A Closer Look
We have picked out a few key points from each of the new parts of Section J starting with J1.3 Roof and Ceiling Construction.
Roof and Ceiling Construction
This appears to be a very short section which should be easy to get one’s head around. If you take out all the lines of text relating to other climate zones it has only 3 lines.
In Line 1 it starts out with “A roof or ceiling must achieve a Total R-Value greater than or equal to— “
That looks straight forward until you read the definition of Total R-Value…..
Total R-Value (m².K/W), for the purposes of Volume One, means the sum of the R-Values of the individual component layers in a composite element including any building material, insulating material, airspace, thermal bridging and associated surface resistances.
The main difference in the 2019 version is the inclusion of “thermal Bridging”. Thermal bridging has been given more importance in the new code but not more prominence. It is buried in the definition of Total R-Value. In the previous code there was a specific clause dealing with adding thermal breaks. Now it is part and parcel of the roof assessment.
In practice, this means you need to know the structural system proposed for the roof at the time of doing the assessment. This information is usually not available at that time, so a typical roof structure is assumed, and it is up to the applicant to check this is valid when the structure is designed later in the process.
Line 3 in this clause introduces a limit on the solar absorption level of the roof. This is very restrictive and if any but the lightest colours for the roof are proposed then a performance solution will be needed.
1. Be aware of thermal bridging and the need for structural information for the Section J assessment.
2. Note than the max solar absorption permitted in the DTS provision is 0.45 which means only a very light coloured roof is possible without resorting to a performance solution.
Roof lights cannot be more than 5% of the floor area they serve.
There is now a mandated U-value for all roof lights which is, 3.9.
The SHGC value is obtained from Table J1.4.
Key points: Roof Lights are not to be more than 5% of the floor area they serve.
U and SHGC values of the roof light must meet the requirements of this section and need to be certified for sign off at Occupancy Certificate stage.
Walls and Glazing
Walls and glazing are now assessed as one system, which is very different to the old code which assessed them as separate elements. The ABCB has released a calculator for this part which is available from their web site.
The code sets up threshold requirements for:
1. Max U-value of the total wall-glazing system
2. The min R-value for the wall component of the wall-glazing system and
3. The Max SHGC value for the wall-glazing system.
It may seem confusing to reference R and U values for walls. Typically, R-value is referred to for walls and U-value is referred to for glazing. But to assess walls and glazing together a common unit must be adopted and that is the U-value. Mathematically the R-value is the inverse of the U value. In other words, the R-value is the level of resistance to heat transfer while U-value is a measure of how easily heat can transfer.
As for the roof assessment, thermal bridging must be assessed. This requires the proposed wall structure to be known at assessment time. If it is not known, a typical wall structure must be adopted. This assumption also needs to be verified or reassessed before OC to make it possible to sign off.
Unfortunately, this is a very detailed clause and it is not possible to go into a lot of details.
Key points: The main thing to be aware of is the information necessary to be able to assess the wall-glazing system.
1. Structural make-up of the wall component
2. Shade elements over windows. Allow shade projections to extend past each side of the window by at least the same dimension as the projection distance.
3. Window schedule.
4. Floor plans
5. Elevations of all facades plus sections of representative locations in the building.
The information that drops out of the assessment will be the:
1. R-value of insulation,
2. R-value of the thermal break (if required), and the
3. U and SHGC values of glazing
The requirements for floors have been revised and there is a new requirement to add the effect of slab on ground or suspended floor with airspace under as the case may be.
Detailed edge beam conditions apply to slabs with in-floor heating.
Key Points: Floors need to be insulated.
The sub-floor space must be taken into account, when assessing floors.
We have covered most of the key changes in the Section J that relate to assessing the building envelope. We have developed an in-house calculator to make the assessment quicker and more accurate. The output from the calculator is then included in the report to show how compliance has been met.
There are more changes in Section J affecting other areas such as lighting and mechanical services. We will cover these in a future newsletter.
In the meantime, if you have and questions on the new code please don’t hesitate to contact us.