Interior Shades for the Passive House

The Passive House Summer Shade

The Passive House is designed to achieve maximum Solar Heat Gain by orienting its largest windows facing south in order to provide at least 50% of the heating demand. In the summer months shading of these windows is necessary and exterior shades and blinds are excellent solutions to balance out the energy needs for the Passive House.

Even though exterior shading solutions are preferred, they may not be possible for a variety of reasons, such as, “we never planned for exterior shades, now it’s too late”, or “we don’t have a budget for motorized shades”, or just simply “we don’t want exterior shades, we want interior shades”.

The challenge with interior shading solutions is to find a shade fabric that provides the best SHGC number. To understand how to do that, let’s look at how solar shade fabrics are measured for performance. The chart below explains how performance in measured by looking at three numbers: TS for transmittance – how much solar energy passes through, AS for absorbance – how much solar energy is absorbed and RS for reflectance – how much solar energy is reflected. These three numbers equal 100% of solar energy.

First let us look at the performance of an exterior shade. This way we have a standard to work with. As you can imagine exterior shades have many advantages over interior shades. One major one has do with heat absorption. With the shade fabric on the outside, and solar energy absorbed by the fabric will dissipate into the atmosphere. See below drawing.

For some concrete numbers let’s take a look at Serge Ferrari’s best exterior roller shade fabric, the Soltis 92 and we find the following results:

White is color #92-2044 and black is color #92-2047. We want to use these two colors as they show the extreme examples. All other colors fall within their range. The chart shows the three categories: TS, RS and AS the total sum equaling 100% of solar energy. TV stands for light transmitted and the way to measure glare reduction. The last two columns show the SHGC reduction for an interior or an exterior installation.

The performance for the #92-2044 in white is as follows: 20% of the solar energy is passed through the fabric (TS), 70% is reflected (RS) and 10% is absorbed (AS). As an exterior installation this color will allow 14% of solar heat gain verses 34% when mounted on the inside.

The black fabric, #92-2047 has significantly different results. 5% of the solar energy is passed through the fabric (TS), 8% is reflected (RS) and 87% is absorbed (AS). As an exterior installation this color will allow 8% of solar heat gain verses 53% when mounted on the inside.

Take note of the difference in the AS number. 10% for the white verses 87% for the black. This works well for the black color on exterior installations since we do not care about the heat absorption and as a result the black color reduces solar energy from entering the window by 92%.

So the bar is high to achieve a 92% reduction in solar heat gain with 0% absorbance. We can start by looking at a popular interior solar shade fabric: Mermet’s E-Screen in 3% open weave. As you can see in the chart below, the number in SHGC for DG (double glazed glass) ranges from 33% in white/white to 56% in charcoal/charcoal. The corresponding numbers for Absorbance goes from 11% in to 91%.

These numbers are not even close to the Soltis 92 performance. In order to get closer we need to work with reflective solar shade fabric. This incredible performer has a thin metallized coating on the street side of the fabric. This allows the sunlight to reflect off the fabric and back out though the glass. The result from the reflective backing is low transmittance, highly reflective and good absorbance numbers.

When a reflective fabric is analyzed, the first thing you notice is that the difference in the numbers between the light and dark colors are negligible. That is because of the thin metallized coating applied to the fabric negates the fabric color performance differences. Therefore, the chart below shows only light colors for simplicity sake.

The white/white transmits 7%, reflects 60% (this is high because of the metallized backing) and absorbs 33% of solar energy. There are great results for the reduction of glare with a 94% (reciprocal of 6) score. The most important number is in the last column, SHGC of 15%. This is more then 2 times better then a non-metallized interior shade, but not quite as good as an exterior shade which boasts results of 8% SHGC.

You may notice the big difference between the exterior and interior numbers are for absorbance (AS). The Soltis 92 has no affect on the AS number since it is an exterior installation, while the interior shade AS number is 33%. Eventually that heat radiates off the fabric though at a reduced rate. However, the SHGC of 15% is calculated with the AS number considered.

The Passive House is designed to achieve maximum solar heat gain by orienting its largest windows with southern exposure with the intent of providing at least 50% of the building’s heating demand. While this works well in the winter months when the low angled sun radiates heat into the space, this solar radiation becomes excessive during the summer months and needs to be shaded. That is why the engineers at InSync Solar have created The Passive House Summer Shade. An ideal low cost alternative to motorized exterior shades, which will provide the best SHGC reduction for an interior shade.