Loft insulation around a complex roof layout

[Lemna gibba]

I'm not sure whether to put this under roofs or insulation as it concerns both.

 

Our house is a 1930s build with an extension in the 1980s. The original gable roof is above half the house and a 1980s build gable roof that joins this at right angles. Both roofs have a similar construction. Timber rafters are fixed to wall plates which are lengths of timber fixed to the tops of the front and rear walls of the original building and
the left and right side walls of the two storey extension. The rafters meet at ridge boards on the apex. The ceiling joists also connect to the wall plates and rafters to
create a rigid section. Intermediate support is provided by purlins which are the lengths of timber fixed crossways underneath the rafters and fixed into the gable. Where the new and old roofs meet, the old roof has not been removed and still has the original slates. The original roof has some boarding nailed to the underside of the rafters, but this is in bad condition and about 1cm thick and probably not insulated. Overall, I think the roof is in good condition and was apparently renovated in 2006. There is some paper that degrades if you touch it and a sarking felt? (anyway some kind of modern membrane) that is in good condition. We have never seen any evidence that it leaks. 

 

The loft is only 1.3m high. The new part is well insulated by some pretty thick layers of mineral wool. Not sure how much, but it looks pretty thick. In the old part, the

insulation consists of mineral wool below tounge and groove boards, but this is badly done. I can see where the boards are cut out there are patches where it is really thin. Also, I can see for a fact that it's not 290 mm ceiling to floor measurement through the roof hatch is less than this. It's pretty clear that the roof boards should be taken up and insulated properly. Where the old roof hasn't been taken down, the insulation is non-existent to badly arranged. 

 

I want to think about the best way to insulate this, and who to speak with. I had thought that I would do this myself, but realise that it might need someone with some knowledge of roofing.  I don't want to insulate in between the rafters as the roof works. And I don't want to raise the height of the floor by too much, as the utility of the loft as a storage space would go down substantially, but would this would be my preferred path. I assume the most logical thing would be to remove the old roof, open up the space some more, lift up all the boards, clean up and lay insulation down. My questions are. Who would I need to speak to to find out how much of the old roof can just be removed? Then regarding the insulation, what to use. I would favour rigid board if I can get a thinner insulation giving the same effect. My dilema is that we plan to do some major renovations on the house next year and including this in with those would be very useful. Not least because if I do add celotex or something between the joists then the electrician will need access to the ceiling next year anyway. However, it would be good for us to have an EPC before then that doesn't require loft insulation. This leaves me a bit torn between an intermediate fix or something that can be lifted to allow access. I would very much welcome any thoughts on this. We are serious about insulating, and want to do a good job. 

 

1930s part. Access to the new part requires a crawl under the purlin!

 

1980s part. I think the insulation is fine here. The wires are from the solar panels.

 

Old roof still in place, as far as I can tell the insulation is virtually absent between this, or is just a few cm thick.

 

[tonyshouse]

I would remove the old slates and sell them! Then join new and old insulation in an OCD fashion, no gaps.

 

i would remove the plasterboard from under the rafters, dispose of it, lift the chipboard floor carefully, install 400mm of ceiling insulation everywhere and keep the loft empty of junk 

[Lemna gibba]

I can see the sense in what you're saying, but we really want the loft as storage space.

 

[tonyshouse]

Then same but use raising pieces to help form a new storage floor 50mm above the top of the insulation , I use 47x50 timber and and deck on top

[tonyshouse]

I am giving you loft storage space safe from mould and condensation with a deck raised above the top of the insulation with 50mm ventilated void, nicely insulated ceilings too

[Lemna gibba]

Thanks for the suggestion. I see the importance of raising the deck above the insulation. My main concern is that this will raise the deck height quite substantially. Possible by 25cm (extra 10cm to bring it back to 290 mm, plus 10cm for the extra insulation you suggested beyond this and 5cm for the void).  If we did this, then access to the old part under the purloin would be impossible, but this might not be a problem if we can remove much of the old roof. Access between rafters would be easier than under the purloin. The height of the loft would reduce from 1.3m to 1.05m. Due to the angle of the roof this will make the useable bit very narrow.  The most important thing we have in there is the inverter for our solar panels, but I worry that access to this would become increasingly problematic. 

 

What do you think about doing something similar with rigid board insulation? This would mean that we could reduce the thickness of it. I guess the problem would be that it won't be as breathable as the mineral wool.