The introduction of the NCC 2019 introduced heating and cooling limits. This meant that some designs would achieve 6 stars, even though the cooling limits stayed too high. 

The heating and cooling load limits will change depending on the NatHERS zone where the dwelling is located and what floor construction is nominated.

Now, we must achieve 6 stars and ensure that our heating and cooling loads are below the limits. 

A house energy rating software uses the Chenath modelling engine to calculate the energy usage of a dwelling for every hour of every day of the year. 

This enables us to gather data on how each zone or room performs. While we focus on the overall yearly performance of a room, you can also nominate any room and any time of the year to see its predictive performance.

With this information, we can analyse the weak points of the design layout and identify the improvements to be made.

The house energy rating software produces glazing values and insulation R-Values, which demonstrate how the building fabric performs. 

Insulation is measured as an R-Value. A high R-Value is better for an energy rating, until a certain point.

Glazing is measured by:

• U-Value measures the glazing or frame's ability to slow down the ambient temperature from outside to inside and vice versa. The lower the U-Value, the better the rating.
• SHGC-Value measures solar rays, directly from the sun. Warm climates benefit from a lower SGHC as it blocks the sun while cold climates benefit from a higher SHGC to welcome the sun in.

It is not uncommon to prepare an energy rating with the glazing results blowing out the budget dramatically. Having to provide a client with a report that specifies thermally broken double glazing is never fun, but is often unavoidable as 95% of the plans and permits have already been completed and the building permit is right around the corner.

When we nominate standard double glazing and maximum insulation and only hit 4.5 stars, we either need to look at the design to see what can be removed or we need to upgrade the glazing to get to 6 stars.

Cost-effective windows and insulation start with the design. Getting the design right and following good passive design principles is a surefire way to keep the window and insulation costs down.

If the extension and the internal works are over 50%, the whole building will need to achieve 6 stars. Regulation 233 of the Building Regulations 2018 stipulates that for any extension work over the last three years that is more than 50% of the original volume, the whole code applies.

You will need to apply for partial compliance, requesting an exemption for the existing walls, windows, and flooring due to the cost of bringing these up to the current 6 star code. The argument for your exemption is that it will cost far more in construction to perform these upgrades than the potential energy savings you may receive throughout the home’s life.

If the extension and internal works are under 50%, a two-rating method can be used. This enables us to perform a 6 star energy rating on the existing dwelling first with all the current conditions added, with an average result of 1.5 to 3.5 stars that can be expected. Then we perform another 6 star energy rating with the proposed extension.

The proposed volume needs to be 6 stars, but this is averaged out with the existing dwelling results.

This method is often more cost-effective as you only need to concentrate on the proposed extension and the existing roof cavity.