Best fibreglass insulation for under floorboards?

[Logan]

I’m looking for recommendations for the best value fibreglass insulation (preferably pre-perforated for joist spacing) that can be used for under floorboards in the crawl space. 
 

My floorboards currently have no insulation underneath and the laminate is very cold to walk on. 
 

I was looking at this as a potential option but not sure of R value required: https://www.wickes.co.uk/Knauf-Insulation-Space-Bottom-Layer-100mm-Loft-Roll---11-08m2/p/109449#

Thanks in advance!

[Iceverge]

Have a look at the kg/m3 figures. The denser the better IMO. Also I prefer slabs to rolls in general.

 

What joist spacing do you have?

[SteamyTea]

It is the thermal conductivity that is the important bit.

The lower the number the better.

[Logan]

Thanks. Not got the money to do the slabs that’s why I’m looking at fibreglass. 
 

This is the cork stuff that was under the laminate - being replaced with 9mm ply and 6mm LVT with a built in underlay. Don’t know if that will make a difference to avoid me having to do the dreaded job?! 😂 I only want to do it if it’s going to make a substantial difference.

[Iceverge]

4 hours ago, Logan said:

4 hours ago, Logan said:

Not got the money to do the slabs that’s why I’m looking at fibreglass.

 

 

 

That what I ment by slabs. That's Rockwool but you can get glasswool (fiberglass) slabs too. 

 

How do you plan on insulating below the floor? Will you lift the floorboards? 

 

 

 

 

[Logan]

Looking to do it from the crawl space underneath but will need to do it before the new LVT goes down which is very soon.
 

The slabs are a lot more expensive than the roll so out with the budget!

[Nick Thomas]

Yeah, the slabs are about 4x the glass wool, it's a significant difference. I got a load of Rockwool RWA45 recently because I was installing it from below and didn't want the glass fibres floating down onto me during insulation. I'd think reeeallly carefully about how you're going to protect yourself while doing it 😬. It's really horrible stuff, and you don't want it in your eyes, up your nose, down your throat, on any of your skin... full PPE? In a confined space?

 

I got the rockwool slabs from insulation4less, which sells the fibreglass stuff too - https://insulation4less.co.uk/products/knauf-combi-cut is a fair bit cheaper than the wickes one per sqm, although there's a delivery charge below £300 (ex vat) which might wipe out the savings if you're only doing a small area.

 

I think you'd need 150mm of this stuff to meet building regs for retrofitted floor insulation (U=0.25).