Replace roof insulation advice 75cm roof joists

[BathDIY]

House is 1970's property with tile roof and traditional felt membrane, we have some condensation issues I now think due partly to low amounts of insulation.

The roof is a cold roof with old fibreglass insulation in place, depth is currently CIRCA 10cm, but it's really patchy (multiple layers / going to scraps in places). I have patch repaired in places but I now think this is insufficient.

The roof joists are on the small side 45mm X 75mm at 40cm centres.

The loft is boarded out partially which compresses the existing insulation down. As the roof pitch is 30 degrees am concerned that raising this area may well inhibit headroom and am not confident in the loft legs I see available online.

Any ideas on how to add a decent amount of insulation while retaining storage space?

Is there anything I can do to stop condensation, for instance, I wonder if I could beef up the insulation with glass fibre and staple Foil Wrap Insulation to the roof rafters?

 

[IT Minion]

I'm guessing you meant 45mm not cm.

Insulation between the joists and Celotex laid over the top. The typical rule of thumb I've seen used is that it's twice as effective as rockwool per mm depth. So to take your ~70mm depth of insulation up to modern spec (270mm) you'd want to stick 100mm thick slabs over all of it. Then tape the joints together and overboard with something, probably something thin and light to avoid excessive weight. It isn't supposed to be taking serious weight or people jumping on it in stilletos.

3x2 is a very small joist, perhaps someone who knows more could chip in on how you'd tell when you've overloaded a roof.

 

[oldbutnotdead]

Condensation in the loft is caused by warm (damp) air leaking up from the living areas. Seal the loft hatch properly, plug any holes around pipes going up into the loft, make sure any extract fans are venting to outside not to the loft and fit proper covers to any water tanks up there.
Insulation- i'm not convinced by those legs either (never tried them but they look v skinny). You'll get more payback (reduced energy bills, increased comfort in the house) by increasing the insulation depth between/above the joists. Modern standard is 300mm or so, if you really need the storage space up there then ripped plywood or fence boards fixed to the joists will give you depth for insulation and something to fix your boarding to (yes you'll lose some headroom)

 

[BathDIY]

I'm guessing you meant 45mm not cm.

Insulation between the joists and Celotex laid over the top. The typical rule of thumb I've seen used is that it's twice as effective as rockwool per mm depth. So to take your ~70mm depth of insulation up to modern spec (270mm) you'd want to stick 100mm thick slabs over all of it. Then tape the joints together and overboard with something, probably something thin and light to avoid excessive weight. It isn't supposed to be taking serious weight or people jumping on it in stilletos.

Yes mm can't see the wood for the trees (edited original post but can't change title ). A great suggestion but that sounds costly, may rethink the use of my garage for storage.

Condensation in the loft is caused by warm (damp) air leaking up from the living areas. Seal the loft hatch properly, plug any holes around pipes going up into the loft, make sure any extract fans are venting to outside not to the loft and fit proper covers to any water tanks up there.
Insulation- i'm not convinced by those legs either (never tried them but they look v skinny). You'll get more payback (reduced energy bills, increased comfort in the house) by increasing the insulation depth between/above the joists. Modern standard is 300mm or so, if you really need the storage space up there then ripped plywood or fence boards fixed to the joists will give you depth for insulation and something to fix your boarding to (yes you'll lose some headroom).

I hate the idea of adding anything that high on a joist that's so piddly in size.

Already sealed the holes thanks for the heads up though. How would you suggest fixing the fence boards, with brackets?

Also, I think I read something about an air gap of 5cm needed between board and loft roll for air flow.

 

[oldbutnotdead]

Fence boards or ripped ply, screw them to the side of the joists (not the rafters)- to avoid confusion, the rafters are the timbers running up to the ridge holding the tiles up, the joists are running across the loft floor and are probably only designed to support the ceiling below.

 

[BathDIY]

Fence boards or ripped ply, screw them to the side of the joists (not the rafters)- to avoid confusion, the rafters are the timbers running up to the ridge holding the tiles up, the joists are running across the loft floor and are probably only designed to support the ceiling below.

Yes, I am fast forming the view that I should empty the loft and just insulate.

 

[oldbutnotdead]

Yep, at least half the stuff up there will be junk you've kept because you can. Def the most cost-effective route is to insulate the loft floor

 

[^woody^]

I've always had a sneaking feeling that there are good insulation properties in boxes of junk stacked up in a loft in leu of some insulation quilt

 

[JP_]

Yes, I am fast forming the view that I should empty the loft and just insulate.

certainly easier. I got a couple of rolls of loft insulation from B&Q a couple of years ago and rolled them out and noticed an almost instant improvement. Took about 60 seconds to open it and push it along with a broom. Can't use that part of the loft though!

 

[BathDIY]

As we are still seeing some condensation issues in the loft is there any value in using a VCL layer under the insulation?

Its what I have been advised to do by one company but am concerned this may cause more trouble than it solves.

 

[JP_]

probably better to find source of moist air and improve loft ventilation