Retrofitting insulation under attic floor

[rangy]

Hi all
I have an existing rectangular attic floor comprised of 5 inch t&g pine planks and been down approx.60 yrs and in very good u molested condition. . It measures approx 8mx4m.At each side, it has the usual space in the eaves and this is fully insulated but the main floor area was never insulated( probably not a thing back in 1963).
Each of the joists on either side have a cross batten between them, I assume to keep the joists straight and also to spread the load from above. Unfortunately this prohibits me sliding in any insulation from either side without taking them out which I'm loathe to do. I'm thinking of taking up one of the planks the full length of the attic as I don't want to take up more than necessary and then hoping to slide in mineral wool batts between the joists towards each of the cross braces.
Does this sound feasible or sensible or is there an easier way and a better material to use.? TIA

 

[freddiemercurystwin]

You wouldn't normally insulate a loft conversion floor.

 

[foxhole]

Should work or you could use loose lay insulation.

 

[rangy]

Sorry, I haven't clarified, it's not a loft conversion just a hobby room, no one living up there. Loose fill may be a bit messier?

 

[robinbanks]

What depth of insulation will you be able to achieve with this approach of going under the boards? I highly doubt it will be sufficient

 

[rangy]

The planks are 5 inches wide so no more than that unless the insulation was more compressable and "fluffed up" a little once in situ.
It's a really good point you are making as I don't want to waste time and money and effort if it achieves minimal gains.
I guess it also depends on what people think is the best type of insulation to use and the requisite useful thickness required
TIA

 

[robinbanks]

I wasn't thinking the width was of any consequence, it's more about the height of the void under the boards. If that void is 4 inch it's about a third of the height needed to meet modern regs; 11 ish inches of wool insulation is recommended. If you use a more performant insulation like PIR (kingspan) you can reduce that. Wool is typically rated 0.033 to 0.044, with PIR being 0.022.. smaller is better, so a build up of 4 inches wool between ceiling joists (3 to 4 inches is a typical joist size) and 4inches PIR over them, will meet regs.

Then re-lay your 5 inch boards or replace them with eg TnG OSB sheets

 

[rangy]

Thanks for that Robin - I have 50mm PIR Celotex boards in between my roof rafters but if I'm only taking up one floorboard lengthwise( 2 max), there's no flexibility in the board to put down so that's a non starter. The gap between lounge ceiling plasterboard and attic floorboards is approx. 8/9 inches so plenty to go at which is why I was considering wool/ mineral bats.
Also, I don't want to take up large chunks of the floorboards as cosmetically it looks really good and doesn't squeak.
It's basically what can I shove into the opening to either side of said opening between the joists and would it be worth it
TIA

 

[robinbanks]

Why not just put more insulation at loft ceiling level then you have a hobby room inside the insulated envelope?

 

[rangy]

That's exactly what I'm trying to achieve Robin, it's more a case of what to use and how much( cost vs performance)

 

[robinbanks]

If you already have 50mm in your loft ceiling, adding another 4 inches should meet regs. Depending on the extent of the area you're upgrading, a building control service involvement may be required. Your local council BC department should be able to advise you via phone or email as to whether an app is required