Laying new loft insulation over old

[DaveItYourself]

I've been upgrading my loft insulation.
1950s house.
I have joists that are about 130mm deep.
In between the joists, there was a thin layer of old glass fibre insulation and what looked like rockwool insulation over that.
The insulation varied in depth and was ~75mm deep at best, ie at least 55mm below the top of the joist.

Following the usual steps:
- inspect existing insulation, is it free from damp and mould etc => yes
- is the existing base layer 100mm deep and within 25mm of the top of the joist => no

So on that basis, I concluded I needed to install a new 100mm base layer before installing a 170mm top-up layer to reach a target 270mm insulation depth.

All the advice seems to say leave the old insulation in place if dry etc. However, with it in place, there wasn't room to lay out the new 100mm layer and get in within 25mm of the top of the joist. So I opted for squashing down the old insulation to leave room for the new 100mm to finish flush with the top of the joist without compressing the new insulation.

Was this the right thing to do?

One might say why bother squashing the old insulation, just let the old and the new stack up, the more depth the better. But I have been installing raised loft boarding using legs, so needed to limit the height of the insulation.

 

[motorbiking]

Did you want to board over it? Typically insulation is rolled out on top of the rafters like a new lawn. The idea is to trap air, so squashing it defeats that.

 

[DaveItYourself]

Thanks for the reply.

>Did you want to board over it?
Yes, as I said, "I have been installing raised loft boarding using legs"

>Typically insulation is rolled out on top of the rafters like a new lawn
Well, it is rolled out in-between and subsequently on top of the joists. The rafters are what hold your roof tiles up. But otherwise, yes.

>The idea is to trap air, so squashing it defeats that
Yes I know that. I wasn't squashing the new insulation, rather I was squashing the old insulation down in order to avoid any compression of the new insulation following boarding over it.