New Zealanders aren’t big on insect screens I’ve observed and I don’t understand why. They are really good at their core job, keeping interior spaces (largely) free of biting, whining insects. But screens also reduce solar gain through the window glass, so they have the potential to reduce overheating. Given the widespread problem of overheated homes and the need to retrofit the vast majority of New Zealand’s housing stock, it’s good to consider all available solutions.
Unlike permanently mounted external shutters or louvres, insect screens are removable: fitted in spring and sent off to storage in autumn. They are also cheap, generally easy to retrofit (depending on the window style) and virtually invisible when you’re looking out from inside. These are pretty appealing features. Insect screens do need cleaning periodically though (but dirty screens, like dirty glass, reduce solar gain so there’s an excuse for the hottest part of the year.)
Modelling impact of insect screens
Model their impact in PHPP in one of two ways. If the screens are permanently fixed, add them as permanent shading in the Shading worksheet or change the g-value modelled for the glazing in line with their calculated impact. Seasonal screens should go in the temporary shading column.
Insect mesh, massively magnified.
Mesh sold for the purposes of insect screening is simple stuff. In this case, the impact on solar gains is roughly proportional to the open area of the mesh. The openness factor is a crude approximation but one that as a certifier, I am willing to consider. It’s a simple case of noting the diameter of the wire and the gaps between them. Work it out from a sample if the data is not listed on the product.
Take as an example, a specific product: black fibreglass mesh external screen, 18×16 per square inch (sorry) with a strand diameter of approximately 0.38 mm (I converted this from a gauge measurement). This results in an openness factor of 0.58 or 58%.
The IAC ratio is a step up in sophistication. You may be able to find the specific product you’re specifying in a table of calculated values. Theoretically the supplier could provide this. Expect the number to be slightly different from the openness factor. The IAC ratio is obtained by dividing the shading by the solar gain without the shading (ie divided by 1 because the open area without the shading is 100%). In the case of the product described above, the IAC ratio is 59%.
Provided there is reasonable margin in the design, using the openness factor to estimate the impact of the screening on solar gains should be acceptable for Passive House certification, but confirm this on a project-by-project basis with your certifier.
The IAC ratio is a relatively crude approximation and more accurate predictions can be made by simulation. designPH should work or engineers might have access to a tool from ASHRAE tool, ASHWAT (ASHRAE Window Attachment), currently implemented in an enhanced version of the ASHRAE Loads Toolkit.
Pros and cons of micro-louvres
Then there are more complicated screens, like micro-louvres, which are designed to do multiple jobs: reduce solar gain, prevent rain from coming in and provide privacy, on top of letting air in and keeping insects out. Micro-louvres may be fixed or operable. Here’s one example:
Micro-louvre showing clear view while at the same time fully blocking the sun coming through. Note the shadow on the windowsill, it’s as if the louvre were opaque! Cool, literally. But only on the equatorial facing (ie north in New Zealand) windows, during the summer. © 2024 Sustainable Engineering Ltd.
All that extra benefit comes with some disadvantages. You can see out very well if you are looking straight out the window. But when looking through the window at an angle (for example because you want to look down to the street while standing next to the window), the louvres will obscure your view of outside. Window orientation also has a big effect. These work best on a north-facing window (in our Southern Hemisphere). The low angles at which early morning and evening sun hit eastern and western windows respectively will go right through the louvres. Shielding from sun on eastern and western faces requires external blinds, fixed louvres like the one above won’t help.
Precise calculation of the impact is reasonably complicated to get right. For complex screening like a micro-louver, I’d recommend modelling it in designPH. Scale up the blind and window by 10 times to prevent SketchUp having issues with very small items. This is pushing at the edge of what designPH is designed for so be sure to sense check your results. With external shading that is very angle dependent like a micro-louvre, the window orientation will significantly impact the results.
Reference:
“Solar Gain through Windows with Shading Devices: Simulation Versus Measurement,” Kotey et al. ASHRAE RP-1311, 2009.