Insulate my old roof!

[jayc89]

1850's house, hipped roof, no felt beneath the slates. A reasonable amount of insulation laid across the joists, but there's certainly cold spots (i.e. ceiling above shower instantly attracts condensation - I know it needs an extractor - waiting to fit MVHR). The sparky made a right pigs ear of the insulation when he was rewiring the house, so the insulation's currently less flat than these images.

 

My plan was to drop all the ceilings, run an airtight membrane across the joists, batten where we'll install downlight and then attach new PB. Which will at least keep the rooms airtight. 

 

But I don't know what to do with the loft itself. My plan is/was to remove the mineral wools whilst dropping the ceilings, place eaves vents all around the perimeter and have something blown in, cellulose? 

 

The wood work is in reasonable good nick, so re-roofing isn't a requirement right now, some of the nails are starting to fail, so we do have a couple of slates come loose every year (you can see daylight in the couple of images from areas where this has happened - fixed now). This potentially increases the risk of leaks, and water reaching the insulation could impact its performance, certainly as it'll likely be weeks/months between a leak occurring and me realising. Could I staple a breather membrane to the underneath of the rafters to protect the insulation from any moisture getting through the slates but still maintaining the ventilation? Is that just over thinking? I guess the membrane would direct any air flow between it and the slates, meaning the wood left outside of the membrane could be prone to moisture?

 

On top of all this, the MVHR unit needs to live up here somewhere. The manifolds can live under whatever insulation I end up using, the unit itself and ducting to/from it will need some insulation. Given the space we have up there, I'm thinking of building it its own insulated box/room to separate it from the cold loft space - would that work?

[jayc89]

Had a quote for Warmcel, 350mm deep which is coming out at approx double the cost of buying new insulation rolls. Is there any major benefit to Warmcel (or any other blown cellulose) over mineral wool, other than ease of application?

[Jimbo23]

Watching this one closely mate because I'm in a very similar situation - your house is a little older than mine, but the roof space looks near identical.