Looking to insulate loft but joists are too shallow at 60mm - stuck on deciding on what to do

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I'm looking to add around 100mm of insulation to the loft after having removed the former occupant's loose-fill and then boarding it. I'm only looking to add this little because I hope to convert it into a bedroom if possible sometime down the line so I don't want to have to remove it after boarding the rafters, but for now I'd like some protection against cold for the coming winter.

The joists are only raised 60mm from the ceiling so I'd have to squash standard 100mm insulation down a lot and potentially prevent airflow. Compounding this issue is that there are eight spots installed in the hallway which aren't fire-rated nor are they IC-rated as far as I can tell (RSA3DBC1), hence cups will have to be glued in place on top of them and insulation cut to accomodate them, damaging the insulation potential.

From here it really looks like I have three options:

1. Forget about insulating the first 60mm - raise the joists by putting in crosspieces at 100mm tall and insulate between these. Install a floor on top. This would prevent me from being able to close any gaps by crosspiecing the insulation unless I raised it again.

2. Fill the 60mm gap with 100mm insulation or loose-fill, then install 100mm crosspieces and 100mm insulation between them. Not fond of this because of the extra cost of insulation and/or the dust that might prevent the loft's use a livable area.

3. Use standard-sized 50mm insulation board between the joists - cut around the boards allowing the cups on top of the spots. Not confident about making perfect cuts on insulation board and, cutting holes into the board might massively reduce its effectiveness.

A final question - how large should the gap be between the top of the insulation and the installed floor be for good airflow? Should I be looking to raise the joists beyond 100mm for 100mm worth of insulation? I think 50mm clearance is given as standard for boarding over the rafters.

IMG_20221124_125541.jpg
 
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You won't be able to build a loft conversion off those ceiling joists. Put 300mm mineral wool insulation in and board on stilts - the savings on your heating bill will pay for the materials within months - if you use flooring grade chipboard you will be able to recycle that for a future conversion.
 
There is an efficent alternative which I used in our last house because I wanted something easy to lay:-https://www.wickes.co.uk/YBS-SuperQuilt-Multifoil-40mm-Insulation-Roll---1-2-x-10m/p/166805?_br_psugg_q=superquilt
 
3. Use standard-sized 50mm insulation board between the joists - cut around the boards allowing the cups on top of the spots. Not confident about making perfect cuts on insulation board and, cutting holes into the board might massively reduce its effectiveness.
That is why god invented spray foam...
 
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No offence but I would not touch spray foam with a barge pole.
 
Why thats its job, a can of spray foam seals insulation boards gaps, after all that's all foil backed insulation board is, a layer of spray foam between two bits of foil.....how else do you think its done...laser cutting of the board to fit...
 
You're confusing joists (on the floor) and rafters (the sloping bits holding the roof up). As @cdbe , you won't be able to do a proper loft conversion on those skinny joists so just chuck the rockwool and legs in for now. Don't worry about airgap between loft boards and insulation, you don't need one. Do make sure you don't block any eaves ventilation with the rockwool though.
 
As mentioned anything you put down would all need to be removed for a loft conversion, in fact you'd have to lay chicken wire over the lathe and plaster fixed to the existing or new joists to work as a cradle so the insulation is retained if the L&P falls down in the event of a fire.
 
There is an efficent alternative which I used in our last house because I wanted something easy to lay:-https://www.wickes.co.uk/YBS-SuperQuilt-Multifoil-40mm-Insulation-Roll---1-2-x-10m/p/166805?_br_psugg_q=superquilt

Thanks I'll take a look at this.

You're confusing joists (on the floor) and rafters (the sloping bits holding the roof up). As @cdbe , you won't be able to do a proper loft conversion on those skinny joists so just chuck the rockwool and legs in for now. Don't worry about airgap between loft boards and insulation, you don't need one. Do make sure you don't block any eaves ventilation with the rockwool though.
I did intend to board the rafters to make the loft warm but it's far easier and straightforward laying it as there's actually two parts to my loft - the other part is only accessible by wedging myself through the rafters. Not sure about the right way of doing that part considering the rafters extend into my neighbour's loft and also that they get far too close to the wall. Perhaps I'd have to make that part cold and board over it keeping the main part warm.

feh_008570_000001_IMG_20221124_125918.jpg


As far as my skinny joists, could I just bulk them up by adding crosspieces or is it because they literally can't support the extra weight? Any suggestions on the legs? Loft Legs seem to be most popular but I think I've seen metal ones.

Mmm, OP best not try and put too much junk priceless family heirlooms up there if you do board it out :)
Is this gonna be a problem in the future?
 
I used original 170mm Loft Legs. No problems.
Screwfix/B&Q do their own Dial make. Saw some negative comments.

Metal ones are probably these ones. Have seen them used and look good.
 
Lath adds a surprising amount of strength to skinny joists (which are probably twice the strength of modern equivalents). As long as you don't go mad with the storage you'll be ok, ceiling might bow a bit but is unlikely to break (I ended up stashing about half a ton of bricks on a similar looking ceiling, no damage caused)
 

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