Internal wall insulation options

Strummer22

Hi,

I'm wondering if anyone can advise on the best method for internally insulating a room for which the walls are stonework outside, then an uneven cavity with no cavity wall insulation, then 5 inch thermalite blockwork. Internally the walls have cement render which is in pretty good condition.

We are considering either:
1. standard internal wall insulation (kingspan-type, not necessarily kingspan), either insulation panels then plasterboard, or combined insulation and plasterboard panels; or
2. wood fibre insulation boards covered with lime render and lime finish.

Option 1 seems easiest and almost certainly cheapest (in labour if not materials). I am just trying to work out whether there will be any tangible benefit to option 2. The walls are already not breathable on account of the cement render.

Could option 1 result in moisture becoming trapped in the insulation between the cement render and the plasterboard, bridging it and causing all sorts of issues? Or does it not need to be breathable at all on account of the existing cement render?

We have other rooms which will require internal insulation which we'll do after this one is finished; these have solid brick walls and the plaster is blown (or at least a bit dodgy) so we'd strip back to brick. Again, which of option 1 or 2 would suit these rooms?

Any input would be much appreciated.

FreeBear

On my house, I have cavity brick walls on the lower half, and solid brick on the upper - The upper half has a thick cement render on the outside, so breathability is out the window. I've been putting 65-75mm of Celotex on the inside and finishing off with plasterboard.

In a little more detail - Fixing 25x50mm battens vertically to the wall at ~600mm centres. Filling the space with 25mm thick Celotex and using expanding foam to fill any small gaps. All joints then covered with aluminium foil tape (after  trimming back any errant foam). A second layer of 25mm Celotex then put over the top with all joints again foamed & taped. Finally, horizontal 25x50mm battens fixed with screws in to the vertical battens on ~400mm centres. The space between, again filled with a third layer of Celotex, plus more foam & tape. 12mm plasterboard to finish. Used a strip of 12mm ply over the top of the window to give something solid to fix a curtain rail to. At the base of the wall, a batten was fixed at ~75mm above the floor for skirting to screw to. Radiators have all been moved to interior walls, and no sockets fitted on those exterior walls.

It is early days, but I'm not aware of any damp forming behind the insulation - I really ought to insert a probe or two to monitor... In terms of saving energy, it has made a big difference with the rooms done so far (although some of that is down to having new windows).

On a solid brick/stone wall without cement renders, and especially on the ground floor, I'd suggest woodfibre or cork with a lime plaster. This would allow the wall to breath, so there shouldn't be any long term problem with damp being trapped.

ThisIsWeird

Strummer22 said:

Hi,

I'm wondering if anyone can advise on the best method for internally insulating a room for which the walls are stonework outside, then an uneven cavity with no cavity wall insulation, then 5 inch thermalite blockwork. Internally the walls have cement render which is in pretty good condition.

We are considering either:
1. standard internal wall insulation (kingspan-type, not necessarily kingspan), either insulation panels then plasterboard, or combined insulation and plasterboard panels; or
2. wood fibre insulation boards covered with lime render and lime finish.

Option 1 seems easiest and almost certainly cheapest (in labour if not materials). I am just trying to work out whether there will be any tangible benefit to option 2. The walls are already not breathable on account of the cement render.

Could option 1 result in moisture becoming trapped in the insulation between the cement render and the plasterboard, bridging it and causing all sorts of issues? Or does it not need to be breathable at all on account of the existing cement render?

We have other rooms which will require internal insulation which we'll do after this one is finished; these have solid brick walls and the plaster is blown (or at least a bit dodgy) so we'd strip back to brick. Again, which of option 1 or 2 would suit these rooms?

Any input would be much appreciated.

Hi Strummer. (What do you strum?)
Do you have a suspended floor? If so, I presume it has vents to the outside? And the uneven cavity will be open at loft level?
If so, there should be little or no risk of any trapped moisture issues. If you wished to mitigate further, then you could always tanking-slurry the internal render first. 
Then the simplest fixing method is to adhere the Celotex-type boards straight on. And, yes, p'board-lined stuff is easier and quicker still.
I understand that some mechanical fixings are also required, but you'd be checking the correct fixings methods in any case.
I lined a single-skin garage wall to make it a habitable space, and used FreeBear's method - battens and stuff. I'd probably go for tanking&gluing in future as it's probably less work, but no big deal either way.
Your existing internal skim is definitely Thermalite? In which case you may not need to go as thick with the Celotex in order to achieve superb results.

stuart45

I guess from the description of the wall's construction it's a barn conversation.
Because the inner walls are often built straight off the slab, any moisture in the cavity can't run down below DPC, so it's worth bearing in mind when you cut off any heat to the cavity which helps keep it dry. 

Strummer22

Thanks for the advice so far.

ThisIsWeird said:

Strummer22 said:

Hi,

I'm wondering if anyone can advise on the best method for internally insulating a room for which the walls are stonework outside, then an uneven cavity with no cavity wall insulation, then 5 inch thermalite blockwork. Internally the walls have cement render which is in pretty good condition.

We are considering either:
1. standard internal wall insulation (kingspan-type, not necessarily kingspan), either insulation panels then plasterboard, or combined insulation and plasterboard panels; or
2. wood fibre insulation boards covered with lime render and lime finish.

Option 1 seems easiest and almost certainly cheapest (in labour if not materials). I am just trying to work out whether there will be any tangible benefit to option 2. The walls are already not breathable on account of the cement render.

Could option 1 result in moisture becoming trapped in the insulation between the cement render and the plasterboard, bridging it and causing all sorts of issues? Or does it not need to be breathable at all on account of the existing cement render?

We have other rooms which will require internal insulation which we'll do after this one is finished; these have solid brick walls and the plaster is blown (or at least a bit dodgy) so we'd strip back to brick. Again, which of option 1 or 2 would suit these rooms?

Any input would be much appreciated.

Hi Strummer. (What do you strum?)
Do you have a suspended floor? If so, I presume it has vents to the outside? And the uneven cavity will be open at loft level?
If so, there should be little or no risk of any trapped moisture issues. If you wished to mitigate further, then you could always tanking-slurry the internal render first. 
Then the simplest fixing method is to adhere the Celotex-type boards straight on. And, yes, p'board-lined stuff is easier and quicker still.
I understand that some mechanical fixings are also required, but you'd be checking the correct fixings methods in any case.
I lined a single-skin garage wall to make it a habitable space, and used FreeBear's method - battens and stuff. I'd probably go for tanking&gluing in future as it's probably less work, but no big deal either way.
Your existing internal skim is definitely Thermalite? In which case you may not need to go as thick with the Celotex in order to achieve superb results.

I strum a Cort acoustic, Fender telecaster, and Gibson SG (when time allows!)

Yes, suspended floor and the cavity is open at loft level. I think we would not need to tank the render. 
I mentioned that the cement render is in good condition because this should allow the boards to be stuck on with adhesive as you suggest. 

Our builder who just put a new window in the wall said it's definitely thermalite and he said the same thing, i.e. we already have some insulation. However the whole house is pretty drafty and we have a heat pump so radiators don't get very hot. We want to keep as much heat in as possible.

stuart45 said:

I guess from the description of the wall's construction it's a barn conversation.
Because the inner walls are often built straight off the slab, any moisture in the cavity can't run down below DPC, so it's worth bearing in mind when you cut off any heat to the cavity which helps keep it dry. 

No it's not a barn conversion but it is an extension to an old farmhouse (done way before we bought the place), and appears to have been constructed using similar methods to a barn that was built at the same time. So it might as well be a barn conversion. 

ic

There's a guy on youtube https://www.youtube.com/@CharlieDIYte that has a lot of videos about renovating his 100's year old cottage, including plenty on insulating as he tries different methods in different rooms.  He focusses a lot on the detail of damp and humidity control, thermal bridging, etc etc.  Well worth a look.

ThisIsWeird

Strummer22 said:

I strum a Cort acoustic, Fender telecaster, and Gibson SG (when time allows!) Nice

Yes, suspended floor and the cavity is open at loft level. I think we would not need to tank the render. 
I mentioned that the cement render is in good condition because this should allow the boards to be stuck on with adhesive as you suggest.
Our builder who just put a new window in the wall said it's definitely thermalite and he said the same thing, i.e. we already have some insulation. However the whole house is pretty drafty and we have a heat pump so radiators don't get very hot. We want to keep as much heat in as possible.

That all sounds good. And Stuart's concern about the concrete slab hopefully shouldn't apply either, as the founds will be way below floor level. And the underfloor void should also be ventilated to the outside - I don't know if the vents are sleeved as they pass through the two skins, but if so (and take expert advice) couldn't they be modified to open into the cavity as well? That should certainly keep them dry.

So, it looks as tho' the added insulation level is easily sorted. Work out how much is 'ideal', and how much you'd actually want to add, given the reduction in room size. 5" of Thermalite is a good start, so a few inches of Celo over this will give you superb insulation.

But, draughts are the heat killer. If you cannot get under your floor to insulate and draughtproof from there, then see what you can do from above. But absolutely make the floor airtight! Seal between each board, and especially around the floor perimeter - the cheapest of frame sealants will do this job. Then see if you can add anything above this - even an 8mm layer of fibreboard underlay will do a lot.

FreeBear

Strummer22 said: Our builder who just put a new window in the wall said it's definitely thermalite and he said the same thing, i.e. we already have some insulation. However the whole house is pretty drafty and we have a heat pump so radiators don't get very hot. We want to keep as much heat in as possible.

The main source of cold draughts here were down to poorly fitted windows (installed back in 1976). Second source was around the perimeter of the ground floor rooms where skirting meets floor. Sealant (either mastic or expanding foam) at floor level sorted them. Replacing the windows was an expensive undertaking, but worthwhile. Opted for 80mm uPVC frames and wrapped the wall insulation around the reveals (just a 25mm layer). This helps to reduce any cold spots forming, and also gives a secondary barrier to any draughts coming in around the frame.

If you do go for insulated plasterboard, be aware that there are two types. EPS/XPS (polystyrene) and PIR/PUR (polyurethane). Polyurethane has a slightly higher thermal resistance than polystyrene so you need fractionally less for the same u-value. Mechanical fixings are needed at ~600mm centres, and the screws could cause cold bridging. Ensuring continuity of insulation at the board joints is also not easy, particularly with the thicker sheets.

Using the warm batten method as described above avoids issues with cold bridging and gaps between the sheets. Multiple layers of foil faced Celotex with offset joints suitably taped also provides barriers to moisture, so you don't need a vapour control membrane (VCL).

Insulated plasterboard would be quicker to install, but is slightly more expensive. The warm batten method, more labour intensive, but you get a VCL for free and minimal cold bridging. You also get to choose where to use ply instead of plasterboard to screw curtain rails to.