Roof, new slates, insulation and solar panels

[Jacob]

Having roof re-slated, plus membrane, insulation etc.
Any thoughts on putting 4" insulation board between the 4" rafters? Maybe skew pin it to the rafters from above as it goes in, or would 2 layers 2" sheets be easier to handle?
Solar panels to follow - would the brackets or whatever be better fixed as the slating is in progress, or as a separate job after the roof is finished?

 

[Sideways]

In roof solar is the neatest solution if you are having the slates re done.

Take a look at the Marley version, but there are others.

https://www.marley.co.uk/solar-roof-tiles/solartile

 

[ChaiLatte]

What provides the best thermal performance?

4" of insulation at rafter level (only the loft space benefits)?

4" extra of insulation at ceiling joist level?

 

[Eshmiel]

What provides the best thermal performance?

4" of insulation at rafter level (only the loft space benefits)?

4" extra of insulation at ceiling joist level?

The thing about roof space with lots of bare wood in it is that it needs good ventilation. If there's not a significant air flow, the rots can get a-hadden o' yer rafters.

If you insulate the very top of the roof (what you call "rafter level"?) then you'll want to keep the heated air below, in the loft space, static otherwise you lose its heat and hence the insulation doesn't do anything to keep the house warm.

If you insulate at loft-floor level, with the air in the loft above left free to move in-out past the rafters, then the house below is insulated but the loft is ventilated and hopefully remains rot-free. Also, you aren't heating loft air just to throw it away outside; or to keep it in there despite the loft not having anyone living in it.

 

[Beau]

If being completely re slated as mentioned it might be worth considering in roof panels. Reading the other day that the cost for in roof PV is little more that just slating as so much labour and materials are saved. The potential downside is you are then tied to a size of panel which may change over the years.

As for insulation I would look at a sealed warm roof construction with full fill PIR between rafters and overlaid PIR then battens. Think you have to mount a vapour proof membrane on the underside with that build up. Not built one yet in that construction but its what I wished I had done on our last renovation project.

 

[Jake]

If you put insulation between rafters you need extreme attention to water vapour tightness (and VCL and tape is expensive and the detailing is pernickity) to avoid condensation in unventilated places causing problems with the rafters.

A warm roof is better (insulation over rafters) but not so practical on a renovation usually.

Insulation under rafters is simpler, at the expense of headroom if that matters. In my attic, which is just for storage, I counterbattened under the rafters and insulated under those, so there is very little area of the cold side of the insulation which doesn't face a ventilated space, but headroom didn't matter there.