To insulate or not (small spot)

[nero44]

Hello,
I live in a typical victorian terrace, with a recessed entry door. I am currently redoing the 1st floor master bedroom, which includes putting new flooring in. Whilst I had the floors up, I've also installed a front door light. Whilst doing so I noticed that the floor (ceiling of the recessed entry) is a simple thin lath and plaster (see first pic).

Now, I've already decided not to insulate the first floor bedroom, to allow for better airflow and to prevent moisture/condensation etc, and also it's internal, so I am happy to have heat being lost and travelling up into the house. My question now is, would it makes sense to stick some mineral wool just on the area above the front door, as that area is exposed to the outdoor and is just the lath and plaster. n.b. the flooring will be glued herrinbone, so there won't be enough space to insulate there. I've indicated the corner in green on the floorpan.

 

[paulc]

As this section is open to the outside, it would be foolish not to insulate. So pack the void with fibreglass or mineral wool to the full depth of the joists. Perhaps add a vapour control membrane over the top to minimise interstitial condensation.

 

[fernicarry]

Our shower room flies over the front entrance vestibule and that's what I'm doing. Its still a WIP so there are gaps and no floor down yet, so a bit disturbing to look down and see outside... (And no ceiling either so you can look up from the front door and see the sky through the roof light...).

Definitely also add a membrane over the top of the joists (my choice would be a breathable one despite it being on the warm side) before you put your flooring down to make it more airtight. If you have taken off the skirting boards then lap the membrane up onto the wall before reattaching them. Try to extend the insulation beyond the green outline by at least its own thickness so that heat loss can't take a "short cut" around the edge of the insulation.

There's an argument to insulating the whole of the bedroom floor since you don't need as much heat in a bedroom as you do in a reception room. You also get sound reduction as a useful side effect.

 

[nero44]

Thanks both that’s very helpful.

Does it have to be a rockwool slab meant for suspended floor, or will a softer loftroll do as well? Like a knauf 150mm loft roll. There are some radiator pipes running there as well.

 

[Cherryburn]

Loft roll is typically not quite as insulating as slabs, but obviously it's better than nothing if you already have some. Roll is around 0.044 W/mK, slabs can be as low as 0.032 W/mK.

 

[nero44]

Thanks. I don't have any spare, hence the question. I don't have any car and most building merchants charge a fortune for any deliveries. Let me take a look, maybe I can find some slabs on eBay or so.

 

[noj]

I think B&Q only charge £6 for delivery. Free if you spend over £75? Worth a look...

 

[a twig]

Someone like zipcar (if available near you) are often worth using for these sorts of situations

 

[paulc]

I think B&Q only charge £6 for delivery. Free if you spend over £75? Worth a look...

Insulation falls under "bulk goods", so B&Q charge £30 for delivery unless you spend more than £100.
Wickes will deliver insulation for £7 (free over £75), so it pays to shop around.

Always worth checking to see how much insulation you currently have up in the loft. If you have less than 250mm, doesn't hurt to top it up - I've got nearly 400mm in places up in my loft.