Rational Design

Building energy compliance made simple
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Do you need assistance with doing fenestration calculations for your building? Or are the calculations forcing you to use expensive double glazing or coated glass? There are alternatives!

We can save you time and money!

Often your proposed building works better than the requirements in the SANS 10400-XA:2021. And if not, there may be a few inexpensive changes you can make to ensure that it will. Expensive glazing is often not required. And as professionals registered as ‘competent persons’ we can use certified modelling software to assist you in demonstrating this.

How the

Rational Design Works

If the fenestration calculations show that you need expensive high-spec windows, you have an option to submit a rational design report. In this report your proposed building is compared to a similar reference building which complies to the standards of having glazing of 20% or less. It is likely that your designer or architect has saved energy waste in other areas or in other ways, and therefore your building works as well or even better than the reference building. If not, we can easily find inexpensive ways for you to do so.  With this report you can save on materials such as high-tech glazing, and you can find other ways to save money, and earth’s valuable resources.

What are

Fenestration Calculations

To satisfy the requirements of the SANS 10400-XA:2021, you need to have a ratio of fenestration area to habitable area equal to or under 20%. After that there is a sliding scale of specifications you need to apply to your windows and doors satisfying required shading and/or U-Values, and Solar Heat Gain Coefficient values. The fenestration calculations show you exactly what you need to specify. These are required by local authorities to obtain approval.

What we do

Our Services

Rational Design Production

in cases where you would like to avoid double glazing.


on changes you can make to better the energy efficiency of your building.

fenestration calculations

we determine the size, placement, and type of windows in a building for optimal energy efficiency, natural lighting, etc.


Compliance Certification

SANS Form 2 Duty 19 – XA


About Us

Tommy is an architectural designer with more than 15 years’ experience designing buildings to comply with the South African National Standards (SANS) including the SANS 10400-XA Energy Efficiency in Buildings.  

He has experience with buildings all over southern Africa, especially in the difficult Cape area where we deal with the ‘Southern Cape Condensation Problem Areas’ (SANS 10400-XA:2021 5.3.6). As a cost and environmental conscious company, we work hard to find ways to safe energy without costing our clients a fortune in extra materials.

What’s the


Please contact us for a quote. A typical house project where a rational design is required costs between R5,000.00 and R10,000.00 depending on the size and complexity. Costs for consulting and fenestration calculation vary depending on the time required and changes made.

get in touch

Contact Us

Please feel free to contact us on email, info@gorational.co.za or via the form to set up a virtual or in person meeting to discuss solutions for your project.

Or give us a call on 074 174 2583