Scott Wardlaw 1/22/21 Scott Wardlaw 1/22/21

PassivHaus Windows Explained

This article is our third in our series discussing Passivhaus 'fabric-first' design principles to improve the energy performance and quality of our buildings. Focusing on Passivhaus Windows, the article highlights the technical requirements of a Passivhaus Window, design detailing and finally discusses the advantages to health and well being experienced in buildings incorporating such systems.

Windows are by necessity a requirement in any building. They have to provide as a minimum daylight and to frame views, but they also play a particularly significant role in the energy performance and thermal comfort of a building. With its emphasis on building comfort, healthy living environments and energy performance, the Passivhaus Standard is particularly demanding when it comes to appropriate specification of glazed elements within your design.

Technical Context

In order to better understand how a Passivhaus Window assists in meeting the standard, it is broken down into various technical performance parameters.

Solar Gains

Designing to optimise solar heat gain in northern Europe is a fundamental principle of successful Passivhaus design. Size and orientation of the window on the site are important in this regard and under the control of the designer. The solar heat gain of the glass (or g-value), a measure of how much heat energy the glass allows to enter the building, is also important. Passivhaus Windows need to have g-value to suit the building location and climate. In northern Europe, this would typically be 0.5 or higher.

Losses

Just as gains are important, so are heat energy losses. The measure of a window's heat loss is its U-value, calculated as an average for the whole window - glass, frame, spacer and installation.

The glass unit in northern European climates is required to have a u-value of 0.7W/m2K or lower to meet Passivhaus Standards. In order to achieve this the glazed unit will often be triple glazed and require an inert gas such as Argon or Krypton to be sealed between the panes. This results in a very heavy window which places particular requirements on the frames and can limit unit size commercially.

The frame is a linear element and as such is given a Psi-value (linear heat transfer coefficient) to determine its thermal performance. The psi-value required of a certified frame must be at least 0.2 W/mK. In order to achieve such values, frame manufacturers often use complex thermally broken frames. Materials used are generally UPVC, Timber or Aluminium.

The second component in determining the psi-value of the frame is the glass spacer. This can be vitally important. In standard double or triple glazed units, often aluminium spacers are used. However, these are highly thermally conductive causing significant heat losses. Passivhaus windows therefore often incorporate plastic or composite spacers to counter this.

Finally, careful interface detailing is required between the window and the wall system to ensure that there are no thermal bridges occurring between the window frame and the wall itself. Consideration of the position of the window relative to the thermal layer of the wall is required to manage this key interface and limit heat transfer through the frame and window reveals. This is vitally important to ensure condensation does not build up promoting mould growth and damage to the fabric of the building.

Once all of the above has been considered, the Passivhaus Planning Package assessment tool (PHPP), will arrive at an Installed U-Value. In northern Europe, this value is set at 0.85W/m2k.

Thermal Comfort Context

But what do all these U-values and psi-values actually mean in the real world?

The thermal performance of a wall is much higher than a window, the window transmitting more heat than the wall. This can cause three main areas of discomfort for occupants;

Draughts. Not caused by wind or poorly sealed windows, but by temperature differential. Warm air in the room contacts the cold window surface and cools down, sinking to the base of the window. This creates the feeling of a draught. Indeed, this is why radiators are often located below windows.
Temperature Asymmetry. If a human experiences a temperature difference of around 4.4 degrees between their head and their feet, then they will feel cold, regardless of the temperature of the room.
Heat loss from our body radiating out towards the colder surface of the window will make us feel cold also.

Passivhaus Windows counter the above through their improved performance requirements over 'standard' window units. A certified Passivhaus Window will only allow a temperature difference of 4.2 degrees cooler than the average internal surface temperature resulting in no draughts, no temperature asymmetry and no excessive heat loss radiating towards the window. The result? Improved thermal comfort.

Health Context

The requirements of the Passivhaus Standard also ensures that the window installation promotes a healthy internal environment. By limiting thermal bridging and condensation, the risk of mould build growth on window surfaces, so often prevalent in 'standard' constructions, is eliminated. The high thermal performance of the window coupled with effective interface design, maintains internal surface temperatures and relative humidity at a level that ensures condensation and mould cannot form. This enhances health and well being for the occupants, but also protects the building fabric, improving durability of components.

Consideration of a Passivhaus Window for your building whilst expensive, will give unrivalled thermal comfort, reduce energy bills, protect the building fabric and will more than likely, last longer when compared to conventional double glazed systems. There are many manufacturers now producing Passivhaus compliant systems. If you would like further assistance in determining whether or not such a system is appropriate for your Project, please simply get in touch by clicking on the button below.

Scott Wardlaw 1/15/21 Scott Wardlaw 1/15/21

PassivHaus Wall Systems

This article is our second in our series discussing Passivhaus 'fabric-first' design principles to improve the energy performance and quality of our buildings. Focusing on the External Wall, the article highlights the general strategies prevalent today with a focus on Timber Frame Structural solutions, the most popular here in the UK.

Design Principles

The Passivhaus Standard has very stringent requirements with respect to meeting the energy demand of the building when compared to more conventional building standards (Primary Energy Demand of </= 120 kWh/m2.yr and Space Heating Demand </= 15kWh/m2.yr). In order to meet such demands the thermal performance of the building envelope has to be optimised to achieve the following values;

U-value of 0.15 W/m2k or less
Airtightness of 0.6 air changes/ hr @n50 or less

The building envelope therefore must have;

Very high levels of insulation
Airtight building fabric
Windtight building fabric
Thermal bridge free construction

Within the standard there is no limitation on how these values and criteria can be achieved, allowing creativity in design and multiple building systems to be considered. By far the most cost effective system for domestic building in Northern Europe however, is Timber Frame Construction. Within this 'system', the parts themselves are interchangeable, providing the opportunity to choose from a vast array of different elements and components to achieve higher levels of performance and economy. Such systems are light weight, easy and quick to transport and construct and are robust. Whilst there are many derivatives of timber frame construction, all are characterised by an internal zone for running services and applied internal wall finishes and an external cladding zone capable of supporting a wide variety of external cladding systems to meet aesthetic and regional requirements.

Broadly speaking, timber frame construction is split into two approaches, open panel construction and closed panel construction. Open Panel construction relies upon simple open stud framework being erected, then insulated and clad on site. Closed panel construction optimises the advantages of offsite manufacture, forming modular panels in a factory complete with an enclosed structure, insulation and protective sheathing layers to the internal and external face of the panel. Open Panel is often slightly cheaper, but Closed Panel construction offers the benefits of improved build quality.

Open Panel Construction

In order to meet the stringent thermal requirements of the Passivhaus Standard, open panel systems will typically be constructed from 184mm deep timber studs, supplemented by additional thermal layers of insulation placed externally or internally to alleviate thermal bridging through the timber studs impacting upon the overall U-value of the assembly. This allows a wide use of insulation material to be considered, dependent upon cost, performance and environmental strategy. A typical assembly comprises the following;

External cladding zone
Windtight, breather membrane
OSB Sheathing board
184mm timber studs at 600mm centres
Insulation infill
Airtight and vapour control layer (membrane or board)
Supplementary thermal insulation layer
Service void
Internal wall finish

Advantages

Low cost
Leveraging common construction skills
Versatile
Medium speed of construction
Flexibility in design

Disadvantages

Highly dependent on site supervision
Required build quality hard to achieve
Slow erection times when compared to other methods

Closed Panel Construction

Optimising the advantages of offsite manufacturing techniques, closed panel construction is becoming increasingly popular. Broadly speaking there are three types of system used.

Closed Panel System 01

Essentially an offsite version of an open panel system, many suppliers now offer what can be considered an entry level closed panel system. Typically the panel is supplied with both external and internal sheathing boards fitted with the panel being fully insulated. In some cases external cladding rails and finishes can also be applied subject to logistical constraints. These systems can also take advantage of manufactured I-Joists replacing the traditional 184mm stud. These can provide a deeper wall, increasing insulation and reducing thermal bridging through the stud.

Closed Panel System 02

Another way of increasing the levels of insulation beyond a standard 184mm stud whilst also minimising thermal bridging through the structural timber elements of the wall is via a split-stud or twin stud wall. In this construction, two lines of studs 90mm wide are placed tied together at intermediate heights of 600mm vertically. This allows any depth of wall to be constructed to achieve the required thermal performance of the construction. Other advantages include optimising the integration of intermediate floors and roofs to minimise thermal bridging and improve the thermal performance of the entire structure. Beyond the structural layer, the system shares the same characteristics of other timber frame systems.

Credit: MBC Timber Frame

Closed Panel System 03

The final closed panel system is a Structural insulated Panels Systems or SIPS for short. SIPS panels comprise high density EPS or Foam insulation and timber studs sandwiched between two layers of sheathing board, typically OSB board. The result is a composite panel of high strength and thermal performance capable of large spans. These panels are able to be used as walls, roofs and even floor cassettes. Manufactured under factory conditions, the panels are quick and easy to transport and erect on site, achieving wind and watertight status very quickly. They are best suited to modularised simple forms, their efficiencies quickly compromised when more complicated forms are desired.

Advantages

Speed of construction on site is high
Whole façade option
Improved build quality
Modular build
Building performance generally improved

Disadvantages

Can be costly
Specialist structural input required
Supplier choice scarce
Can be inflexible due to modular build strategy
Can require specialist erection team

In June of 2019 the UK became the first major economy in the world to commit to producing net zero greenhouse gas emissions by 2050. Enshrined in Law this means that all industries, including construction, must operate at net zero carbon by 2050. Indeed in Scotland, the devolved administration has committed to a date of 2045. Some cities such as Edinburgh and London, have even more aggressive targets of 2030 in mind. To meet these targets the built environment, in particular our homes, will have a continuing requirement to meet increasing energy efficiency targets. The net result of this will be a continued drive towards off site manufacturing techniques such as the ones described in this article, to ensure post occupancy energy performance targets are met.

The above systems are all capable of providing an energy efficient façade and are becoming increasingly popular in the UK, in particular in the Self-Build Home sector. Choice is available. All are capable of delivering a building performing to the Passivhaus Standard. Choice will be dependent upon a combination of factors including cost, speed of construction, technical expertise of your team and complexity of design.

If you have found the above article useful and would like to understand more about the Passivhaus Standard, we would encourage you to read the other articles in our blog available at www.novo-design.co.uk/blog. Alternatively you can join our mailing list below.

Scott Wardlaw 1/7/21 Scott Wardlaw 1/7/21

PassivHaus Foundation Systems and Details

This article is the first in a series discussing how integrating Passivhaus ‘fabric first’ principles vastly improves the energy performance and quality of our buildings. Focusing on foundation design, the article highlights the three broad design strategies employed to optimise the thermal performance of the building.

Context

As energy performance goals begin to rise, the potential for heat loss through the base of buildings has come in to sharp focus. Concrete basement walls, stem walls and slab edges can no longer be left exposed to winter air or frozen soil near the surface compounding thermal bridging, causing significant condensation and mould growth at key interfaces. Infiltration of cold air up through suspended floors has also been identified as a problem.

In answering such design challenges, designers proposed insulating the building perimeter down to a sufficient depth that the temperature differential between the soil and the foundation system was no longer considered a problem. Alternatively, insulation could be extended out horizontally from the building perimeter just below grade level. This protects the soil mass directly under the building perimeter from heat loss to the surface. Combination of these techniques is still in use today.

To meet Passivhaus standards in harsh climates such as those experienced in central and northern Europe, it is necessary to completely isolate the foundation system from thermal contact with the soil. In addition, perimeter drainage systems are required to reduce fluid pressure against such foundation systems as well as protecting the integrity of embedded barriers and materials within the system from soil that would otherwise be saturated with water.

Design Strategy

The core design challenge is to maintain thermal isolation between the structure and grade whilst transferring loads from the building via walls and columns to the foundation. The most straight forward approach is to surround the entire foundation in insulating material. This approach allows the mass of the foundation to hold the building down assisting detailing with all connections to the slab within the thermal envelope. It also has the added benefit of the slab acting as a thermal store for the building, enabling internal temperatures to be regulated more easily. Design challenges present themselves at interfaces with door openings and external landscaping, but these can be easily resolved via simple flashings and appropriate detailing to protect the integrity of the exposed insulation at the slab edge. There are numerous systems like this on the market presently from suppliers such as Kore Insulation and Isoquick.

**Strategy 01**

Insulated Raft Slab Foundation

A second strategy available for consideration is more of a hybrid of traditional strip foundations and the insulated slab outlined above. In this strategy, a conventional strip foundation, common in the UK, is installed with insulation extending down both sides of the sleeper wall (whether cast concrete or concrete block on a footing). This does not eliminate thermal conductivity through the base of the footing, but it does reduce thermal losses at the key interface with the ground floor. This option is popular as it is familiar with traditional foundation practices and can reduce concrete quantities. However, interfaces with other systems such as membranes and fixings are more complicated and difficult to control.

**Strategy 02**

Insulated Strip Foundation + Ground Slab

Where large amounts of concrete are considered undesirable or not possible for logistical or financial reasons, suspended timber ground floors with a ventilated air space below to control moisture from the ground are still possible to meet Passivhaus standards. With similar design challenges presented by the hybrid solution outlined above, care should also be given to maintaining integrity of the membrane and perimeter insulation at the interface with the required ventilation slots to promote airflow to the subfloor. This adds construction complexity, but if executed well, can provide a robust lightweight ground floor construction that performs well. Care should also be taken to ensure infiltration of cold air from the subfloor is eradicated through the inclusion of an airtight layer of appropriate board or membrane.

The three broad strategies outlined above if executed correctly, will provide a sound thermal bridge free foundation from which to realise your Passivhaus building. It is important to understand that the foundation is the first area where the thermal integrity and, by extension energy use, of the building is challenged. It is vitally important that clear dialogue of the building's energy strategy is communicated to the design team, in particular, the Structural Engineer and Architect, to ensure that all the hard work undertaken in the design and performance of the Superstructure is not lost at the key interface with the ground.