sheathing - so it remains warm and dry.) I suspect this will become standard with EWI in the future, because it’s best done together to create continuous air control and thermal layers around the eves of the building, as well as continuity across neighbouring properties.

A bit of a long response, but I hope this helps.


This looks great Frank. One question: when fixing the battens for the interior plasterboard, presumably up and down the length of the rafters, will the fixings not penetrate the air control membrane, and, if so, how will Ben avoid thereby compromising its airtightness?

Thanks for detailed reply Frank. The big question is who is going to do this for me! The only people I can find are spray foam cowboys!

Any general contractor with experience with loft conversions are able to do it. Usually two people teams, who have a reputation in your neigbourhood to maintain, are fine with interior work like this.

You could start with comparing a few suppliers or manufacturers with the membrane/tape system you need. Get a few samples and the supporting documents. Show them to the contactors and let them choose.

You can do quality control, or get someone in to check key milestones: insulation fit and continuity being one, tape adhesion and air tightness being the other. Your issues will be down by the wall/roof junction around the eves, or gable windows. Probably worth contacting your local officials who will need to sign off on stuff before comitting.

Unfortunately I don’t know how to negotiate the contract and price. Maybe others have a better idea.

If you can’t use 400/600mm batten spacing directly on the rafters, you can also use horizontal.

You fix onto the rafters, tapes/membranes seal around nails, especially when backed up and compressed by battens.

I’m sure there’s a bunch of details I don’t know though. Anyone who’s worked with plasterboard more will, experience would make the most optimal option obvious.

ref (I’m not a member, just lifted this image):