Insulation in old barn at pitched roof level

[jfb]

I am converting an old barn into an office space and am considering the best option for insulating the pitched roof at rafter level. A new roof was put on it just before I bought it to ensure that it wouldn't fall foul of a planning timescales. So I am very reluctant to reroof but that makes it harder to work out a sensible insulation plan.

 

There are no plans for cooking facilities or shower but there will be a toilet/basin.

 

At the moment the structure is:

new clay tiles (tight fitting)

battens

breather membrane (2 types, don't know what sort, but modern, not felt, so I presume is breathable)

rafters (100mm)

purlin (minimum 150mm, max 200mm)

 

Rest of structure is 500mm thick stone walls where I will put on lime render/wood fibre board and a limecrete floor.

 

For the pitched ceiling I was hoping to do something like this:

 

tiles

battens

breather membrane

rafters (100mm full filled with fibre glass)

graphite eps (75mm)

intello airtight/vapour control layer

batten (25mm)

plasterboard +skim (15mm)

 

This leaves all the purlins exposed as I want and as much insulation as can be had but doesn't have any ventilation under the breather membrane. Except that the build up is breathable and there will be some ventilation through the tiles.

Even if I try and reduce the rafter insulation so it is not fully filled air getting in at the eaves wouldn't have any where to ventilate without installing some sort of tile/ ridge vent. Even then there isn't any cross ventilation that you would have with counter battens.

 

In an ideal world I would have the roof off, cover with 100mm eps, breather membrane, counter battens,  battens, tiles.

But that is going to be so much more work building up walls, planning issues, reproofing that I would really like to find a simpler solution.

 

Any thoughts appreciated.

[Mr Punter]

You need a continuous gap from eaves to ridge for airflow to prevent condensation.  Perhaps use 75mm Celotex cut between rafters with 25mm air gap above.  50mm Celotex under with ali tape over joins and finish with plasterboard.  It won't win any prizes for low energy but is fairly cheap and effective.

Edited January 15, 2019 by Mr Punter
air gap

[Ed Davies]

Between the breather membrane and the tiles, are there any counterbattens up and down the roof or just horizontal tile battens? If you had counterbattens, so a ventilation path all the way up and down the roof, the rest would be fine, I imagine.

 

If you've only got horizontal tile battens there would also be the concern that that would be relying on the membrane sagging between rafters to allow any water under the tiles to run down so if you stuff mineral wool under the membrane you might dam the water against the tile battens.

[jfb]

no counter battens, just horizontal battens straight onto the rafters. Not much sag either.

 

[Ed Davies]

I don't think much sag is needed to allow water past but perhaps somebody who really knows can give advice on the wisdom of putting in insulation which might push the membrane up against the battens.

[jfb]

i don't mind the idea of 75mm of solid insulation between rafter to allow a gap but I still don't quite see how that actually provides ventilation since at the moment it doesn't have anywhere to go at the ridge (though clearly has to be much better than no gap). If using celotex between and below the rafters and taping joints presumably I can do without a vcl/airtight layer but am losing a breathable build up.

If using 75mm EPS between rafter, 75mm EPS below rafter and intello  the u value not quite so good but more breathable.

[Ian]

@jfb

What type of clay tiles are they?

"Plain" tiles don't generally need any extra ventilation but the larger format interlocking tiles do need the extra ventilation provided by counterbattening.

[jfb]

not sure of exact type - look like this Ian

[TerryE]

Put a small horizontal rafters bridging across between the pitched rafters leaving at least 100mm between the top and the ridge beam.  You can now insert 50mm Celotex flush with the base of the rafter leaving a 50mm up-and-over airgap under the breather membrane.  And add another 75mm celotex bonded plasterboard or equiv across the rafters.  Note that you really need to dry line the room with  50 or 75 mm Celotex bonded plasterboard as well to get acceptable thermal performance.  Nowhere near passive, but at least not bloody freezing in winter.

[Simplysimon]

@jfb if you are rendering it, what about Diathonite to improve thermal properties?

[jfb]

Terry, I like the idea of a small horizontal ceiling near the apex to allow a bit of ventilation under the ridge beam.

I am looking at 60mm wood fibre board insulation as IWI to keep the wall build up breathable. Not the best thermally but less risk of interstital condensation and it has worked very well in our house.

Hadn't heard of Diathonite but ill take a look Simon.

[Ian]

3 hours ago, jfb said:

not sure of exact type - look like this Ian

Those are “Plain” tiles and wouldn’t normally need counterbattening to create a ventilation zone.

 

Edit: Also, if you are planning to install a vapour control layer on the warm side of the insulation you are already mitigating most of the risk. 

Edited January 15, 2019 by Ian

[TerryE]

2 hours ago, jfb said:

Terry, I like the idea of a small horizontal ceiling near the apex to allow a bit of ventilation under the ridge beam.

 

Good.  You understood what I was trying to explain.  Jan read it and said that I was clear as mud. ?

[jfb]

looking at comparable prices I can get graphite EPS 75mm @ £13.37 a board compared with celotex at around £34 a board.

I think i know which one I will go with………...

[Mr Punter]

You would need EPS about 50% thicker than Celotex to get the same insulation values.

[jfb]

thats true of normal EPS with a conductivity of 0.037 but graphite EPS is 0.030 compared with 0.024 for celotex.

But I take the point.