Insulation efficiency when squashed

[DJM]

I will be insulating a ceiling and loft after some rot treatment, and am just wondering whether 100mm insulation is more effective than 170mm squashed between rafters.

The reason is that in some areas there is only 100mm height, but I will get a better discount if I buy all of 170mm. So if I get the 170 and squash it into the 100mm height areas, will it be worse than the equivilent 100mm insulation?

 

[simonjay]

I will be insulating a ceiling and loft after some rot treatment, and am just wondering whether 100mm insulation is more effective than 170mm squashed between rafters.

The reason is that in some areas there is only 100mm height, but I will get a better discount if I buy all of 170mm. So if I get the 170 and squash it into the 100mm height areas, will it be worse than the equivilent 100mm insulation?

The insulation is due to the trapped air, so if you squash insulation down, it will be worse than the same size non-squashed.
You can cut if down a bit if needed, but can be messy / itchy !
Simon.

 

[DJM]

The insulation is due to the trapped air, so if you squash insulation down, it will be worse than the same size non-squashed.
You can cut if down a bit if needed, but can be messy / itchy !
Simon.

Simon,
I know that like for like it will be less efficient, my question though is:

Is 170mm insulation squashed to 100mm less efficient than 100mm insulation unsquashed.

My gut feel is that it probably is, but if the insulation difference is minimal then the cost difference isnt.

 

[mointainwalker]

I can't remember where I saw it - or it may have been talking to the Tech dept at Rockwool - but the answer was that squashed insulation gives same thermal insulation as unsquashed insulation of same finished thickness ( clear ? ) although it will probably give you better acoustic insulation.