Effect of Window Size on Energy Consumption

Passive House Design centres its philosophy on a number of key principles. These are: ensure high air tightness, provide high levels of thermal insulation and optimise internal gains by providing large high performance solar collectors (windows) in the optimal solar orientation (i.e. towards the equator).

There are often issues of summer overheating and glare for consideration in the design as a result and this is often overcome by providing shading in some for or other that does not compromise solar gain in colder periods when it is most needed.

Large windows provide extra solar heat gains but they also have a much poorer thermal performance than other building elements which is a challenge for designers looking to achieve maximum solar gains without compromising thermal comfort.

This often means that walls that are not facing the sun typically have very small windows or none at all resulting in a lack of daylight and outside perspective for the rooms on these sides.  

A study was carried out in Gothenburg, Sweden in 2001 of 20 terraced houses built to passive standards built with large south facing windows and small north facing windows to maximise solar gain and minimise thermal loss (see below). The total floor area is 120 m², and the original window area is about 16% of this, which is more than what is recommended in the Swedish building regulations for sufficient lighting conditions.

The study ran simulations on how varying the window sizes (south reduced north increased) would affect the energy consumption and maximum power load to maintain an indoor temperature of between 23 and 26 °C. They used a simulation tool DEROB-LTH (Dynamic Energy Response of Buildings LTH) which was capable of modeling both the diffuse and direct sunlight effects on these buildings for the various window sizes as well as orientation and window type.

The simulations showed that neither the size nor the orientation had a significant influence on the heating demand in the winter, but is relevant for the cooling need in summer (see results below).

Therefore it is possible to enlarge the window area facing north and get better lighting conditions, provide more aspects for the user to view and to provide extra cooling in the summer to reduce the cooling load on the HVAC system. This means that it is possible to have greater flexibility when planning window areas and glazing layouts in low energy buildings and passive house designs.  

While the results of the study are particular to more northerly situated countries (approximately 60 degrees latitude) the principal findings of the study remain applicable to passive house design where heating demand is expected to exceed cooling demand.  

In conclusion over glazing can lead to increased energy demand for cooling and poor internal lighting and aspects for the user. Reducing the window area for solar collection slightly can be beneficial to summer cooling energy costs, lighting and views without compromising on winter head demand.

Refurbishing Irish Houses to Passive Standard Case Study in Wicklow, Ireland

 

The Passive House Popularity

Society has recently developed a growing consciousness of the need for energy efficiency for cost savings, sustainability and environmental protection. The passive house construction method delivers massive energy reductions ticking all boxes.

The passive method reduces home energy costs such that heating systems can be virtually eliminated through key considerations. Currently, there are over 25,000 units constructed in the EU and it’s spreading worldwide.

Contribution to Energy Demand in Ireland

While a welcome evolution in building standards the majority of the 1,500,000 houses in Ireland are massively energy inefficient while less than 1000 A rated (low energy homes) were registered in Jan 2011. Compared to the 2005 building regulations the passive standard is 80% less energy intensive and 94% more airtight. 

Arguably with minimal demand for new houses the passive standard is unlikely to contribute to national energy demand reduction for the foreseeable future. But recent refurbishment projects have shown that passive standards can be successfully applied to current stock giving them a new future proof lease of life. 

Case Study: Glencullen, County Wicklow, Ireland

Viking House is undertaking a retrofit of a 1960’s bungalow consisting of a 300mm thick cavity wall, strip foundations, and standard pitched roof construction. This is typical of current housing stock in Ireland, particularly those with a BER E rating or less. While the build consisted of a passive house standard extension we will focus on the existing section of the house for the purposes of this article.

Key Components

The 15 kWh/m2 heating demand for the passive standard was achieved by:

·         External wall -improved to U=0.11W/(m²K). 100mm cavity filled with EPS bead insulation plus 200mm of EPS external wall EPS insulation. 

·         Floor -improved to U=0.08W/(m²K). 400mm polystyrene, wooden floor

·         Roof -improved to U=0.11W/(m²K).  300mm cellulose insulation.

·         Windows –new triple glazed units U=0.08W/(m²K)

·         Heat Recovery Unit

Thermal bridging was a particular issue at the junction of the external wall and the floor as shown in the PHPP (Passive House Planning Package) graphic below.

The solution wa to extend the insulation 500mm below finished floor level (FFL) externally and 300mm in the cavity as per the next PHPP graphic.

As you can see the thermal bridge is minimized with the surface temperature now at 15.5°C which will eliminate condensation and add thermal comfort.  Note: as damp proof course was 300mm below FFL the issue became much simpler to resolve.

 

Other Applications

In the UK a number of social housing projects are being undertaken to the passive house standard.

Potential Problems with Residential Passive Retrofit

·    Achieving Optimal Orientation & Internal Layout for Solar Gains

·    Overcome orientation issues by adding extensions in right orientation

·    Solid floors and walls : insufficient insulation &  excess thermal bridging

·    Services passing through building envelope

Best Case Scenario for Passive Retrofit

·     Cavity wall & suspended floor construction. 

·     At least one major wall facing south.

·     Space to extend laterally and vertically if required (for solar gains etc).

First Steps Blog

The passive design process should be used to determine appropriate solutions for your particular region. Details are not typical throughout different regions as climactic and geographic conditions vary which affect the design e.g. Californian design techniques that were applied in Europe gave poor results.

The principles are based on Amory Lovins concept of reduced investment from energy efficient design. Energy efficiency means simplified HVAC systems through insulation, solar gains etc.  If peak load is kept below 10W/m2 then a separate heating system isn't required.

The methodology for passive house solutions always remain the same:

• Use passive technologies to reduce peak load demands 
• Use heat recovery for good indoor air quality and heating 
• Maintain high comfort levels
• Use affordable, cost effective, simple solutions to minimise energy demand. 
• Always provide insulation
• Provide shading in areas with high solar radiation
• Provide heat recovery in all climates for heating, cooling and dehumidification 
• Geothermal systems can provide heating and cooling depending on the region

The next step is a computer based parametric study of the design solutions checking energy demands, costs and indoor air quality.  A traditional design can be used to start, modifying elements step by step.  Also consider exterior element colours for solar absorption, longwave emissivity and varying internal mass on internal loads.  First principle models used are Derob, DYNBIL and Energy Ten. PHPP is a validated simplified model based on EN832. Contact the Passive House Institute for help in performing parametric evaluations.