Another loft/roof insulation Question

[padstar]

More so did the house heat up quicker and then retain the heat when the heating goes off.

At the moment our house never really heats up fully and then the heat dissapears very quickly when we switch the heating off. We are in the process of insulating the external walls and the loft has minimal insulation in it so it is not surprising just want to be able to heat the house and then keep it constant with minimal effort

 

[IT Minion]

More so did the house heat up quicker and then retain the heat when the heating goes off.

At the moment our house never really heats up fully and then the heat dissapears very quickly when we switch the heating off. We are in the process of insulating the external walls and the loft has minimal insulation in it so it is not surprising just want to be able to heat the house and then keep it constant with minimal effort

To let your house get to temperature and then stay there with less work from your boiler, you need to stop the heat escaping.

It doesn't matter if you stop the heat going out the top by putting 300mm of loft roll over your upstairs ceiling, or insulate the rafters and stop all the drafts (like a loft conversion).

BUT, doing both half heartedly won't work well. A bit of insulation in the joists will reduce the heat loss to the roof, but any drafts in the loft space (which are normal in cold rooves) will drop the loft temperature to the same as outside, which means you're effectively just benefitting from the 100mm roll.

If you want to do your rafter insulation approach so the loft stays warm then don't muck around with 70mm, it isn't enough. You will need that and additional layers (total depth something like 125mm?) underneath the rafters to stop cold bridging and additionally as woody says to stop all drafts. In that case the loft roll between the joists is a waste of effort.

 

[padstar]

Have been up there again today clearing more of the loft space and having a good think about it. The roof space is not that cold and far less drafty than the old roof was.

I am now starting to think just insulate the joists. 100mm in existing joists. There are some cross braces up there already 100mm deep so i could add to these to drop the centres down and install a 2nd 100mm layer of mineral wool then board over.

I know it's not 300mm but would it do the job. Or I could cross brace in 6x2 and add 150mm on top.

 

[JohnD]

200mm plus an airspace and board is pretty good. I have cross battened in a similar way. When screwed down tight, the battens, and the boards, make the floor more rigid.

I don't have the figures handy, but each 100mm you add does not have the same value, the returns diminish.

e.g. the first layer might reduce 60% of the heatloss

the second might reduce 60% of what's left

the third might reduce 60% of the remainder

so, for each 1kw of heat loss,
you'd save 600W by the first layer

then 240W

then the third would only save you 144W

(these are imaginary figures)

 

[phatboy]

I did 100mm between the joists, then a LoftZone kit, with 200mm laid between it, across the existing insulation

https://www.loftzone.co.uk/

Cost me about £1000 to create a 6.5 x 6.5m deck, with insulation about 10m x 9m, which is massive for storage