Internal Wall Insulation installation method

[Claudatious]

Hi there,

I live in a terrace house with a solid front and rear elevation made almost entirely of Bungaroosh (lime, flint etc). Im trying to get my head around the many different approaches to insulating internally. The Bristolian guide to solid wall insulation (https://sdfoundation.org.uk/wp-content/uploads/2018/01/2015_bristolsolidwallinsulationguidance.pdf) suggests not exceeding 60mm in depth of insulation so to allow some amount of warmth to the solid wall to allow for moisture dispersion. Additionally, many of the suggested methods require a lime render finish on the internal final finish, which to be honest, is a bit specialist and hard to get my head round.

Ive come up with my own idea! which I am asking your good selves if you would consider and scrutinise. My idea is to use either flexible hemp or flexible wood fibre insulation laid against the internal wall horizontally held against the wall with a 3x2 stud wall. The insulation side of the stud timbers could be wrapped in foil tape to prevent contact with any amount of moisture content within the insulation. Vertically in-between the stud timbers I would leave empty and use this void for electrical points. Then final finish would be plasterboard and gypsum plaster. To allow the wall to breathe I would insert round vents (approx 30mm diameter) in-between every vertical stud, both close to floor level and ceiling level.

Can anyone think of why this idea is floored? It seems sound to me!

 

[fernicarry]

You really want the warm side of the wall to be airtight but breathable. Allowing air from the warm side of the room into the wall structure through your little vents is a bad idea as warm air holds more moisture and will cool down and condense on the very surfaces you are trying to keep dry.

Leaving a service void isn't necessarily a problem, its how new builds are done, but new builds have a vapour control layer between the service void and the insulated wall structure to separate the warm/wet habitable space from the cooler, drier wall. They work on the principle of not allowing water into the fabric under any circumstances. Now I think vapour barriers are a bad idea for traditional properties because they aren't that water tight on the outside so water always gets driven in and needs a way out, even if thats through the inner face. Instead I'd fix a breather membrane between the inner face of the insulation and the studwork. That would obviate your foil tape too.

Better would be to eliminate the void space altogether by packing the insulation around the stud work, you'll lose less floor area too. Electrical services could be run in conduit through the insulation but may have to adjust cable sizes to suit this installation method. Attach the back boxes to the studwork and pack the insulation around them and the conduits. The hemp stuff is pretty good to work with in this respect as it conforms to most shapes whilst staying quite self-supporting, I really like it as a material.

In addition the insulation should be in good contact with the external masonry, again to avoid void spaces. If its very irregular (mine is stone rubble so it is), then add a parge coat of lime plaster (some brands have a bit of thermal insulation value too) to get it mostly level, doesn't need to be perfect, the hemp will take up the imperfections. Then build up your studwork and as above pack the insulation into it. This is exactly what I am doing in my front box room.

If the wall tends to be pretty dry anyway I'd take my luck with a plasterboard finish over it all. Just not the silly foil or EPS backed stuff. It'll be somewhat breathable, maybe sufficiently. I might chance my hand doing this on my own room because my plastering skills are too rubbish to take on an area any larger than a 10p piece and I'd like it done sometime this decade. Ask me in 20 years time whether it worked out...

 

[Claudatious]

Fernicarry, thanks for a superb reply. I wasn’t sure anyone would reply but you smashed it! So appreciated.

a question!

You say better to pack the insulation around the stud. Do you actually mean wrap the studs or just place insulation within the void of the stud, or both? The former seems odd as the plaster board would be screw up against the stud with insulation squeezed between plaster and stud. Also, the studs would also no longer have neither a VCL or a breather membrane? Or have I misunderstood?

Lastly, the walls are flat enough but have been painted over the year with non breathable paints. What do you think, knock of the 140 year old internal render and render afresh with 1 part lime 3 parts sharp?

Thanks for your time once again.

 

[fernicarry]

I mean behind and between the studs, so the insulation is touching on 3 sides, front face left clear where the boards are fixed.

The guide you linked to is pretty good btw, everything you need to know is in there somewhere and I'm glad to see my own home-spun ideas are consistent with theirs.

The breather membrane was more to act as an airtight barrier between your service void and the insulation. If you're not going to have a void I'd say its not strictly necessary but as per the guide its useful to put a membrane over the floor joists (if you can easily get the boards up) and continue that up into the wall in one continuous piece so that you seal up the junction between floor and wall (sec 6.3.2.1). Preferably the IWI should continue below floor level rather than just sitting above the boards.

Lastly, the walls are flat enough but have been painted over the year with non breathable paints. What do you think, knock of the 140 year old internal render and render afresh with 1 part lime 3 parts sharp?

Ideally you want to remove that but you will be setting yourself up for a lot of extra work. I'm using Breathaplasta Thermal to parge my wall but I have a lot of gaps to fill and bumps to even out so its necessary. Its nice stuff to work with, just add water and knock it up in a rhino tub with a cordless and a paddle. I was making steady progress until the cold weather stopped play, maybe get going again later this week.

 

[Claudatious]

yes okay, I’m getting the picture. I guess the only concern I now have, is using upwards 150mm of insulation and wether this allows enough warmth to keep the walls dry.

Knocking of the 140 year old render which has been painted and is also blown in places suites me fine tbh. It never felt right to leave it there.

On the front and rear elevations, the stud walls will be secured in place by the floor joists above and the floor joists below. So no need to secure the timbers against the wall.

When I come to do the side walls (Terrace house), I’ll use 2x2 or 3x2 at the most. In this case it will be less a stud and more a frame to hold the plasterboard. If I use screws that don’t rust along with resin and glue and whatever else is necessary, this would be okay right?

 

[paulc]

I've insulated a couple of the walls here (still have a few more rooms to do). The first wall I did, fixed 2x1 battens to the brick walls with a strip of DPC material between the batten & wall. The insulation then filled the space between battens, A second layer went over the whole lot, and held in place with yet more 2x1 battens screwed to the first ones creating a sandwich. A final layer of insulation fixed between the battens, and then topped off with plasterboard.
The next wall, I didn't have the space to put three layers on, so just covered the wall with insulation boards and fixed 2x1 battens over the top. Frame (hammer) fixings held the battens in place, and again, space filled with insulation and topped off with plasterboard. Did not use stainless steel screws for any of the fixings - If they rust away, I'll be long gone and it will be someone else's problem 50-100 years down the line.

 

[Claudatious]

What was the thickness of each layer?

 

[johnbanshee]

What do you put on top of the studded insulation layer?

 

[paulc]

What was the thickness of each layer?

25mm of Celotex. But the solid brick walls I did are on the first floor and have a cement render on the outside dating from the time the house was built.

What do you put on top of the studded insulation layer?

Regular British Gypsum plasterboard. One ceiling that I wanted a lime finish on, I brushed on some DeBond Blue Grit followed by a 3-5mm skim of lime plaster. It seems to be holding up quite well, and despite what some say, gypsum is breathable. Maybe not as much as lime, and it will turn to mush if it remains damp.

 

[fernicarry]

using upwards 150mm of insulation and wether this allows enough warmth to keep the walls dry

How do you get to 150?

the stud walls will be secured in place by the floor joists above and the floor joists below. So no need to secure the timbers against the wall.

3x2 studs will be a bit bouncy at that length. You'll want noggins and maybe some fixings back into the wall although that introduces cold bridging. Personally I don't think our buildings will ever be at the level of performance where that starts to matter but others might be more knowledgeable on that.

 

[Claudatious]

Two answer all points. I was going to lay 75mm on its side essentially stacked between the stud frame and the wall, then fill the 75mm void making it 150mm. I would use noggins. I don’t think I need to fix to wall though I agree with taking a realistic approach and not expecting ultra low u-values.

My house will never be ‘passive’, but it will be an enormous improvement on its current state.

 

[paulc]

When you come to put the finishing layer on, fix a strip of 12mm ply above any window & door openings. Extend either side of the opening by 6-8". This will give you a nice solid surface to attach curtain rails to. If plastering in lime, slap a bit of Blue Grit or DG27 on the ply, and it will give the plaster something to grip.

 

[Claudatious]

Good shout

 

[johnbanshee]

When you come to put the finishing layer on, fix a strip of 12mm ply above any window & door openings. Extend either side of the opening by 6-8". This will give you a nice solid surface to attach curtain rails to. If plastering in lime, slap a bit of Blue Grit or DG27 on the ply, and it will give the plaster something to grip.

Good idea, I will need to do this on mine. Definitely a good idea to preempt by putting sturdy wood in place for the future.
Has anyone ever tried fixing curtain pole holders to those hollow metal lintels modern houses have - they are just an absolute nightmare., B&Q give you little 15mm screws - how you are actually supposed to use them I have no idea.

 

[fernicarry]

I was going to lay 75mm on its side essentially stacked between the stud frame and the wall, then fill the 75mm void making it 150mm

See this video about the 24 minute mark -- or slide 13 on the first presentation.

Rethinking IWI with Natural Fibre Insulation - The Alliance for Sustainable Building Products

ASBP and our Natural Fibre Insulation Group took a deep dive session into thermal upgrades to solid walls using natural fibre insulation internally. Our experts explain the impact of insulation depth on energy savings, thermal comfort and humidity balance within the fabric. The event aims to...

asbp.org.uk

You'll get 90% of the energy savings with 75mm of insulation that you would with 150. So the extra 75mm is very marginal in terms of reducing running costs but doubles the installation cost and the loss of floor space.

 

[Claudatious]

Thanks for video link. A bit of a dry presentation (exude the pun).

Yes that certainly is the dilemma. I had someone do a mock up on a software called Wufi.

He recommends placing the insulation between the studs and between the stud and the solid wall. So the stud would be against the wall which would then cause some amount of cold bridging. He also insists on a VCL between stud and plasterboard.

So complicated