Internal wall insulation options

Our house which was built around 1910 has solid brick walls. (Basically two layers of brick, but no cavity).

Internally the walls don’t seem to have plaster on them I think its been described as sand/cement render.

About 12 years we asked a builder to insulate a couple of our bedrooms. He simply attached studwork to the internal face of the external walls, filled in the space with 100mm of loft insulation (rockwool) and then placed plaster board over the top which was then skimmed and painted.
This has worked well, and those rooms are now much more comfortable.

We would like to do the same with some more rooms now and I’m after advice on whether this is a good way to add Internal Insulation. It seems like a reasonably cheap and effective solution but I’m open to other options.

Any advice or suggestions on better approaches are welcome.

Also I’m pretty sure that there was no vapour barrier of any sort fitted, but as I say we haven’t had any issues in 12 years so not sure if one is a good idea or not. If we go down this route should we have install a vapour barrier ?


Yes, this is a good way of adding insulation but with wall batts not loft insulation, which will gradually sag. Also more than 100mm if you can manage it. Do make sure to insulate between floors and ceilings and where possible between
rooms. These precautions will reduce thermal bridging.

I suggest assembling an “inner wall” of batts clamped in place by battening and then another layer of batts between the battens. Again, this is to reduce thermal bridging. Use batts half the planned depth of insulation and stagger the joints/overlaps.

I like the idea of two staggered layers, but how do i fix the battening securely enough to fix my plaster board to, without over compressing the first layer of insulation ?

I was originally assuming that I would screw the studs directly to the wall and fill in the spaces with insulation.

Now I’m thinking maybe I effectively need to build a stud wall 50mm away from the existing wall with insulation batts behind it and then fill in the gaps between the studs with more insulation batts. And fix the stud wall to the ceiling, floor and walls at either end.

That should give a solid structure to fix plaster to. My description was of the barest essentials, perhaps not even for a habitable room.

Be sure to hold the inner insulation firmly to the wall so that there is no air movement behind or around the insulation.

If you have any timber in the floors or walls, does someone need to do a calculation here of the likely temperature of the inside face of the masonry (much colder than it previously was) and of the likelihood of interior moisture passing through the plasterboard and insulation? On hitting the cold interior surface of the masonry might there not be a risk of that moisture condensing? I am not an expert, but interested to hear the answer anyway, if I am wrong. Maybe a bit of interstitial condensation would do no harm in your situation, particularly if you have no timber joists to rot, or wouldn’t happen.

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Here you need to remember the mantra “Build tight, ventilate right”. In fact water ingress is more likely but interstitial condensation does need to be allowed for. There are calculations to do, the most serious of which is known as WUFI. I confess to not knowing the details. As the walls will be cooler and there will be less internal evaporation any absorbed water will take longer to dry out. For this reason a breathable rain inhibiting layer is a good idea. Unless your home is made of a pure limestone with negligible silica content I suggest Stormdry.

Came to highlight the floor joist rotting, it’s a big deal. There are ways in which you can insert metal plates underneath the joist to provide a capillary break and a targeted thermal bridge to warm up the end grain if you’re worried. Maybe even treat it chemically to help fend off the rot.

It would be worth also considering having interior wall insulation on the front facade, to keep the aesthetics of exposed masonry, but go for exterior wall insulation of other elevations. Not sure what the planners would think of that. But it’s the only way I can think we can get our old building stock up to the performance we need tbh. You’d then need to think about some compromised detailing on the corners of the building where the thermal control layers are discontinuous.

There’s a moisture management hierarchy. Prioritize them in roughly this order before worrying about thermal resistance. A phased approach over a long period may be the only way to do this, depending on your budget. Get a very good bricky. High performance retrofits of interior insulation on old masonry mass walls is the hardest thing to do to. It goes a little like this:

  1. Prevent the water getting on your masonry in the first place, through a combination of - larger overhangs (hard to retrofit), better pointing including replacing damaged bricks (they are not created equally, modern bricks are way better, testing may be necessary), brick treatments to repel more water (may need to be re applied more regularly to the parts of the building that see more weather). Setup up a maintenance schedule with said bricky. All this generally increases the durability of the building anyway.

  2. Worry about moisture transport through air leakage before vapour diffusion. Vapour diffusion is orders of magnitude smaller compared to moisture transfer due to air movement. Liquid applied air barriers are okay, but parge coats are better because they flatten the wall for the insulation. This includes making sure your insulation cavity doesn’t have a lot of convective loops going on - transporting moisture on the air from the inside to the condensing surface of the masonry is probably your main concern. I doubt there’ll be much vapour drive inwards if you do this well. Your installer will probably not respect tight-fitting insulation enough, as it’s usually thrown in with reckless abandon.

  3. Think about a vapour throttle/retarder, or smart membrane, before considering a vapour barrier, which is totally closed. I suspect the best place to put a smart vapour throttle would on the internal side of the insulation. These smart membranes make life easier, because they allow drying inwards, put prevent vapour pressure outwards. Maybe the manufacture of the product with have guidelines for how best to use their system in your conditions. At least you’ll know what to ask.

  4. Expect failure and design using moisture resistant materials. Insulation that is both vapour open and moisture resistant. I suspect mineral insulation would be best here (no foil, anywhere). Avoid using untreated timber batons in direct contact with the masonry. I think it’s possible to use rigid insulation fixed to the wall with the batons of the service cavity. I like using 25mm plaster board for sound proofing and thermal mass - as thermal mass needs to be in contact with the conditioned air to be useful to regular diurnal temperatures.

As you can see. This is not easy. I would not take my word for it. The issue with DIY retrofits is that you have to become a bit of an expert. This is why community retrofits, followed by long periods of monitoring, are needed to gain trust and confidence. If you want to be a hero in your community, put sensors all over your enclosure, in the most sensitive areas, and monitor how it goes, to compare with simulation. Some other sod can push that to failure with more thermal resistance another day.

Or use expert design/consultation and get warranties, which costs a lot.

In conclusion, if it were my home, I’d go with exterior wall insulation first, if possible, on as many facades as possible. Then treatment to the masonry to avoid water absorption as much a possible. Followed by an interior parge coat/liquid applied air barrier that acts as a vapour throttle (0.1 to 1 perm) edit, actually, not sure about this, continuous mineral rigid insulation system, smart membrane allowing drying inwards but resisting vapour diffusion outwards, followed by a 35mm insulated service cavity and your plasterboard to maintain integrity of at least one cavity after services are installed. I would say 100mm of insulation in total is about as much as I’d be comfortable risking, even 70mm would be fine. More insulation = more risk of failure.

To reiterate the concern about your floor joist, the vertices and edges joining walls to floor to roof systems etc is where all the detail is most important.

You may also have to consider getting better heat exchangers in the building to use more energy more cost effectively (lower temps = cheaper heat). A ventilation system with a whole house dehumidifier can keep relative humidity down during sensitive parts of the season, no need to go above 35% imo. Thermal energy storage and heat networks are probably our only hope to do this to everyone’s place fast enough, but that’s an entire different discussion.

All a bid daunting, but that’s what it takes.

Good luck!

Might jump into sketchup and post the assembly later. The product research would be interesting, I’m a little out of touch with what’s available these days.

I’m generally in agreement, particularly with sequencing.

I disagree with target humidity. The target range for relative humidity should be 40-60%, according to the World Health Organisation and Passivhaus standards.

In a perfect world, yes, but in retrofits you can’t always have that. 35% may only be necessary for part of the heating season, just to help the masonry wall stay dry enough. The research I’ve read is that people can’t tell the difference between 35% and 60% in a blind test. You might get slightly drier skin over time, but just moisturize and use lip balm.

I agree that the human body is a very poor device for measuring humidity. Likewise for temperature. I didn’t say what felt best but what distinguished researchers have shown to be best.

Having said that, in winter I regularly let my dehumidifier reduce the bathroom humidity to 35% or less when I’m using the room to dry the washing.

It is extremely sensitive to temperature, or more accurately - temperature gradients, and air pressure (drafts). Not so for RH, people really can’t tell. Sound proofing and different colour temperature lighting will have more of an impact to comfort than a bit more RH.

If you have exterior wall insulation below 200mm, then no problem in our climate zone because the wall is no longer a condensing surface.

Again, in retrofits, with interior wall insulation, you have to have a very good reason to try to maintain high RH - such a serious asthma etc.

My understanding is that it isn’t so much (or not only) a matter of personal comfort, but also of human health. The optimum range for minimising pathogenic microbial growth is the 40-60% RH range of the WHO recommendation. The 40% lower figure is specified largely for that reason, I believe, as certain pathogens are increasingly more likely to be harmful as the humidity drops below around 40% (and others above 60%). But there are no sudden effects - figures somewhat outside this range for periods of time are still relatively unlikely to cause ill effects, iiuc.

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Airborne viruses are more easily transmitted at RH <40% (very topical at the moment) whilst airborne bacteria and moulds/fungi are more prevalent with RH >60%.

That is what the human body gauges as temperature, which is why, for example, a comfortable temperature in winter feels cold in summer.

It’s hard not to fall down the rabbit hole here. I’ll just say that last time I looked into this, the benefits of RH to public health are marginal, and we have to be careful not to fall for marketing that leads to building failures.

In a hospital, even marginal improvements make sense when the occupants are likely to be sick, but in residential, I’m sceptical.

If we were to ventilate to keep all rooms below 450ppm CO2, and kept 60%RH. We’d have both building failures, and high energy demand, despite high thermal resistance. I personally care much more about CO2 concentrations than RH.

In retrofits, we cannot expect to be perfect all the time. If you did want a durable enclosure, and keep perfect conditions, you simply put less insulation in to allow for more drying, or back ventilate the masonry wall and thermally break the floor system, essentially building a new enclosure within the building. There’s no free thermodynamic lunch.

I mean within rooms, from your head to your toes, or between floors, or between rooms - from sunny to shaded side of the house. You can detect those gradients very easily when you move across them. RH you just simply cannot tell in blind tests.