Filling The Gaps
The Thermal Performance of Framed Walls
The space for insulation in our walls is often significantly impacted by other materials being used to fill difficult areas, usually for construction and buildability reasons, and mostly where there are relatively small gaps between other assemblies.
In a site visit last week, we saw just such a detail constructed between two large window openings. The space, probably deemed onsite as too small for the practicable inclusion of insulation between the steel structure and the window apertures, was roughly filled with timber. We sent the image to Ruth Williams of InsideOut to discuss some of the implications for thermal performance and moisture management.
As we see, the practical and thermal make-up of our walls, often designed to include insulation between timber or steel framing, can often not be as ideal as first envisaged – and once a building is complete, we can no longer tell what is behind the internal lining.
Thermal Bridging
We all know that the percentage of timber or steel in our walls has serious implications for thermal bridging between the indoor and outdoor conditions. These can affect energy efficiency, thermal comfort, and moisture accumulation at the façade. Obviously, structural materials conduct heat more readily than insulation. The more timber or steel in a wall, the lower the effective R-value of the construction and the higher the risk of low temperature surfaces attracting condensation.
Percentage of Timber Framing
In 2020, BRANZ published research showing that whilst typical timber framing percentages have historically been assumed to be between 14-18% the reality for many timber-framed homes is 24-57%. [1] PlaceMakers more recently undertook a larger study, including both housing and commercial properties, and found framing percentages ranging from 23-64% with an average of 38%.
This finding has been taken up in the proposals for the latest update to NZBC H1 (Energy efficiency) where a consistent value of 38% timber framing is proposed. [2] Although still under consultation, this would mean that the H1 reference R-value for a wall would change to R1.6 rather than the existing R2.0 to match the expectations for a typical wall’s thermal resistance.
Total Thermal Resistance
NZS 4214 :2006 [3] is used to calculate the R-value of a wall including for the thermally bridged sections where timber or steel is present. We can easily look at how much the R-value changes with framing percentage, assuming a simple timber framed wall with:
fibre cement cladding
ventilated cavity
timber framing infilled with R2.0 insulation
plasterboard internal lining
Simply by changing the percentage of timber from 15% to 38%, as outlined above, we drop the wall R-value from R2.0 to R1.6. The R2.0 value would match the current NZBC H1 Schedule method minimum for housing. If we use a more realistic framing percentage we would significantly under insulate using R2.0 insulation although the resulting R1.6 would match the proposed H1 update.
If a steel frame is used instead of timber, including for the R0.25 thermal break required for housing (E3 AS1) [4], it would take a framing percentage as low as 7% to reach the current schedule value of R2.0 for the wall.
Building Performance Impact
Looking at the impact on a whole building, where windows and structure form part of the wall, high percentages of thermal bridging can occur in the wall space, and the windows and structure are already a weak point in the thermal envelope. For buildings with highly punctuated wall area, the wall construction becomes a thermally weaker area of the overall building envelope than we might expect, and details, such as the one we observed, could have a larger impact on the thermal and moisture performance of the building.
Energy Efficiency
Energy efficiency is typically factored into overall building performance, as it forms a key part of the building code requirements. However, wall R-value is only one part of the building envelope, the R-values of glazing, roofs and floors, and the g-value (solar heat gain coefficient) of glazing also need to be considered. The efficiency of a building is influenced by its geometry, the orientation and size of each façade, and the window-to-wall ratio, all of which affect surface area exposure and solar gain. Although it is important to identify construction methodologies that may weaken the performance of the building, such as uninsulated wall areas, these need to be taken into consideration against the whole design to assess their effect.
Thermal Comfort and Overheating Risk
Beyond the energy efficiency of the building, we often know little about the thermal comfort and overheating risk within smaller interior spaces. These conditions are influenced not just by air temperature, but also by the radiant environment. For example, cold or hot façades can make occupants feel cooler or warmer than the actual air temperature would suggest. Where the R-value of walls, roofs and floors is not what we might expect because of construction limitations, like additional thermal bridging, it is even harder to guess what the impact on comfort and overheating might be. We can heat or cool a building but not feel all the benefit of using that power if the façade lets us down.
Building Envelope Specification
There is a common expectation that the more insulated a building is, the better. However once other aspects of the design are set, such as the area to perimeter ratio, the building’s orientation, and the window to wall area ratios on each façade, it is often found that there is a sweet spot for glazing performance and insulation in each element of the building. We should remember that H1 isn’t a design tool and that the only way to really understand the energy efficiency and thermal comfort of the building is by dynamic thermal modelling.
So, we must ask why. The answer is that using the H1 schedule levels of insulation, with a moderate window to wall area ratio and solar gain, the extra wall insulation isn’t giving us more for our money. In this case the thermal envelope R-values are beyond the specification sweet spot for this geometry and orientation.
Of course, like investments, sweet spots can go up as well as down but, in many buildings, using the H1 schedule as a design tool can result in expenditure on materials that won’t improve energy or comfort and can indeed make these things worse than they need to be.
Condensation And Moisture Management
It’s important not to forget other important aspects of thermal envelope design, such as the management of moisture that is also affected by air-tightness, ventilation, and heating. These all need consideration alongside insulation.
There is a greater likelihood of condensation risk at thermally bridged parts of the building envelope where condensation can form on surfaces that drop below the dew point temperature of the surrounding air. Highly conductive materials, such as steel, increase this risk significantly – a risk that for housing and communal residential building is managed to an extent by a thermal break on the outer side of steel framing. For other building types and materials our building code does not specify acceptable solutions and doesn’t offer a verification method [4]. We can use hygrothermal modelling to assess performance, but the best way initially is to design out those cold bridges.
Alternative Insulation Options
In this article, we have been consistently discussing typical timber and steel framed buildings but there are other options. Reducing bridging by insulating externally is a good alternative or avoiding the potential for thermal bridging in as many locations with Structural Insulated Panel (SIP) construction could also be considered. Where either of these alternatives is chosen there is a more consistent thermal envelope with more even energy exchange and thermal comfort, and less risk of cold spots generating condensation risk, certainly worth some thought on your next project.
If you want to fill the gaps for better thermal performance and moisture impacts in your next project, get in touch with us today.
With thanks to Ruth Williams, director of InsideOut, for her summary of the issues. Please feel free to contact her with questions regarding energy efficiency, optimising building envelopes, thermal comfort, overheating, and moisture management. She is a specialist in dynamic thermal and hygrothermal modelling and the effects of design choices.
027 508 8024 ruth@insideout.co.nz www.insideout.co.nz
References
Measuring the Extent of Thermal Bridging in External Timber- Framed Walls in New Zealand. Verney Ryan, Guy Penny, Jane Cuming, Ian Mayes, Graeme Baker, Beacon Pathway Inc. – t BRANZ ER53 [2020] https://d39d3mj7qio96p.cloudfront.net/media/documents/ER53_Thermal_Bridging_in_External_Timber-Framed_Walls.pdf
H1 consultation 2025 - Insulation requirements in housing and other buildings. MBIE Building Performance https://www.mbie.govt.nz/have-your-say/changing-building-and-home-insulation-requirements
NZS4214:2006 Methods of determining the total thermal resistance of parts of buildings. Standards New Zealand https://www.standards.govt.nz/shop/nzs-42142006
NZBC E3 AS1 Internal moisture. MBIE Building Performance https://www.building.govt.nz/building-code-compliance/e-moisture/e3-internal-moisture