April 3, 2018 Dennis Guichard

The truth about low-e glass

In colder climate locations single-glazed low-e glass can sometimes be used to reduce heat loss in winter. Normal clear glass allows most of the heat radiation coming from inside your house to pass through to the outside. By using low-E glass this radiant heat is largely reflected back into the room, thereby reducing heat loss through windows.

Low-e glazing can also reduce the amount of the sun’s heat and UV light coming in through the windows and thus reduce overheating and fading of furnishings in summer but can also reduce the amount of free passive heat your house can soak up in winter.

BRANZ research (New Zealand) shows low-e glass can reduce heat loss through single glazing by up to 25% (in a super cold winter climate zone), and this can be a useful statistic for making it seem like the product has tremendous value in a South African (or any general) context. The truth for us however, substantiated through our now four years and hundreds and hundreds of real-time local-specific building science analysis, is that on a well-designed (optimally shaded) dwelling fenestration actually plays a very small part in the holistic gain and loss of heat.

In a warm climate zone like Durban or Nelspruit the use of low-e glass might only influence the holistic performance of any moderately glazed dwelling by perhaps 3-5 % overall. So the reality, translated into real-term values, is that a client might invest a R200,000 capital cost overspend (over rather just using single-glazed clear glass) for a meagre R1,400 per annum operational cost saving on hypothetical heating and cooling. So that gives the capital investment a 143-year payback period before it is even able to pay for itself and substantiate its cost!

In a colder winter climate zone like Johannesburg, Pretoria or Cape Town the use of low-e glass might only influence the holistic performance of any moderately glazed dwelling by perhaps 5-9 % overall. So the reality is that a client might invest a R200,000 capital cost overspend (over rather just using single-glazed clear glass) for a meagre R2,500 per annum operational cost saving on hypothetical heating and cooling. So that gives the capital investment an 80-year payback period before it is even able to pay for itself and substantiate its cost!

On the downside (aside from the substantial cost) single-glazed low-e windows can also suffer from increased condensation which then largely negates the effectiveness of the low-e coating. As the coating reflects heat, the glass itself becomes colder and condensation is more likely to form on your windows. Like condensation on a bathroom mirror, the condensation means the glass surface is no longer reflective. While condensation is present, the low-e surface provides very little benefit over normal glass. Once the condensation has dried up, the performance of low-e window film is restored.

Low-e glazing falls into two broad categories: soft coat and hard coat. Both applications involve depositing a thin, transparent coating of silver or tin oxide on the glass surface to allow short-wavelength sunlight to pass through while blocking long-wavelength heat radiation. The difference between the two coatings lies in their application, which affects the glazing performance and durability.

Soft (or sputtered) coat: is the most common type of low-e glazing. In this application, the layer of silver is deposited onto the glass through a sputtering process after the glass has been manufactured. Although it provides the best U-value available, this type of coating is fairly delicate and has to be protected within an insulated (double-glazed) glass unit to prevent scratching.

Hard coat: Added during, rather than after, the manufacturing process, pyrolytic or hard-coat low-e glazing incorporates a thin layer of tin oxide into the glass while it is still hot. Applying the tin at this stage welds it to the glass, resulting in a more durable coating. Hard-coat glass can be used in single-glazed windows with better resistance to scratching but their emissivity is not as low as that of soft-coat glass. Because the glass has a higher solar heat gain coefficient (SHGC) it works well for houses that rely on passive solar heating.

Low-e windows may generally also block radio frequency signals so buildings without distributed antenna systems may then suffer degraded cell phone reception.

In summary, given the relatively low impact that fenestration has been shown to play on holistic building performance in the South African context (3-9% of holistic building performance), single-glazed clear glass is always the substantiated (cost vs return on performance) optimised building solution given the substantial capital cost saving (and paltry return on performance) that can rather be invested into items that do indeed rather provide a real bang for your clients buck (LED lights internally or a most efficient hot water heating system).

The key to great architecture remains vested in great architects designing great architecture by getting their first design principals correct… back to basics… shading is king… optimally placed shading from screens, verandas and roof overhangs to keep high altitude sun out of windows and off the thermal mass of walls in summer yet still indeed permitting solar ingress for passive warming of buildings from low altitude winter sunshine.

On my soapbox I will forever provocate that Regulations and legislation should encourage great architecture, not hinder it. Competent rational design energy modelling with a thorough location and altitude specific analysis of how any building design might perform is thus an invaluable vehicle for ensuring that optimised cost and energy efficient architecture is ensured for your clients.

Let great architecture be king…!

Leave a Reply

Your email address will not be published. Required fields are marked *