Skylights: can be evil

Skylights generally make the space below them too hot or too cold, reduce thermal performance of the overall building much more than alternatives and can have mould issues. So why would you?

Originally, I titled this article, ‘Skylights are evil’.

I’ve backed away from that punchier headline because if:
(a) you know what you are doing;
(b) you install a triple glazed skylight (or better);
(c) you can get the performance data you need; and
(d) design the install details well

then it’s not quite evil. Just difficult.

You need a very high-performance skylight to provide good thermal comfort. As an aside, you are almost always better off using a clerestory.

We’ll get to the numbers, but first it’s important to point out that the thermal comfort criteria evaluation in PHPP only considers the winter overnight cold conditions when there’s no sun. That is, it’s concerned about winter chill.

In the summertime, when sun directly impacts on the glass, that skylight is probably going to be even more uncomfortable, but PHPP does not check overheating. The too-hot problem could be fixed with exterior blinds, but those are rarely paired with skylights. As for internal blinds, they aren’t effective with high performance glazing as the heat is already inside the building before it meets the screen.

Skylights don’t only create issues with thermal comfort. There are also quantitative problems when they are going into high-performance buildings, eg Passive House. There are glass, frame and the installation details to be calculated but just obtaining the performance information is too often difficult.

Glazing

The performance of double and triple glazing always decreases as it is tilted from vertical. For example, the table below is of a typical skylight: double or triple glazing with 16mm of 90% argon gas space and a very low g-value/SHGC-value, as would be called for in a skylight. As double-glazing is tilted from vertical to horizontal, energy transmission through the glass increases by 64%! The effect is less with triple glazing, but still significant, with energy transmission increasing by 50%.

How? When the glazing is vertical, one large circulation loop forms in between the glass panes. But when it’s tilted, lots of smaller circulation loops form and that produces higher heat transfer.

Triple glazing suffers to a lesser degree because the circulation loops are split in half, but 50% worse is quite a bit in a high-performance building. And that’s just the glazing.

Frames

The skylight frame performance (measured in Uf) can vary significantly even for the same brand. Good quality skylights can be imported but it’s hard to tell the good from the bad as performance data is so hard to get. Even a triple glazed skylight can cause a project to fail certification (Passive House, EnerPHit or even PHI Low Energy Building); the frame must meet the Passive House Institute (PHI) fRSI=0.25 minimum temperature criteria for your climate region. Otherwise, the risk of mould is too high. That’s why you should care even if your building is not going for certification.

Installation details

Current installation practice sees a skylight sitting atop an upstand of 45mm thick timber, with a nice flashing kit so the roof doesn’t leak. This is not designed for thermal performance. It’s suitable … for a milking shed. Maybe.

Trying to retrofit this detail to improve performance is quite difficult. Sticking 10mm of foam insulation behind the plasterboard can sometimes, just barely, meet the PHI fRSI=0.25 minimum temperature criteria, at least in Auckland and further north. Colder climates need a better install detail, whether you’re planning to meet the certification target or not because the solution to mould is not poisonous paint (the stuff with the anti-mould additives).

Performance and materials data

Unfortunately obtaining the performance data from many manufacturers is like pulling teeth. A rough overall performance number and maybe the glass performance (but not the standard it’s calculated to or angle) is the best you can hope for the first time you ask. For weird reasons I am still figuring out, the manufacturers (or rather the importers) won’t tell you what glass is actually being used or what the frame performance numbers are. Without this information, you are stuck guessing at the performance. Additionally, the flashing kits they stock are designed for current (poor) practice and push you into a low-performance install. Push back.

Advice

Our advice, if you absolutely need a skylight?

1. Use a triple glazed skylight. Even in Auckland.
2. In Queenstown and Central Otago, seriously consider quad-pane.
3. Do not pay for them until you have the required performance data you need.

This would include an actual glass data sheet per EN673/EN410 (preferably for the angle at which the skylight will be installed), glass, brand and model of edge spacer, a frame performance calculation per ISO10077-2, and an installation detail and flashing kit which will allow you to meet the Passive House Institute fRSI=0.25 minimum temperature criteria.
If you can at least obtain details on the exact glass/spacer and frame details/materials, we can calculate all of this for you, but it will be an additional cost to the standard certification fee.

A clear alternative

A clerestory is a high section of wall that contains windows above eye level. It delivers much the same benefits as a skylight: it brings in daylight and fresh air and can be used to dump heat.

In my opinion, clerestories are much better than skylights for three reasons:

1. much better thermal performance for the same money, thanks to their vertical orientation in the wall;
2. much better sun control; and
3. they are more weathertight.

See the figure above for why glazing works better in a vertical format!
It’s much easier to fit exterior shades to clerestories compared to skylights. Clerestories’ vertical orientation means more radiation passes through the glazing in winter compared to summer, which is clearly ideal.

Finally, clerestory windows tend to be more weathertight than skylights because the vertical seals and flashing systems are just like standard windows. No special flashing kits required. And if you leave a clerestory window open in the rain, well that’s much less of a problem than if you leave a skylight open.