Planet friendly architecture, guidance and inspiration​
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Taking the fuss out of architecture and building projects

From damp prevention to better air quality, the benefits of breathable buildings

Following last month’s hefty 8400+ word Newsletter [that surprisingly didn’t seem to put people off!] you might be glad to hear that this month’s Newsletter is a bit shorter. 🙂

Today, we’re looking at breathable buildings - trying to explain what this actually means + why it matters. This should assist anyone planning a new building that isn’t a traditional cavity walled building, such as a timber framed building that uses lots of natural organic materials.

It is also vital to understand this concept when renovating or retrofitting old buildings with solid walls as they behave in a similar way. For example, if you decide to insulate an old solid walled building you may disrupt the natural moisture behaviour of the existing wall and create problems.

Understanding how breathable buildings are intended to perform may also help you understand how alternative modern sealed constructions work and how these are often at odds with a breathable construction.

Breathable buildings explained + guidance on how to things properly

So, what does breathable actually mean in the context of buildings? Does the building breath like I do with air coming into my lungs and back out again as carbon dioxide?

Well, no, not exactly.

Breathable building materials are permeable enough so that any moisture from humid air and moisture from surface wetting can be absorbed and can then easily evaporate during dryer warm periods.

A lot of natural, traditional building materials have inherent properties that allow moisture vapour to pass through them, they are vapour permeable. These materials can also absorb and retain moisture in vapour form [like moist damp air in the background or from a bathroom], this means they are also moisture buffering [the scientific word for this is ‘hygroscopic’]. When looking at these materials in even greater detail [under a microscope] the pore structure can also enable liquid water [not just water vapour] to move through by other means, this is sometimes called ‘wicking’.

This is very different from some modern, synthetic building materials that are made from fossil fuels and plastic and are heavily processed. These are the opposite and therefore classified ‘vapour closed’ or ‘non-vapour permeable’. They don’t ‘breathe’.

There’s a whole load more scientific explanation and detail that I could go into about physics, how to quantify different levels of vapour permeability, ų-values, SD values, and materials science, but I don’t think it’s necessary and it would probably bore the pants off you [and me for that matter]. The descriptions above should explain enough for what we need to know for now.

For now, we are just interested in the vapour permeability. With the chart above the light blue is best [lowest numbers] as these materials are more permeable and let more moisture vapour though the material. The dark blue is the worst [highest numbers] as these materials are not permeable [synthetic] and do not let much moisture vapour through. As can be seen, the natural organic materials perform the best, while the synthetic materials do not perform as well. For example, I understand that wool can absorb up to about 35% of its weight in water vapour before the effects of this much moisture retention would be noticeable.

How does this work in practice?

Here’s a sketch I prepared earlier. This is based on a wall but similar principles need to be adopted for roof and floor build ups too, but less so for a floor as the water vapour moves slightly differently.

This drawing is a vertical cross section through the external wall build-up of a timber framed building. A cross-section drawing is a vertical slice through a building. A bit like cutting a slice of cake and then looking inside the cake to see the layers of cake and icing.

Thermal insulation [layer 1] is positioned in between the vertical structural timber members [timber studs] + an extra layer of insulation on the outer face of the timber studs [layer 2] to max out the thermal performance. On the warm internal side of the insulation or the inside face of the studs, is a layer of structural plywood. A polypropylene sheet membrane [blue dashed line] is then fixed to the plywood which helps create air tightness so heat isn’t lost through unwanted holes or drafts. This is then covered over with a small timber batten framework to form an airspace that can house cables and pipes and then finally covered over with plasterboard and skim or an alternative support board with lime or clay plaster.

On the cold external side of the insulation is a different type of polypropylene sheet breather membrane [green dashed line] primarily to act as a secondary weather barrier from wind driven rain etc. This is then covered over with vertical timber cladding and an air cavity [called rainscreen cladding] to drain off any penetrating rainwater. Other forms of external cladding or finishing could also be used, such as horizontal or profiled timber cladding, brick, stone or render etc.

The insulation used here is natural wood fibre that has good vapour permeability and moisture buffering properties. Other natural insulation products could also be used, see the ‘Scale for vapour permeability’ image above. The internal sheet membrane, blue dashed line, [also called a vapour control membrane] is also vapour permeable. The external sheet membrane, while water resistant from the weather, is also vapour permeable.

This means that background water vapour is intended to enter the constructional build up, it’s not intended to stop it entering. Over the colder, damper winter months, with higher internal humidity, a certain amount of water vapour moisture can enter the build-up and is temporarily stored in the insulation. Then, over the warmer, summer months, when the internal humidity drops, the stored water vapour moisture can evaporate harmlessly into the room.

It’s also worth mentioning that these internal vapour control membranes are ‘smart’ humidity variable membranes. This means they let less moisture through in the damper winter months and more moisture through in the warmer summer months.

As both internal and external membranes are breathable, they also allow a degree of moisture movement and evaporation in both directions.

There is a limit though, too much water vapour would eventually saturate the natural insulation and create condensation inside the build-up, which is not desirable! Read the section below about how to reduce the risk of unintended consequences, like damp and mould.

Please note, fire resistance performance isn’t covered here, if this is required, then the build-up might need to change slightly, with different fire boards or alternative natural insulation products [such as hemp blocks], used as part of the build-up.

What about non-breathable sealed buildings?

A good question, I’m glad you asked.

Here’s another sketch that I also prepared earlier.

This drawing is also a vertical cross section through the external wall build-up of a timber framed building. The position of the components and overall build up is essentially identical but the performance of some of the materials is very different.

The internal sheet membrane [orange dashed line] is not vapour permeable, it is formed as an airtight vapour barrier, intended to stop any water vapour moisture from entering the constructional build up, while also reducing heat loss through drafts.

A similar external sheet breather membrane is still used [green dashed line] that is vapour permeable and also acts as a secondary weather barrier from wind driven rain etc. No change on this bit.

The insulation used is synthetic, such as phenolic or PIR insulation [a lá Kingspan or Celotex etc.], lined with foil, that is not vapour permeable. Not breathable natural insulation such as wood fibre, cork, hemp, sheep’s wool or straw etc. Do you see the irony in using synthetic insulation materials, like Kingspan and Celotex, that are made from petrochemicals / fossil fuels, with the aim of reducing our reliance on fossil fuel energy.

This means that any background water vapour moisture is intended to be blocked or stopped entirely, from entering the constructional build up, throughout the year, at all times.

Sealed buildings can work but as they rely on internal vapour control membranes that are not vapour permeable, they need extremely good site workmanship [should that be ‘workpersonship’?] to ensure the insulation is fitted tightly without edge gaps and the internal membranes are installed fully sealed without any gaps or holes. In reality it’s likely there will be gaps, as I understand it is impossible to ever build a 100% fully sealed construction.

Over time, any building will move slightly [seasonal expansion and contraction etc.] and ‘flexes’ slightly. This means the joints can open up slightly and lose their fully sealed nature, so the theory fails slightly. DIY tasks with new screws and nails in the external walls, can also damage and puncture the vapour barrier, years after the building was finished. On this basis water vapour can then enter the construction. Hopefully any water vapour ingress is minimal so the risk of damage is very low. However, if the moisture entering is too high or over a prolonged period of time [possibly a few years] the water vapour can then condense in cold areas [usually the cold side of the insulation], and form water droplets, which can’t be seen and can’t escape, until it’s too late. If this happened, then these damp conditions could create blackspot and mould, resulting in an unhealthy indoor environment. We really don’t want that.

The vapour permeable breather membrane on the external side should help to remove any condensation but it’s possible that there will be a lot of condensation still remaining in the wall build-up.

It’s also worth noting [in my humble opinion], that a sealed build-up should not be used on historic, solid walled buildings. For example, retrofitting with internal wall insulation or external wall insulation. Adding in a sealed build-up will change the vapour open behaviour of the old walls and is likely to lead to walls remaining damper and wetter with less opportunity to dry out. This could also cause problems, with damp and blackspot. This is another huge topic in its own right so keep your eyes open for a future newsletter all about this.

If a build-up is vapour permeable doesn’t that mean it’s not airtight?

Another good question, I’m glad you asked this one as well.

If the correct products are used as intended then a vapour permeable construction is also airtight and does not let out heat through uncontrolled ventilation air paths - aka drafts.

Primarily, the internal, vapour permeable, vapour control membrane, indicated with the blue dashed line on the first sketch above, creates an airtight barrier that doesn’t let out warm air but is also a ‘smart’ vapour open membrane so the construction can breathe, and lets through moisture vapour in the form of a gas.

In solid masonry wall constructions, the airtight barrier is often formed by an existing lime plaster finish, which is also vapour permeable. Over time this plaster airtight barrier is habitually damaged. Plaster that is several decades old will move and sometimes start to come off the main wall behind, and mechanical fixings puncture the plaster to support shelving or other items of fixtures and fittings. If retrofitting the solid walls with insulation, the type of insulation build up, may change the airtight and vapour open barrier positions and will need to be carefully considered to deal with this appropriately.

Is this something new?

No, definitely not.

Historically, traditional solid walled buildings were built as vapour open constructions and had no physical barriers to moisture penetration. No DPC [damp proof course] and no DPM [damp proof membrane] etc. They absorbed and released moisture and easily evaporated any damp rising from the ground. Towards the end of the 19th century, brick buildings sometimes used slate damp proof courses at the base of walls, but they essentially remained breathable solid walled structures.

Modern cavity walled buildings, usually dating from the late 19th century or early 20th century onwards, were built as sealed constructions. They employed entirely different principles from the breathable buildings of the past. Moisture transfer was intended to be blocked at every possible junction and surface. Any form of damp and water vapour is essentially prevented from entering the building and constructional build ups. In a cavity walled building condensation can occur on the ‘safe’ side of constructional build ups, such as the external leaf of a cavity wall.

This is how I was originally taught at college and university, but I’ve since learnt more fully about the breathable buildings of the past [and the future], and how these techniques can continue to be used for new buildings and upgrading old buildings. Historic, breathable buildings, if faithfully repaired, maintained and upgraded with traditional breathable materials and techniques, should be able to continue to perform this way, without damp problems, even in the 21st century.

Having said that, I did actually investigate breathable constructions in a basic way while researching my dissertation back in the late 1990’s. Things have certainly moved on since then and I now understand more fully how breathable buildings are meant to function.

What are the benefits of a breathable building using natural materials, why bother?

• Less chance of water vapour and condensation getting permanently trapped in the constructional build up, if within acceptable limits. There is opportunity for the moisture to dry out during the warmer summer months. Vapour open buildings adapt to changing weather patterns and seasonal humidity fluctuations, making the building envelope more robust against climate stress.
• Enhanced indoor air quality, as natural breathable materials absorb and release water vapour, naturally regulating the internal humidity, creating healthier environments. This means there is less humid air hanging around that needs to be dealt with by other means, such as increasing the ventilation and the internal temperature.
• They rely on natural, organic materials which should involve less processing and harmful chemicals during manufacture. Less embodied carbon and less embodied energy. This factor alone is good news for environmentally friendly buildings that rely less on heavily industrialised processes.
• Certain natural insulation materials also have a heat absorbing property, such as wood fibre and cork insulation. The insulation can absorb a certain amount of heat during the warmer summer months and then discharge it slowly when it’s cooler, without causing discomfort. When used in conjunction with other cooling methods this can help reduce overheating discomfort during the hotter, dryer summers we are now experiencing.
• The flip side of this is that if using synthetic insulation materials, these do not have a heat absorbing property. Any timber framed building that uses synthetic insulation may suffer from excessive overheating and could create an uncomfortable environment, unless other overheating reduction measures are used.
• It’s about working with moisture, not working against it and trying to stop it, about regulating and managing the moisture within reasonable limits.

What could possibly go wrong, aren’t natural insulation products amazing and flawless?

Natural insulation products are good but they do have limitations, sometimes unintended consequences can occur. As mentioned earlier there is a limit of how much water vapour can be absorbed by a natural insulation product. If too much water vapour enters the construction the insulation would eventually become saturated and condensation would form within the build-up. If this happened, then these damp conditions can form the ideal environment for mould and blackspot spores to grow. These can spread undetected behind plasterboard or cladding, affecting indoor air quality, which can result in an unhealthy indoor environment. This creates health risks for the building occupants, people like you and me. This is not good.

Therefore, it’s essential to prevent mould from occurring. Ensure condensation risk analysis is completed for the constructional build-ups of all major components, such as floors, walls and roofs. This a special calculation where the specific site conditions, and materials are assessed. If these condensation calculations confirm the risk is too high then the design needs to be modified to reduce this risk down to acceptable limits.

It’s worth noting that when completing condensation risk analysis for existing old solid walled properties, then a certain specific type of condensation calculation should be completed that assesses these risks more comprehensively. Especially where you might want to insulate the walls to improve thermal performance. It’s often better to have a thinner layer of insulation with slightly less thermal performance to reduce condensation risk. While thicker, super insulated retrofit insulated build ups can be used, these are effectively independent timber framed walls that incorporate the intelligent membranes mentioned earlier. To do this you need a lot of space, say like a barn conversion or an old Victorian warehouse conversion; and a lot of money.

Using thinner layers of thermal insulation to reduce damp or condensation risks in historic buildings with a vapour permeable construction is also specifically identified in the Building Regulations Approved Document Part L Conservation of fuel and power. This demonstrates the government also recognise potential issues that may occur when upgrading older historic buildings.

Condensation risk analysis is a big topic in its own right and could form a future newsletter, so keep an eye out.

Another unintended consequence that could result from the build-up getting too damp is that any timber could suffer from accelerated decay and degradation. Metal components could also suffer from corrosion.

Insulation that is too damp from too much water vapour ingress can reduce the thermal performance of these elements. Wet insulation isn’t able to resist heat as effectively. This can lead to increased heat loss and cold spots, with higher energy bills.

Building contractors not fully understanding the differences and reasons for selecting specific types of vapour permeable airtight membranes and potentially swapping out the specified products for something else, easier and cheaper to obtain. If this happens without any consultation with the design team and the wrong products are used this may mean the construction does not perform as intended and may also create a build up of condensation.

Natural insulation products are not suitable for use in areas subject to prolonged water exposure, such as below ground or below any damp proof course in an external environment. This can cause the natural insulation to degrade and rot and ultimately the building envelope thermal performance would reduce. In these areas, synthetic insulation does need to be used that is resistant to water, such as XPS or EPS insulation. While I prefer to use natural insulation materials as much as possible, I do recognise there is sometimes a place to use synthetic insulation products, albeit as sparingly as possible.

Using the incorrect type of paints could also affect the breathability of these build ups, especially to walls and roofs / ceilings. This leads nicely onto the next section that is all about paints…

What about the internal paint? Does that make any difference?

Absolutely, yes, it does.

It applies to both new build and old buildings with a breathable construction. Older buildings can often have their breathability disrupted when more modern impermeable paints are used on the historic building fabric. This can lead to the paint bubbling and blistering and then wrong assumptions made of rising damp or other damp issues, when it’s just that the wrong paint has been used.

While mainly aimed at the internal paints, this can also apply to external paints, if used directly on a historic solid walled building, such as brick, or render.

When you have a building with a vapour permeable build up do not use an impermeable paint such a standard vinyl matt emulsion from your regular DIY store local to you. You will need to source breathable or microporous paints. These are slightly more specialist [and expensive] paints. These types of paints will allow the full build-up to continue to perform as a vapour open construction, so water vapour or liquid or both, can continue to pass through unimpeded, preventing too much moisture building up, as described above.

A few specialist breathable paint manufacturers worth checking out are:

• Graphenstone
• Earthborn
• Beeck
• Farrow and Ball
• Baumit Klima

Financial impacts

While there are clearly some benefits to using natural, vapour open materials and systems of build one noticeable issue I’ve observed is that they currently cost more than the synthetic alternatives.

At the moment, I think this is due to manufacturers scale of economies for the synthetic materials. The natural material manufacturers don’t seem to have such large overall manufacturing capacity and the cost of raw materials is higher. This may change in the not-too-distant future though, possibly led by higher oil prices, influenced by global forces and instability, that the synthetic material manufacturers can’t control.

While cheaper synthetic materials aren’t suitable for certain buildings, such as historic solid walled properties, they can be used for new builds successfully and are cheaper, but please do re-read the section above about the benefits of a breathable building using natural materials.

If you are able to use natural, vapour open materials and systems of build, while it may be costing you slightly more financially, it is very likely having an increased positive impact on the planet, and that’s worth shouting about. Well done you!

Breathable construction used on a low carbon project in Sheffield

This project involved the transformation of a typical 1950s brick house through a bespoke rear extension designed around the changing needs of a family + a Magnolia tree! A key objective from the outset was to minimise the environmental impact of the project by reducing embodied carbon and using more sustainable, natural materials wherever possible.

The clients are super enthusiastic about creating a low-carbon extension and I worked closely with them to adopt a range of environmentally conscious design choices, including:

• A low-concrete foundation solution that reduced concrete volume by approximately 7 tonnes. Originally, we tried for no concrete foundations but the ground conditions weren’t favourable.
• Timber frame construction instead of traditional masonry.
• Natural wood fibre insulation in place of fossil fuel-based insulation products.
• Hemp-based internal boards instead of conventional gypsum plasterboard.
• Timber windows and doors rather than uPVC or aluminium.
• Vertical larch timber cladding.
• Recycled wood-chip kitchen worktops [Foresso].

The extension was thoughtfully shaped around an existing magnolia tree that holds huge sentimental value for the family, so a corner window and seat were designed in, overlooking the tree, turning it into a focal point throughout the year.

The breathable constructional build ups are based on a tried, tested and guaranteed* set of floor, wall and roof, full system assemblies, provided by Soprema Pavatex [a French / Swiss company]. The wall system is fire resistant to a certain degree as well. All materials were obtained from a single source UK supplier called Unity Lime and they provided a fantastic service during the design and construction. [Again, for transparency, no commission from this, just knowledge sharing!]. For reference, there are other suppliers on the market selling similar products and systems.

*As long as certain stringent criteria are met, such as purchasing all materials together as a package and installing as the manufacturer instructions.

Other notable features include:

• Large roof overhangs to provide summer shading and to help reduce overheating.
• Angled walls to reduce visual impact on neighbours.​
• 45-degree internal walls to improve circulation and spatial flow.
• Ceiling height sliding doors creating a flexible ‘broken plan’ layout.

The extension is intended to achieve thermal performance slightly better than the minimum Building Regulations requirements, about 10% better. I understand that most of the materials used emit fewer harmful VOCs which helps to create a healthier internal environment and improve occupant wellbeing.

This project is currently on site and I understand the external watertight envelope is nearly completed. I mentioned this project in a previous newsletter and it has come a long way since then. While there have been a few technical challenges to overcome, I understand the Clients are pleased and excited with their project.

So, going from vague university research in the 1990’s where I didn’t fully understand how breathable buildings work, to now, where I do understand and where I’m working on new buildings and old historic buildings, that use these principles. Exciting times!

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Like the sound of a breathable building? Want to use it on your next project? Got questions? Well now, get in touch and…

New places

If you are new to my newsletter then each month, I share a photo of somewhere I've been. Just sharing something about my love of buildings and nature. I also love travelling to new places and cultures. Whenever I go on holiday, I try to take a few photos of buildings and places I find fascinating and interesting. I’m by no means a professional photographer but I’d like to share some of these places with you, hopefully they capture something of what I experienced.

Last month’s photo of a peculiar spiky building was of Casa dos Bicos, Lisbon, Portugal. Casa dos Bicos means ‘House of the Beaks’ [or 'Spikes’]. When we visited a few years ago, I didn’t know what it was or the history, I had to do a bit of post-holiday research. Apparently, the ‘House of the Beaks’ is an unusual house built in the 16th century, in the Alfama neighbourhood of Lisbon. The curious façade of spikes is meant to be influenced by Italian Renaissance palaces and Portuguese Manueline styles. It somehow survived the major earthquake of 1755, that incidentally destroyed a lot of Lisbon. Over time it was abandoned as a residence and used as a warehouse. Following a 20th century renovation, it became the headquarters of the Jose Saramago Foundation and one of the sites of the Museum of Lisbon. The building is located in a Special Protection Zone which is a buffer zone designated around architectural or national heritage sites, to preserve their historical and aesthetic integrity and merit. I think this is very similar to a UK Conservation Area but with perhaps a bit more protection to buildings in the zone.

So, what do you think this is a photo of and where was this photo taken? If you think you know where the photo was taken or like the photo and want to find out more, hit reply and let me know.

You’ll have to wait until the next newsletter to find out where this photo is from. I know, I can feel the pent up excitement from here.

What's next

Next time we will look at overheating in buildings. Why does it seem to be more of a problem nowadays? What methods are available to reduce overheating with thoughtful building design, including retrofit upgrade techniques to existing buildings. I will also share my thoughts on where I think the future of building design might be heading in the UK to reduce overheating. Keep an eye out to find out more.

That’s it for now, hope you enjoyed this and I look forward to sharing more with you soon.

By the way, in case you are wondering, you’ve been sent this email because we’ve spoken in the past, perhaps discussing your project, or you have asked me a question via the website, social media or we have met in real life. I do hope you found this email helpful, entertaining and inspiring. However, if this is not the case, please hit unsubscribe at the bottom. I’ll try not to be offended, although I probably will be a little bit! I know, I know, it’s me, not you [it usually is].

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