Insulating suspended timber floors - bearers?!

[mwebber]

After hours of trawling posts here and elsewhere, I’ve decided to insulate the suspended timber floors of my Victorian semi by slinging breathable membrane between joists and laying 120mm of rockwool batts in between. I understand the various pros and cons of this compared to using PIR - I’m happy with my decision and am not really here for opinions on that.

The joists are 5” (127mm) deep so I’m going to keep the membrane up slightly from the bottom of the joists in order that the 120mm batts are firmly pressed up against the planks when they go back down.

However, I’ve not been able to find much discussion that specifically addresses the impact of filling the gap between joists may have on the wooden bearers that run perpendicular under the joists - the insulation will virtually sit on top of the bearers (just a small 7mm gap).

Can I just sling the insulation so that it is virtually touching the top of the wooden bearers even though this drastically reduces the overflow over the top of them?

At the moment I’m working on the basis that the insulation reduces the airflow along the side of the joists and nobody seems to worry, so presumably this isn’t a concern for the bearers on their top side?

The only information I did find somewhere was someone suggesting to either halve the depth of insulation over the bearers, or to use PIR for those sections in order to maintain airflow, and then carry on as planned elsewhere.

Any thoughts?!

 

[mwebber]

Should’ve said - the bearer wall below the bearers has good spacing to allow airflow through the wall.

 

[Deluks]

If you mean a brick sleeper wall, that the joists sit on, and it is 'honeycombed' (gaps in the brickwork) then that will be fine.

Unwanted opinion: Loft roll will work out cheaper than batts, but if you've already bought them then don't worry.

If the wall isn't honeycombed or there's not enough holes, then you could screw some low profile ducting to the top of the wall plates, yes it will mean slightly thinner airflow at the point, but underfloor ventilation must be maintained.

225 Flat Channel

A 1m length of flat channel that's easily cut to size for easy installation in bathrooms or kitchens. The low profile is perfect for concealed fitting in wall or ceiling voids, or above wall units - an efficient and reliable option for short and simple duct runs.• Rectangular duct - 234 x 29mm...

www.toolstation.com

 

[mwebber]

Ah that looks like a great option. I’ll double check how much honeycombing there is in the bearer wall but may do the ducting as well just for belt and braces - we’ll be fitting an overlay UFH system on top so this is the one time I intend to be dealing with the subfloor!

Thanks for tip on rolls vs batts - hadn’t got the batts yet and hadn’t really looked at price - the main attraction was the fact that I can got the batts in 60mm thickness and double them up to 120mm… a very brief search made me think I was unlikely to get roll in a 120mm thickness (or combination of rolls to make 120mm)… Maybe the extra 20mm isn’t worth it - I could do 100mm roll and help with the ventilation over the bearers at the same time!