Floor insulation with a ventilated crawl space

[AronSearle]

the article refers to walls (vertical) where there is an air gap between the surfaces and the mineral wool.

I can't find non paywall articles discussing floors.

The same mechanisim happens though, airflow can occour through mineral wool, caused by several different mechanisisms, mostly heat forced convection or air flow around vents (it is a ventillated airspace).

When installing mineral wool suspended in wire mesh, you could have an airspace above and below the mineral wool which is why I link that article as it discusses how these arispaces will then cause airflow through convection through the insulation.

and the wool is to be stuffed tightly to block draughts.

Mineral wool is not airtight, I don't know why you think it is, I even linked to the whitepaper showing how airflow through it around vents significantly affects its performance.

batts are more airtight, but still allow some air movement.

 

[JohnD]

Mineral wool is not airtight, I don't know why you think it is

You should not attempt to put words into my mouth. I have not said that mineral wool is airtight. It does however block draughts when stuffed tightly into gaps. Since it is conformable it is easy to stuff tightly into gaps, which rigid boards are not.

 

[geraldthehamster]

You should not attempt to put words into my mouth.

Or indeed mineral wool .

Cheers
Richard

 

[AronSearle]

You should not attempt to put words into my mouth. I have not said that mineral wool is airtight. It does however block draughts when stuffed tightly into gaps. Since it is conformable it is easy to stuff tightly into gaps, which rigid boards are not.

It is not airtight, it does not block draughts, it ******* them, membranes and sealents block draughts.

It may reduce airflow so that it is not a noticeably perceptible air leakage (to human perceptions).

I am not putting words into your mouth, *you* said it blocks draughts = air = You cant block airflow with something that is not airtight, maybe you should be more precise.

 

[jabuzzard]

It is not airtight, it does not block draughts, it ******* them, membranes and sealents block draughts.

It may reduce airflow so that it is not a noticeably perceptible air leakage (to human perceptions).

Look anyone who thinks that air permeability through mineral wool insulating a floor over a ventilated void is a significant mechanism of heat loss from a house is on the crack pipe. If you really do think it is significant then put some floor lining paper down before you lay your carpet. I personally do this anyway because it stops underlay sticking to the floor boards and being a pain to remove, and it is as cheap as chips.

Far more significant by several orders of magnitude is the forced convection cooling that will happen regardless of the insulation type that you use.

Searching around shows that for a ceiling going from 100mm to 270mm which is more than replacing 100mm mineral wool with 100mm rigid foam has a payback over 10 years and that is cheap roll out wool, with a ceiling that has higher heat loss than a floor. Switching from 100mm mineral wool to 100mm rigid foam is going to be a 20+ year payback and that is assuming that the cost of capital deployed (aka you could have invested the money on something else) is zero.

If you have the floor up or have a huge crawl space you can stand up and can afford the capital outlay then by all means retrofit a rigid foam insulation. It will provide the best result. For most people however fitting mineral wool to the depth of the joists will make a huge difference be easier and cheaper to fit and have a much shorter payback period.