Home  »  Resources  »  TPA FAQ's

TPA FAQ's

Search a range of topics to help troubleshoot common issues encountered by Thermal Performance Assessors.

Can’t find the answer you’re searching for? Contact us at TPASupport@designmatters.org.au to submit your question to the support desk.

How do I make assumptions in FirstRate5 about window awnings?

SUBJECTS: Awnings, FirstRate5
REGIONS: All States

QUESTION:

I’m curious to know a bit more about the assumptions made in the thermal software package, FirstRate5 about window awnings. Obviously, there are many different awnings on the market. Could you advise what the packages assume (in terms of projection, vertical offset, shading %, etc)?

The reason I ask is that I’m modelling a dwelling and we want to see what type of awning would best shade for the morning sun (which makes a room quite hot).

ANSWER:

My understanding is that awnings in FirstRate5 are generic retractable type on offset brackets or similar, that have heavy shade/blockout fabric. If nominated it is assumed that they are operated as for internal blinds to assist thermal performance at appropriate times. They are fully retractable when passive gain is beneficial and extend down vertically to shade glass from low angle solar exposure.

Accordingly, not all varieties of awnings will be modelled with exact precision but we know that vertical shading on east and west windows in summer that ensures the sun does not hit the glass is highly recommended. Sun hitting those windows in cooler months is beneficial.

Any horizontal shade device can be dealt with as an eave over windows that can have seasonal shading percentage defined.

How do I apply a skillion section for a roof?

SUBJECTS: Roof Vents
REGIONS: NT

QUESTION:

I have been asked to provide NatHERS certificates, to a number of coastal NT remote area housing projects (climate zone-1).

The skillion section below, shows roof ventilators with a damper. I have asked the Architect for clarification, on the function of these vents.

If they do penetrate the ceiling and as they will have sealable dampers fitted, can they be assessed similar to an exhaust fan sealed?

ANSWER:

NatHERS does not deal comprehensively with thermal chimney ventilation.

If this vent is intended for that purpose the benefit will not be fully modelled by the energy rating.

At the moment a sealed exhaust fan is the best fit data selection

How do I achieve acceptance with an RBS for an energy rating for an extension to an existing dwelling?

SUBJECTS: DTS Extension, FirstRate5
REGIONS: Victoria

QUESTION:

I am in discussion with an RBS regarding acceptance of an energy rating report for an extension. The proposed extension exceeds 50% of the original dwelling floor area and volume and has been rejected by the RBS.

I understand that in these circumstances approval can not be granted on the volume calculation formula alone, and that additional evidence is required to allow partial compliance which was provided in our report. As we have used this report format in the past which has been acceptable to other RBS, could you please advise if there has been a change in the regulations or the required method of assessment?

In addition I would also like to know if a NatHERS assessment is classed as a ‘Performance Assessment Report’ as requested by the RBS, and what is the position if the project were assessed under DtS and produced a ‘Fail’ condition in the Glazing Calculations Table.

ANSWER:

The clear understanding is that the logic of the adjustment formula means that is can be used for all alterations and additions projects to take into account the under-performance of the existing portions that are deemed unreasonable to upgrade on a cost benefit basis.

NatHERS Ratings are one of the two Deemed to Satisfy pathways for demonstrating compliance of building fabric thermal performance. The other being Elemental DtS.

Unfortunately the issue you have raised is a known to both Design Matters and the VBA, however it requires the VBA to provide clarification ultimately – something we have been pushing for repeatedly.

It is likely that the only way to overcome this particular enquiry as it currently stands would be to have someone at the VBA advise the RBS that the adjustment formula is suitable for this project. You may wish to head down the DtS path in this instance.

What is the best way to provide an energy rating for an extension to an existing dwelling?

SUBJECTS: DTS Extension, FirstRate5
REGIONS: Victoria

QUESTION:

I have been asked to provide an energy rating for an extension to an existing dwelling. They are proposing to add another two bedrooms plus a rumpus room. The extension is 68m2 in size which equates to approximately 37 % of the existing building.

Can you please guide me on the best way to approach this assessment?

Do you have a template or information on what I need to provide for an elemental deemed to satisfy the report? May I have some guidance on how to complete a deemed to satisfy report?

ANSWER:

The response will depend on the level of interest the client has in knowing the detailed thermal performance of the house.

If simple regulatory compliance is the desired outcome, then an elemental Deemed to Satisfy report will be quick and cost effective.

If a NatHERS DtS approach is required to actually quantify the energy demand profile, then before and after ratings employing the VBA Practice Note 55 adjustment formula to determine the required star rating will deliver a suitable result.

Design Matters has available to members in the TPA Practice Notes section a Sample DtS Report, TPA 008-2015.

It is important that anyone using the form has developed an understanding of the benchmarks in place for Elemental DtS compliance thresholds.

How do I create walls in split level houses?

SUBJECTS: FirstRate5, Walls
REGIONS: All States

QUESTION:

When you create a new wall below the wall dividing the split level (reducing the above wall height to the remainder of the lower level ceiling height and making the new lower wall the balance of the height to the lower floor level) and then do similar for the upstairs as per the user manual you are then referencing a level as the 0 point.

What I’m wondering is can it be correct to use either level as a reference level? For instance, set the new bottom wall as zero and reference up from the lower split level or set the new bottom wall as negative and reference from the higher split level?

Do I need to go into the zones/walls and set the remainder of the walls in the non-reference level to the new datum? Common sense tells me I should, but the user manual doesn’t mention it.

I do not need to worry about adjusting the base height of the remaining walls on the changed level?

ANSWER:

The thing to keep in mind to assist in dealing with split levels is that NatHERS programs measure two dimensional heat flows. This means that where a wall is split into horizontal bands a lower portion of wall will have connection to sub-floor the mid-section will be connected to the wall in the adjoining zone and an upper portion will be connected to roof space or external air.

Accordingly the base height does not hold any relevance for split walls between zones and may be ignored.

Following the procedure in the FirstRate manual should achieve the required relationships.

I can’t seem to find the Heating and Cooling loads for Zone 65 at Cargo NSW?

SUBJECTS: Heating and Cooling
REGIONS: NSW

QUESTION:

I can’t seem to find the Heating and Cooling loads for Zone 65 at Cargo NSW. The NatHERS list has n/a for that zone?

ANSWER:

The ABCB Standard NatHERS heating and cooling load limits document clause 2.1 Limitations, indicates that the limits do not apply in NT, NSW, TAS, parts of QLD and WA. Accordingly n/a appears against those climate zones. In NSW the BASIX provisions apply.

I would like to know if NatHERS can assess Class 1b dwellings?

SUBJECTS: Class 1b
REGIONS: All States

ANSWER:

This response is dependent on the software used.

NatHERS Deemed to Satisfy approach is suitable for all Class 1 buildings.

A class 1b building is a one of the Class 1 building types. A class 1 b hostel cannot exceed 300m2 and is envisaged to be a residence for a number of occupants (12 max) with a number of bedrooms plus shared lounge and kitchen. Accordingly it will have a similar thermal performance profile to a large house. If a Class 1b building design indicates multiple kitchenettes then a NatHERS will not be suitable to substantiate compliance.

HERS is for Classes 1a, 2 & 4. If you want to use the HERS software then it will need to be used as a Performance Solution. If there are other kitchenettes, as in a Class 1 dwelling, then they become Day Time zones.

How do I address a request for double glazing for a BESS Assessment?

SUBJECTS: BESS, Windows
REGIONS: All States

QUESTION:

I am negotiating with an ESD Officer of a metropolitan Council and they are insisting on double glazing for a BESS assessment.

The client is hesitant to pay for another NatHERS rating and would just like an idea of how much of an improvement would be achieved?

ANSWER:

There is on occasion crossover in the BESS assessment between building fabric energy performance and IEQ measurement. The IEQ pass mark gets a boost from ticking double-glazing.

This is regardless of whether the building fabric achieves a suitable level of performance at 6 Stars, or even 6 Stars plus 10% required by some municipalities, by applying good thermal design principles which may include single-glazing.

Double-glazing will usually deliver superior results but with the wrong glass can impact negatively by comparison to single-glazing for both thermal and acoustic performance.

Unconditionally demanding double-glazing could potentially cause in warranted housing affordability impacts and deliver inferior performance outcomes.

In some instances it might be prudent to run both scenarios to determine the outcome to advise the Client, then an informed decision can be made before cost implications of revised certificates are endured.

How do I best model a dwelling with custom skylights?

SUBJECTS: FirstRate5, Skylight
REGIONS: All States

QUESTION:

We are currently modelling a dwelling whereby the client is looking to use custom skylights for the top level. The manufacturer has provided the following thermal performance values for the units (U = 2.0 , SHGC = 0.50). My understanding is that as the only way we can model this within energy rating software for NatHERS is to use the default double glazed skylight, which has the following thermal performance values (U = 4.22, SHGC = 0.72).

Our issue arises as there is quite a large jump in the performance values that can be modelled when compared with the values that would actually be used, and this is reflected in the rating. When the model is rated with the default double glazed skylight values we get a rating of 5.4-Stars. However in a test we completed with velux fixed skylights (U = 2.6 and SHGC = 0.24) the rating jumps to 6.6-Stars. We therefore believe that the actual rating using the preferred skylight would fall in the middle and would therefore meet the minimum 6.0-Star requirement. As there are limited options for skylights, is there anything we can do test/show that the proposed design compiles?

ANSWER:

The options for roof windows in FirstRate5 software are very limited.

The best way of getting the closest match possible for the intended product will be to select a combination of 2 or more of the options available and allocate proportionate areas of each option to approximate the overall performance of U-2.0 SHGC 0.5. The result will necessarily be conservative for U-value and reasonably accurate for the SHGC number.

A summary of the procedure should be included in the additional information comments panel so that the NatHERS Certificate contains a clear explanation as to why the modelled values are different to the specified product.

How do I model default double clear skylights in FirstRate5 for BASIX compliance?

SUBJECTS: BASIX, Skylight
REGIONS: NSW

QUESTION:

We have a NSW project that has been assessed for BASIX at DA stage and has been modelled with the FirstRate5 default double clear skylights (U=4.22 & SHGC=0.72). We are now reviewing the design for CC and the client would like to use the Velux FCM product (U=2.9; SHGC=0.28). There are no other changes in the design between DA and CC that would warrant re-modelling so we are trying to determine if re-modelling would be required due to this skylight product change.

The BASIX Thermal Comfort Protocol (refer Section 6, Table 3) states the following for skylights, in particular clause (d), which would imply that re-modelling would not be required due to a change in the skylight product, as long as an approved skylight was incorporated into the model.

ANSWER:

NatHERS protocol provides the understanding that window substitutions are acceptable if U-value is equal or better and SHGC is within plus or minus 5%.

Without seeing the detailed design it is not possible to provide a categorical response.

It might be effective to run the calculation and verify the effect. Making the change could potentially have 0.1 star change, and will change heating and cooling loads, which might impact on BASIX (or the new ABCB) caps.