How to improve 1970s / 80s filled cavity wall insulation

Bathsheba309

We're in the process of upgrading our 1970s bungalow. The roof is very well insulated, and we installed an ASHP earlier this year. We're installing low profile UFH room by room, and our plan has been to dry line and insulate the exterior facing walls of each room as we do it, along with installing four wall mounted HRV units strategically placed through the house to sort out condensation / mould issues - at the moment our house is pretty well sealed without effective ventilation. We already have filled cavity walls but our EPC is only a D. To put in an extra layer of insulation is expensive (though not so bad when we're redoing a room anyway) I'm trying to work out whether it's worth it. All the information I can find is on further insulating solid walls, nothing on adding insulation to filled cavity walls. Any thoughts / advice appreciated.

FreeBear

Bathsheba309 said: and our plan has been to dry line and insulate the exterior facing walls of each room as we do it,

To put in an extra layer of insulation is expensive (though not so bad when we're redoing a room anyway) I'm trying to work out whether it's worth it.

If you are paying someone to do the work, the extra cost of insulation is going to be minimal by comparison. Doing the bulk of the work yourself, I'd suggest getting a stack of 25mm Celotex/Kingspan type boards and some 25x50mm battens. Stick one layer of Celotex to the wall, fill any gaps with expanding foam (use a foam gun), and tape the joints with aluminium foil tape. Using hammer frame fixings, fit battens to the wall at say 600mm spacing (countersink the battens). Fill the space between the battens with another layer of Celotex, again filling & taping the joints. Finally, fix plasterboard, screwing to the battens - Some grab adhesive on the back of the PB will stop any "bounce" if you want a really solid surface, but I've not found it essential with 12.5mm thick boards. Tip - If you plan on fitting curtain rails above the windows, use a length of plywood ~150mm wide instead of PB. Extend ~150mm either side, and you'll have a solid surface to screw in to.

Insulated plasterboard is a little more expensive, but quicker to put up. It still needs mechanical fixings to secure it to the wall (600mm centres with mushroom heads on the hammer fixings).

Depending on the size of your window frame profile, it is worth wrapping a layer of 25mm Celotex around the window/door reveals. I have 80mm uPVC frames here, and with 25mm insulation and 12mm ply, just 20-30mm of frame is visible. Not so easy if you have trickle vents in the top of the frame though...

Bottom line - The extra cost is not huge. It will improve the EPC rating, save money on heating, and should add value to the property.

Bathsheba309

FreeBear said:

 Stick one layer of Celotex to the wall, fill any gaps with expanding foam (use a foam gun), and tape the joints with aluminium foil tape. Using hammer frame fixings, fit battens to the wall at say 600mm spacing (countersink the battens). Fill the space between the battens with another layer of Celotex, again filling & taping the joints. Finally, fix plasterboard, screwing to the battens - Some grab adhesive on the back of the PB will stop any "bounce" if you want a really solid surface, but I've not found it essential with 12.5mm thick boards. Tip - If you plan on fitting curtain rails above the windows, use a length of plywood ~150mm wide instead of PB. Extend ~150mm either side, and you'll have a solid surface to screw in to.

Thanks very much for the advice, we are doing it ourselves. Can I check, do you mean cover entire wall with insulation board and then nail the battens to the wall through the first layer of insulation board, so that the wood is on top of the insulation board rather than the wall, and then put another layer of insulation board between the battens? Thanks for the tip about curtain rails, we definitely would not have thought about it until too late!

FreeBear

Bathsheba309 said: do you mean cover entire wall with insulation board and then nail the battens to the wall through the first layer of insulation board, so that the wood is on top of the insulation board rather than the wall, and then put another layer of insulation board between the battens?

Yup, exactly what I meant.

Do a search for "warm batten method", and you should find plenty of images.

Edit - Image of just one wall I've been insulating.

Bathsheba309

Thank you so much! We will do that - it sounds like it's much better than the standard dry lining that we were intending on.