Creating a single room thermal envelope

We currently have one very problematic bedroom. It’s SE facing and has two cold solid brick walls and a very leaky UPV window.
This room is either uncomfortably hot, freezing cold or damp.
Looking at the problems, we have come to realise there are many complexities towards making this room more temperate.
Apart from the usual household blockers of disruption and cash flow, how to remedy the problems in this small bedroom is proving a conundrum.

The draughts from the window is straightforward to remedy with a new well fitted double glazed unit.

The penetrating damp from the failed pointing is also a straightforward remedy too - we repoint and have found the source of the water ingress.

The condensation and reoccurrence of mould is slightly more complex as we realise the two cold external walls and the gypsum plaster wall coverings in this small space are a big part of the damp problem.
We have recurring black spot mould on these cold and damp to the touch walls. To manage it, we treat the black spot mould and keep furniture away from these walls to enable the airflow, but it’s not enough as the plasterboard offers poor u values, isn’t breathable and as is notable with gypsum plasterboard - it doesn’t release the moisture it absorbs. Hence we are thinking to replace the gypsum plaster board with lime plaster on the external facing walls to manage the moisture .

However, the problem of creating a more temperate and comfortable room feels much more complex. Increasing the u-values in the walls means we need to insulate and that is a bigger expense.

Yes in winter we can heat and reduce heat loss , but reducing the temperature in summer feels like our biggest problem and we’re thinking about internal insulation or dare I say it - a small air conditioning unit.

I think our biggest question is around how we safely create a thermal envelope in this room with internal insulation and not cause more of a moisture problem in this room.

It was pointed out to us, how the solid external walls will remain a channel for stack effect from downstairs up into this little bedroom, therefore achieving a fully insulated internal envelope in this room feels like it could easily fail, so do we forgo the attempt at internal insulation and source a low energy air con unit for the extreme heat instead?

One thing I’m most mindful of is effectively managing the moisture and maintaining the ‘breathable’ fabric of the Victorian building.
External wall insulation isn’t an option here for us.

On the flip side of things - a local builder with a similar gable end property said he’d just installed the polystyrene backed / insulated plasterboard without any kind of membrane and his house is fine … leading me to ask myself if I’m complicating this bedroom issue ?
The same builder also rubbished expanding insulation tapes in favour of builders foam for insulating around windows :grimacing:

It’s hard to make good choices when so many contractors are against it! I also had that feeling while speaking to this builder that I should just do what women do - buy what you like and let the men do the work and the thinking !! :face_with_raised_eyebrow:

Any feedback from within the group would be gratefully received - it’s the only place I ever get a considerate answer which will resonate with my understanding of what I’ve learned here.

Thankyou in advance

Hi Carla (sorry for getting your name confused), can you clarify a couple of points:

  • what is the orientation of this rooms (SW or NE etc)?
  • what room is located below this room?
  • what ventilation strategy are you using for the dwelling?
  • it is worth adding improved ventilation as part of the package of measures needed to deal with the damp / condensation in this room.
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@pottyone72, Carla, The cold and damp in this room will improve somewhat as the effects of repointing take effect, however being a solid wall there is still the problem of the high U value and resulting condensation.

Is this room needed for normal use? If not I would be tempted to strip off the plasterboard and anything you find behind it. Then leave the room to dry out as much as it will before replastering with lime plaster or maybe hemp/cork plaster (also lime based) for slight insulation. If the wall is in a bad way it may be worth parging before plastering or changing the window. While the plaster is off would be an ideal time to change the window and make airtight to the brickwork. I always ask window fitters not to use trim and say I will make good. It is the only way (apart from watching all the work like a hawk) to ensure a proper airtight finish. Even if you decide against IWI I suggest insulating the window reveals.

If you need to isolate this room from the room below to stop the stack effect you could put an airtight membrane under the floor, possibly with a little insulation. It probably isn’t worth the cost of a breather membrane unless you have plenty left over from your previous work.

What are your long term plans regarding ventilation of the house? I seem to remember you contemplating a balanced Aereco system. Whatever your current plans you need to take them into account when planning ventilation of this room. For now it might even be sufficient to leave the window ajar and the door closed. I don’t recommend trickle vents. They are a curse to airtightness planning.

Is the window contributing to the summer overheating problem? If so a bris soleil may be useful. I have seen Victorian buildings with original or copy sun blinds over the windows, so they aren’t necessarily out of character for the age of properly. You need to go wider than the window itself to allow for changing angles to the sun as the earth rotates.

Why do you trust the men to do the thinking? Who knows the intricacies of your home best? Unless you are very lucky you probably know a lot more about retrofit, and certainly in relation to your own home, than your tradesman.

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Hi Dom :grinning:
The front face of the house where we have solar panels, is where the bedroom is situated. On our solar survey it states that it is minus 45 degrees from the South.

The house is a Victorian solid wall semi.
The bedroom is in the gable end .

Under the bedroom is our hallway which is also situated to the cold wall side of the house - today the hot furnace wall !!

Re ventilation in the room , currently the leaky windows provide constant airflow , but not an air brick in sight from the inside or outside.

Our options considered are windows with trickle vent or an air brick - but what about heat loss ?
We wondered about a single room mechanical ventilation unit too with heat exchanger.

Our main concerns are creating a comfortable healthy sleep space for our eldest girl still at home .
We’re at the ventilation versus heat loss debate and how to achieve a room which is for the most part temperate. The room being like a furnace is currently as much a concern as the cold , damp and draughts which is why I’m thinking along the route of breathable wall coverings and @Tim_Gilbert - some underfloor insulation for this room too.

Thankyou for boosting my confidence about me knowing my home better . It’s very true Tim.
I have to say it’s not about men - it’s just that a lot of tradesmen I speak with seem to have an issue with me (a she) trying to have a conversation about my home . I actually do see that it’s these tradesmen who ‘re not ready to think out of the box they’ve been shut into and having me try to communicate otherwise adds further insult to them!!
Needless to say, I can’t work with a contractor or a tradesmen who won’t communicate with me.

We’re temporarily moving our daughters bedroom to our lounge.,
Stripping the room back to brick on the external walls is the plan , as well as replacing the window.
We’re going with Eurocell for the window and have measured the window from the brick opening outside. We’re happy with the window spec and the price, but Eurocell give the impression that legally they have to add a trickle vent .
We’ve not ordered this window yet!
We also want it fitted with airtightness tapes (Contega have been recommended).
In terms of drying the walls out - we can do this as we intend to strip back to the brick and have a good dehumidifier.

About parging - that’s new to me and I had a little read. It seems common to do this for strengthening foundations and improving the integrity of a damaged wall , but what about keeping the walls breathable - can parging be done in different types of render or is it something done in cement - I think I’m questioning this cause I’m wondering how non breathable and breathable layers work together.
Is cement breathable?

What are the rules for a layered approach to the walls .
Is insulation with a vapour control layer behind lime plaster an option . And if so what type of insulation would we use - breathable or non breathable?
I get very stuck around managing water vapour and dew points and to maintaining a breathable fabric do all the layers have to breathe.
I know that water vapour gets into a buildings fabric more easily than air because it’s smaller molecules and that’s why it’s important to allow for breathability to help buildings dry more effectively when they do get wet.
I’m just struggling to join all the dots which in this case is layers that work together !!

Tradesmen just don’t like being told what to do. I have experienced this first hand although they are generally chauvinistic enough to resent a woman more.

There is no need for a trickle vent when replacing a window. You are allowed to change like for like.

Parging can be done with lime cement or even lime plaster. The cement would be slightly more waterproof but still permeable. It could make a good base around the window as airtightness tapes will adhere better to the smooth surface than to rough brick. The point of parging is that the paste used is forced into the surface pores, cracks and voids of the substrate (bricks in most cases) to give a very nearly airtight finish. If you don’t parge, the plaster sits much more on the surface and there is potential for air movement. The more porous the substrate the more important to parge. Aerated concrete absolutely should be parged as it is the epitome of porousness. (Not relevant to you, I think, but included for other readers.)

Regarding layering, I would parge first if the surface warrants. This also helps the adhesive for the insulation to stick better. Remember to do this down to the ceiling below (or damp course for the ground floor) and to the floorboards above or roof insulation. This minimises cold bridges.

Starting from the outside you want:

Parge (if required)
Insulation with adhesive and fixings as per manufacturer’s specifications
Airtightness membrane (optional when applying at least 2cm plaster)
Lime based plaster
Decorative finish if not using coloured plaster.

The problem with the airtightness membrane is that the plaster may not adhere properly, so it is more relevant when dry lining.

Don’t fix anything to the insulated walls afterwards.

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Would it be okay to use PIR board insulation against lime cement parge?

Or would you recommend another type of insulation?

I’m actually wondering about insulation?
Reading about lime plaster and considering the already small space . I’m wondering how much more insulation a good layer of lime plaster will offer - versus adding insulation too.
Reading the link below about lime plaster, I’m wondering about dry lining and suitable insulation .

The Lime Plaster Guide | Conserv®.

PIR is not vapour permeable, so the extra cost of lime plaster would be wasted.
If the room is so small that the extra thickness is an issue and of course price is a major factor then you have a big problem. Thinner insulation for a given U value is always more expensive.

For the seriously wealthy the answer would be aerogel bonded to magnesium silicate board to dry line, with a lime skim. That gives a vapour open effective thin insulation.

For the rest of us it is a matter of juggling cost and effect. Raised polystyrene is vapour open and good on a budget. Wood fibre has lower embedded carbon but isn’t as insulating and is more expensive.

For a price, that I haven’t investigated, there are lime based insulating plasters, such as Diathonite, however for a good U value they have to be applied very thickly and loosing the space advantage

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Tim might have some views on this but in my mind an alternative (vapour permeable) would be breather membrane against bricks, (say 50mm) hemp or wood fibre batts and finished with clay boards. A further alternative would be cork insulation products.

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I just really appreciate the conversation, shared information and alternative options.
I’m processing it slowly and methodically and will need to look at sourcing and pricing.
@zapaman @Tim_Gilbert - it’s a heartfelt thankyou from me to you both.:pray:t3: You are helping me so much .

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Your solution looks good to me. It isn’t one I would have thought of myself, which broadens the options and my knowledge. :+1:

However it will be more expensive, so more prone to the householders’ budgetary constraints.

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Yeah as Tim says the clay board product is more pricey than normal plasterboard. However, if this is only a small room then maybe it becomes affordable, especially in the context of the humidity / condensation issues you have identified.


Yes 1no Aereco vent (in combination with an overall ventilation strategy for the house) in this room might well go a long way to relieving the humidity issues in this room. Other vent systems are available of course, but I am familiar with the Aereco vents and they work nicely.

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If any layer doesn’t breath then it blocks vapour flow. You need a strategic decision for one or the other. There is no mix & match.

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This raises a potentially important issue. You don’t need to use the same solution throughout your house provided you can abut the insulation and join the airtightness layers for continuity of effect and to avoid thermal bridges/air leakage. You should also try to avoid big changes in U value, as the side with the higher U value will always seem cooler in winter and warmer in summer. (Remember, low U value is good. If your entire house could achieve a U value of 0W/mK you would overheat in winter with no heating system. Body heat and appliances would be far more than adequate.)

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After over 125 years of weathering how porous or permeable are the bricks themselves?

From personal experience I can recommend Stormdry masonry cream. It was easier to apply than I imagined.

As it is invisible when dried it is suitable for spot treatments without giving the wall an odd appearance.


Your comments are really helping me build a clear picture here.
If it weren’t for knowing the plaster is so affected and the seasonal temperatures so extreme in this room, I’d opt for new window, fix the pointing, storm dry on the masonry, add a vent and not bother with the vapour open lime plaster -

But despite not seeing the mould on the walls , you can smell it .

Re ventilation and Aereco - what type of vents are these .
I have a single room Vent Axia mechanical heat recovery vent which we bought for the bathroom, but didn’t fit yet because of structural disruption.
I’m wondering about using and fitting this in the bedroom or would it be too much ?

Re ventilation throughout the house, I think single room vents and open windows are what we’re looking at . Re heating - eventually it will likely be a heat pump or single room heaters - we are insulating and airtighting where we can. On the ground floor , under floor insulation is something we intend to complete eventually after a damp course fix. The mortar pointing at sub floor level is very crumbly too and I wonder if / how we fix this before we fit any insulation under the floor.

The VentAxia in the bathroom permanently running at a low level for background ventilation (and boost mode on humidistat or on the light switch) will be a big improvement for the house as a whole. Basically you need background ventilation to effectively or best deal with humidity build up.
The Aereco we have are the EHT in wall vents and we have a VentAxia extract Tempra fan in the bathroom (similar to your unit but without the heat recovery). So basically a background extract fan such as these Vent-Axia units and in wall room Aereco vents work well together. The Aereco vents provide the in coming fresh air (passively humidity controlled) to balance out the air extracted by the Vent-Axia. I would keep the Vent-Axia units to just the wet rooms, ie no need to fit in the bedrooms. Whereas the in-coming air is needed to control that balance between in/out air-flows.

Opening windows is fine and it is good day to day practice but having a good background ventilation system (such as described above) helps the overall management of the humidity in the property.

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I agree with @zapaman for a possible long term solution but you already have the MVHR unit. You could fit that in your target room, possibly moving it between rooms in the future. I seem to remember that you bought the higher powered and diameter model, which makes swapping out with something else a bit of a problem. That model is really for a large or damp room. Hopefully after your retrofit you will end up with a small dry room, so the prospect of reducing the diameter of your vent and making good might put you off the temporary solution.

My preference is for suitable MVHR whenever practical, with no other permanent vents (in or out). If that is your aim then you will need a smaller unit for the smaller room.

Edit: oh dear. I see you bought the MVHR in November 22. Definitely time to get it fitted. But bathroom or daughter’s bedroom? That is the question. Even having it in the bathroom will help a little in the bedroom, as some of that condensation may well originate as vapour in the bathroom. Given the system specification I think it should go in the bathroom asap and then go for a smaller unit in the small room.

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