Roof Insulation Between Joists

We’ve watched your video on YouTube and wonder if you could give us some advice please…. roof insulation between joist

Last year we bought a barn that was converted in the 1980’s.  It already has foil-backed insulation in the roof but only 35mm and we would like to increase this – without taking the roof off, as it’s simply too expensive.

As far as we know the current roof construction is – from the outside in – slate, barrier, 35mm of a foiled insulation board, covered internally with approx 5mm plywood laid horizontally.

Our plan is to add insulation from inside the house. On the ceiling we were thinking of adding in between the joists because we want to keep them showing by about 10-15mm.

On the walls the beams are very mixed materials and not great quality so we plan to plaster over, keeping only the A frame main verticals exposed. We are thinking of putting 50mm Kingspan in between the joists on the ceiling, then covering with 12mm board, skimming and painting. The walls we are planning on insulating, boarding, skimming and painting.

Is this the right way to approach things or will we have a problem if we don’t put in a vapour barrier? Does the foil on the Kingspan work as a vapour barrier?We would really appreciate any advice.

Thanks very much,

Kind regards,

Hi I’m going to be posting a video shortly addressing this problem, however haven’t quite got my mind around just how to do it because there is the way correct way and a way of doing this that you may get away with! I can’t advise but can try to explain both ways and then its down to you.

An engineer would do some calculation and refer to building control and look at some known detailing and advise from there. As you haven’t got any ventilation on the outside and you are introducing insulating inside the fabric of the building he would probably tell you to start again from the outside (sorry).

Now the way you are thinking of doing this is not correct but has been carried out for years by people like yourself just doing what they think is best. It works most of the time. The problem is its still an expensive operation to do and if doesn’t work  removal of it and possible replacement of rotted timbers would be expensive.

If it was my place would I do it or not? I would look into thing a bit more would like to see the exact buildup IE remove some tiles see the felt they used and the quality of their work then I would probably make a decision.

However you need to look at the gains what you are thinking of doing is not that good not a lot of insulation  I don’t think you will make that much improvement. Now strip the roof then insulate from the outside using some space jackets and then insulate between the joist with ventilation above would make a difference.

Speak soon,

Steven Dickison