Loft storage project

[officialsimon]

I'm wanting to put boarding on the loft floor for storage, but am strengthening the existing 3x2 (75x50) joists of what will be the floor with another 3x2 on top and fastened to the two floor purlins about a third of way in from the bottom of the roof with joist hangers. I'm doing this so it can be walked on and so that heavy things can go up there.

I then intend to put 100mm Kingspan between the joists as we have removed the horrible loose fill insulation between the joists which looks like it has been there for hundreds of years! The two areas beyond the two floor purlins, going to the front and back of the house, I will fill with rockwool or similar as I will wall these two ends off with Contiboard or similar.

Can anybody tell me, when installing the Kingspan to the joists, which being two 75mm joists on top of each other, ie 150mm deep, does the Kingspan board have to go to the top of the joist gap or to the bottom. I saw a video on YouTube which seemed to infer that they should go to the top and therefore require battens at 100mm from the top to hold them?

Also, does anybody know anything about a required ventilation gap? Obvously with 100mm boards to a 150mm depth will automatically leave one, but again, should this be at top or bottom of joist gap? Again, the video I saw would infer gap at the bottom. But is a gap for ventilation all part of the correct procedure and requirements? What does all this say about condensation etc and where it will go and what it will do or cause?

Also, if there's Rockwool pushed up to the other side of the floor purlins to a good depth, then surely although there is a gap at the bottom of the Kingspan on the main side, there wouldn't be much ventilation going on, ie it would just be a sterile airspace void thingy in the main underfloor area? Does any of that matter?

 

[officialsimon]

Nobody can tell me anything about this then?

 

[kbdiy]

Why do you want to use Kingspan between the joists? Surely rockwool or similar is easier to handle and lay in these fairly restricted spaces - and much less costly. The loose fill insulation may well have been vermiculite which could have remained in situ and new insulation laid on top. Ventilation, to avoid condensation refers to general venting of the loft space, not about leaving gaps above or below insulation between joists (if I understand your question correctly). The important thing is to ensure there is a good air gap around the eaves so air flows around and across the roof space.

I think the YouTube vid you refer to may have been about insulating between the rafters, which is where Kingspan would have been used and the battening/airgap would be relevant to that.