Thermal Modelling: How Window Treatments Improve R-Values in NZ Homes Redgraves Curtains, Blinds & Shutters

R-values, U-values and why windows matter

R-value is a measure of thermal resistance: the higher the R-value, the better a construction element is at slowing heat flow. U-value is its reciprocal, expressing how readily heat passes through a material or assembly; lower U-values mean better performance.​

In New Zealand, typical window R-values are much lower than those of walls, floors and ceilings, which is why glazing is a primary focus in energy modelling and H1 compliance. Current guidance suggests that even with good frames and low-E double or triple glazing, windows often sit between about R0.3 and R0.8, still significantly weaker than insulated walls.​

How thermal modelling includes window treatments

Thermal modelling software used for H1 calculations can incorporate the combined effects of frame, glass and additional components like internal coverings to estimate an overall R-value for the window system. By treating window furnishings as an additional resistance layer, modelling can demonstrate how a carefully detailed curtain or blind reduces heat transfer and improves annual energy performance.​

Designers can compare scenarios—with and without specific window treatments—to quantify changes in heating and cooling loads. This allows for more informed decisions on where to invest: higher-spec glazing, optimised shading, or high-performance furnishings such as thermal-backed curtains, honeycomb blinds or shutters.​

The impact of different window treatments

Not all window coverings are equal from a thermal perspective, and their effect on R-values depends on design, fit and materials. Key principles include creating a still air layer between the window and the treatment, minimising gaps, and using insulating fabrics or structures.​

• Heavy, lined curtains (especially when full-length and well-fitted) add substantial resistance and are widely recognised in NZ guidance as an effective way to improve window performance.​
• Honeycomb/cellular blinds can deliver R-value improvements in the range of roughly 0.6–0.7 for the blind itself, dramatically outperforming bare single glazing and meaningfully supplementing double glazing.​
• Well-fitted internal shutters also create a robust barrier and sealed air cavity, adding to the effective R-value and assisting with both winter heat retention and summer heat exclusion.​
  
  

Approaches for architects and energy assessors

For architect-led homes, it is increasingly common to consider window treatments at concept or developed design stage rather than leaving them as an afterthought. By nominating performance-focused furnishings, designers can model realistic operating conditions and potentially avoid overcompensating with more costly upgrades elsewhere in the building envelope.​

This might include documenting assumptions such as “full-height lined curtains with pelmets” or “cellular blinds with specified thermal performance” in the thermal model, alongside glazing and frame selections. For high-performance projects, the incremental R-value gained from window treatments can help bridge the gap between baseline compliance and genuinely comfortable, low-energy homes.​

How Redgraves supports thermal performance

Redgraves’ custom curtains, blinds and shutters are designed with both aesthetics and thermal performance in mind, using quality linings, interlinings and carefully considered hardware to enhance the effective R-value of window systems. Products such as thermal-backed drapery, tightly fitted Roman blinds and insulating honeycomb or plantation-style solutions can significantly reduce winter heat loss and summer heat gain when modelled as part of the building envelope.​

For architects, energy modellers and homeowners, working with Redgraves means access to practical guidance on how specific window treatments will support energy efficiency goals, comfort and compliance with evolving NZ Building Code requirements. By integrating window furnishings into thermal modelling early, projects can achieve better-performing, more resilient homes without compromising on design intent or interior aesthetics