Good morning everyone, my name is Keira Proctor, Managing Director of the A. Proctor Group, and welcome to our 14th webinar of 2021. We’ve been running these since April last year, and if you’ve missed any, you can go back and view the full series on demand whenever suits you.

You can find them right here on our YouTube channel, or through our learning hub at, where you can also request product information and samples or book in for follow up meeting with our team of experts. Remember you can also register for CPD certification for todays presentation using the link in the chat.

This morning we are once again joined by a very special guest presenter, David Somervell, secretary of the community led organisation CHOISS - Cohousing In Southern Scotland and advisor on sustainable architecture.

David will be taking through the principles of cohousing, and discussing how social and technical factors can be combined to deliver sustainable communities and build towards a net zero future.

After that I’ll follow up with a brief introduction to the effects on insulation placement on the hygrothermal characteristics of high performance fabric envelopes, and take a look at how this affects design for both refurbishment and new build structures.

We’ll then finish as usual with our Q&A session where David will be joined by our panel of technical experts to take you questions.

I’ll now pass over to David to begin todays webinar.

Hygrothermal Section

Many thank to David, and we’ll pick up with him shortly for the Q&A session.

Before that though, we’ll take a brief look at the hygrothermal issues surround structures with very high thermal performance. Although super-insulated buildings are nothing especially new, as we move toward net-zero, very highly insulated thermal envelopes are becoming far more common.

Whether the project is new build or retrofit adding a lot of thermal resistance to a structure necessitate careful consideration of the effect on moisture movement. If the vapour transfer properties of the envelope are not equally well considered, high levels of thermal insulation and airtightness can give rise to all manner of moisture problems.

So lets quickly recap on how condensation problems originate in the building fabric.

As we move outwards through the building envelope, the temperature will drop according to the thermal properties of the various layers in the building fabric. Alongside this temperature gradient we can also calculate the dew point at any given position. This is the temperature at which air becomes saturate and loses it’s ability to transport moisture vapour.

Once this dew point temperature is reached, the airborne moisture vapour turns into liquid water and can lead to interstitial condensation if not carefully managed.

Understanding how these two temperature gradient lines are influence by the properties and positions of the materials used is the key to a robust and healthy building envelope.

To illustrate this we’ll consider two basic wall constructions, an existing solid stone wall, commonly found in refurbishments project, and a more modern cross laminated timber panel wall. CLT panels are finding increasing use in passive and low energy new builds, where the combination of sustainable, low wastage offsite manufacture and flexible design options are particularly useful.

In each case, we will look at how the hygrothermal situation varies whether insulation is placed internally or externally. We’ll also consider how the thermal and vapour transfer properties affect the relative positions of the temperature and dew point graphs within the wall.


In a cross laminated timber panel, the most common configuration is to place the insulation, and any associated construction membrane on the outside of the timber structural panels. Shown here is a typical build-up using our wraptite air barrier adhered to the CLT to provide the airtight line, a fibrous-type insulation, our Fireshield vapour permeable membrane and finally a time cladding system with a fully ventilated void behind.

In this construction the vapour resistance decreases fairly consistently as we move outwards through the construction, so the dew point and temperature gradients remain well separated meaning there is little or no risk of condensation occurring under normal circumstances.

In fact because the vapour resistance on the warm side is sufficiently high relative to that on the cold side, the specification of the insulation can be varied with little chance of introducing a condensation risk. This is because the moisture vapour can escape the element before it cools enough that it condenses.

If we vary the thermal and vapour resistance characteristic of the insulation. it will affect the relative positions of the temperature end dew point lines, but the temperature will not reach the dew point. This makes externally insulated CLT a very robust construction method from a hygrothermal standpoint.

On the other hand however, if we flip the construction around, so our insulation is internal to the CLT panel, the situation is a little different.

Now there is more vapour resistance on the cold side, so the temperature in the construction drops down too quickly relative to the rate of moisture vapour escape. The moisture passing through the insulation is hitting the relatively cold inner surface of the CLT panel where it can condense.

The more insulation we add, the colder that surface will become as less heat from the internal environment will reach it. This reduced temperature will in turn make the condensation risk at this interface worse.

In order to remedy this, we need to limit the quantity of vapour entering the construction, which on paper can be achieved by adding a vapour control membrane internally. This boosts the level of vapour resistance on the internal side, increasing the gap between the dew point and temperature and reducing the condensation risk.

There is a complication with this approach however. While a vapour control layer, or a foil faced insulation board with a similarly high vapour resistance, will reduce the dew point, its “as build” performance may not match it’s performance on paper.

If the membrane is not sealed properly at laps and junctions, or is damaged either during insulation or by following trades, it may allow more vapour through than it should, reintroducing the risk of condensation occurring.

Increasing the vapour resistance like this also requires careful consideration of indoor air quality, and the provision of fresh air to the living spaces. With this, too much vapour resistance can make internal spaces unpleasantly humid and in extreme case even unhealthy.

So while in other types of construction such as a traditional timber frame wall panel, there may be other advantages that mitigate this problem, in a CLT construction it far easier to place all the thermal resistance externally and keep this vapour resistances as low as possible throughout.

This “breathing wall” type of construction helps ensure very highly insulated building envelopes can both limit the extent to which powered ventilation is necessary, and have flexibility in the specification of insulation materials.

This is important in more ecologically focussed design where material such as hemp batts or recycles fibre products typically very high levels of vapour permeability, and may require a lot of depth to meet u-value requirements.

An attendant advantage of this approach is that air barriers such as our Wraptite can be used externally to the CLT panel, with all service runs positioned internally. This provides a very robust airtight layer with minimal penetrations, ensuring the low air leakage rates required by passive or near passive standard buildings can be reliably met.


In this case we’ll consider a solid sandstone wall, common to many older properties. Sandstone is a relatively soft sedimentary rock, which means walls built of sandstone can be particularly sensitive to hygrothermal effects such as moisture absorption and freeze/thaw cycling.

This means that in addition to considering the flow of moisture through the construction, and any condensation risks, we must also consider the capacity of the stonework to retain and store moisture.

In this case we can use the same analysis tools that we used for the CLT panel, a method defined in BS EN ISO 13788, and also know as the “Glaser method” to obtain a basic view of the condensation risks at the interfaces.

Once again this will show us that a higher vapour resistance on the cold side will lead to an increased risk of condensation, and the higher levels of insulation internally can exaggerate this effect.

As before we can reduce the risk of condensation by placing the insulation externally and keeping the stonework warm, but in historic properties this approach may not always be possible or practical due to conservation considerations or external features like gutters, downpipes and overhangs.

There is however an important limitation to the Glaser assessment that we should also consider, particularly if dealing with sensitive historic materials such as lime based plasters and renders.

The Glaser method considers the moisture flow only under steady state conditions, and does not account for the capacity of material to retain moisture in liquid form. This means that any wetting/drying effects, to both the indoor and outdoor environments, are ignored. The effects of weather such as driving rain being absorbed are also not taken into account.

Fortunately however, there is another calculation method we can use that will account for these effects, and this is outlined in EN 15026: Hygrothermal performance of building components and building elements - Assessment of moisture transfer by numerical simulation.

This standard, which allows for the dynamic assessment of moisture movement, including weather and moisture accumulation effects, is specified in the BS 5250 code of practice for assessing the hygrothermal performance of solid walls.

So if we now take another look at our internal wall using this method, the results become a little more nuanced.

The internal insulation case where a permeable insulation like our spacetherm aerogel is used can show a higher degree of condensation risk under the Glaser method, but if we include moisture storage and inward drying in our assessment, this moisture accumulation can be accommodated in the fabric until condition are more conducive to drying.

This effect also means the overall moisture related risk can, counterintuitively, be at it’s lowest when vapour control layers are omitted.

If we flip the insulation around to the outside again, while this is still poses a lower overall condensation risk, we still have to ensure the stone of the wall can dry out effectively and that the passage of moisture vapour is not disrupted.

Many older properties are finely balanced systems, where the balance of heat and moisture transfer has remained more or less for many years. Adding modern materials and applying modern performance standards can therefore disrupt this balance and can cause damage to the existing fabric. Lime based plasters and renders are particularly prone to this type of problem as lime is significantly more permeable than most modern mortars.

If the retrofit work is not well considered and the moisture content in the stonework changes dramatically, it can cause a variety of problems. If the moisture content goes to high, this can result in damp problems arising internally, or the expansion caused by freeze/thaw cycles can cause cracking and spalling externally.

Conversely, if the structure dries out too much, relative to it’s historical baseline, material can shrink, or become too brittle, again leading to cracking and deterioration of the building fabric over the long term.

The EN 15026 assessment not only allows the full range of performance characteristics and environmental effects to be incorporated into the model, but it allow this to be run for extended time periods. This allows the identification of small problems that may increase year on year to grow into larger ones over many years.

This knowledge and detail allows designer to ensure retrofit works is sympathetic not only to the aesthetics but also the hygrothermal characteristics. In this may we can ensure upgrade work is sustainable not only in terms of environmental impact, but also in terms of long term robustness and durability.

It allows the energy performance improvements to be maximised while limiting the scope for long term problems to arise with either the building fabric or the health of the occupants.


Whatever construction methods are use, and whatever the development type, the key consideration in designing for net zero is ensuring that the building fabric and systems are suitable. Simply taking existing designs and increasing the levels of insulation and airtightness will not always provide the optimum result.

The simplest and most effective solutions are those which consider not only the fine tuning and optimisation of the building fabric, but also those which take into account the wider social context in which those buildings are used. New ways of thinking about how we interact with the built environment will likely prove just and important a fitting new technology into existing practice.

This Webinar Includes
  • Principles of Cohousing
  • Developing sustainable communities
  • Building fabric solutions for net-zero
  • Factors influencing material choices
  • Community-led development case studies