Dormer room insulation

[PeterR]

Hi, I’ve got a chalet style house built in 1960s. The upstairs bedrooms are really cold in the winter and I’ve opened up one of the walls to see what’s behind it and found out that there is no insulation whatsoever. Behind the plasterboard there is a gap and the the rafters/studs are only 5-7cm deep. There is some old darling felt and then plastic cladding. The question is how to insulate the walls and the sloping roof from the inside? Because the gap is quite small I can’t fit the 100mm insulation boards between the studs/joists and also don’t want to loose over 10cm of floor space in the rooms as they are quite small. Do I need to leave an air gap behind the insulation in the walls? Also, can I stick insulation board on top of the existing plasterboard and will I need a VCL between any of the layers? Thank you

[Dave Jones]

you have a cold roof, this means it MUST have airflow under the tiles to remove moisture. You can insulate upto 50mm of the bottom of the underlay below the tiles as long as the eaves and ridge are open to airflow.

[PeterR]

Hi Dave, thank you for your reply.

Do I need airgap in the wall as well, where the hole is on the picture and also around the window? Or can I fully fill the gap with digit foam boards and stick plasterboard on top? 
Thank you 

[tonyshouse]

No ventilation gap needed on walls, ?

 

for small bits of roof there are exemptions for ventilation gap. I have seen no gaps and no problems lots of times

 

add as much as you can between rafters, fill walls then 80mm sheet insulation under rafters and plasterboard , as much as will fit into the window without covering the glass bead 

[PeterR]

Thank you Tony for your help ?

[Faz]

How far are you intending to go on this?  Ideally, all of the plasterboard would come off and PIR sheets cut to fit between joists.  Then new insulated PB would go back on top.  A proper rip out job - of course, you could do it a room at a time rather than gutting the place.

 

Putting PIR on top of the existing PB isn't going to be the answer - you would have to board over it anyway and you would only get a very thin sheet in there anyway - clearly you would replace the PB with insulated PB rather than doing this. In which case you get a chance to stick PIR in the trusses!

 

 

[PeterR]

That’s what I want to do.

Rip out old plasterboard, stick PIR between studs,seal it with expanding foam and then put new insulated plasterboard on top of that.

I just wasn’t sure if I can fully fill the gap between plasterboard and sarking felt or I have to leave some gap for the air flow.

 

[Faz]

Best leave 25mm if you can - you don't know what the performance of the felt is - probably not a breather membrane in a 1960's build mate.

 

Edit - when I say 'probably' I meant certainly!

Edited December 11, 2021 by Faz

[PeterR]

That’s the problem, the thickness of the joists is only around 50 mm so I can only fit 25 mm PIR board, which is far from the recommended 100mm. Is there any solution for this?

[joe90]

Remember any  insulation is much better than none, the first inch gives the biggest benefit and further inches give diminishing results. Yes get 25mm foamed between 50mm rafters leaving a 25mm air gap fully fill vertical studs then add as much insulation over the joists as you can manage without compromising room space then plasterboard/scim this. 

[Faz]

Optim R is probably the best performance you can get in the minimum thickness - vacuum sealed though so it has to come in the right widths as you can't cut it. 

 

You need to assess what you are looking to achieve - you are in a 1960's build with virtually no insulation throughout presumably - nothing in the cavities or floor I imagine.  You can achieve 50mm PIR in the attic rooms between the sheets and the insulated PB - that will be the same as 100 - 150mm quilt. That will make a significant difference.  Or you can knock the place down and start again.

[PeterR]

Thank you very much guys, it’s a big big help ?