Loft insulation and metal cradle

[Number57]

Hi all, first post but I think I'm going to end up a regular here!

I've recently moved into a house that needs plenty doing, and one of the first bits is to sort the loft out. I've attached some pictures, I hope. My questions are:

1. I seem to have some kind of metallic cradle between the joists, where normally I'd expect nothing, or chicken wire. I'm happy to put wool insulation in there, but if I use PIR board which also has a metallic coating on both sides would I see condensation occur between the cradle and the PIR board?
2. Can I add mineral wool insulation, on top of PIR board?
   1. I want to put 100mm PIR between the joists and then lay mineral wool on top
3. What do I do at the edges where the joists meet the roofline?
   1. Do I just go up as high as the rafter, and keep about a foot between the insulation and the felt?
4. I have cavity walls that are not filled, yet. Do I stop the insulation at the edge of the joists, I remember reading somewhere I should push some down just into the top of the cavity which makes sense to me...

Thanks for any help, I've also added a picture of a lovely wasps nest I found and a damp beam - just to show what I'm up against.

 

[Mr Chibs]

Cracking nest... !

On your first picture... 'metallic cradle'... are you sure it's not foil backed foam board? Towards top right of photo, it looks like a bit is cutout and you can see the thickness of the board.

Are you putting the insulation on behind the ceiling or under the roof?
If roof, think you need to leave an airgap of around 50mm between felt and foam.

 

[Number57]

Top marks @Mr Chibs , and thank you for your response.

Spotted from the corner of a photo when I, stood in the actual loft, couldn't even see that!



So it's 15mm thick PIR board by the looks of things. I've no way of pulling it up to see what's underneath unfortunately but I'm happy that there's something there that I can work with.

Same question still applies, can I just add more PIR board on top, as it will have the same foil backing?

I'm not insulating the roof no, just laying on the joists to have a cold roof. But my concern is if I add 200mm on top of the joists I'll be touching the felt at the edges.

 

[Mr Chibs]

If all you want to do is boost the insulation, I'd just buy normal loft insulation... it should work out cheaper.
I would stop at the inner skin of brick, where your roof trusses are resting, as you want to keep the ventilation going. Try not to touch the felt, if there is condensation, it may drop onto the insulation.

 

[Number57]

That is all I want to do - but I want to boost it quite a lot

I'll definitely be doing fibreglass, as I can't really do afford all of the PIR board I want. But occasionally PIR board comes up for cheap locally and I was wondering if I could lay it between the joists and still have the fibreglass on top of it?

In total I'm shooting for about 400mm fibreglass (or equivalent). So if I had 100mm of PIR between the joists and then 200mm fibreglass on top I'd be a happy chap.
If I did just fibreglass it would be 100mm + 200mm + 100mm. Three layers of the stuff makes it all a bit more complicated to fit, especially if I'm trying to put any storage space in.

Condensation is already dripping onto the insulation unfortunately, ventilation is something I need to get sorted pretty quickly!

 

[endecotp]

It's easier to put fibre (of whatever sort) between the joists and PIR boards over the top, because then you don't have to cut the boards to fit the joists and seal the edges. For light storage you can put chipboard directly over PIR resting on joists.