Two skylights can both have low-emissivity glass and still behave differently in a room. The useful comparison is in three ratings: U-factor for heat transfer, SHGC for incoming solar heat and visible transmittance for daylight. They are not three ways of saying energy-efficient.
Natural Resources Canada's rating guide explains these measures. Keep the actual product code beside them, because changing the glazing, frame or operating type can change the figures.
Mike has three figures on the page and one pencil. "Which problem are we trying to improve?" The homeowner points at the desk that becomes uncomfortable every sunny afternoon. That is a more useful starting point than circling whichever number is smallest.
U-factor: heat moving through the unit
A lower U-factor indicates less heat transfer for the same area and temperature difference under the stated rating conditions. Check the units before comparing: Canadian information may show W/(m2 K), while other literature uses Btu/(h ft2 F). Two bare decimal numbers from different unit systems are not a fair contest.
Also check whether the number describes the whole unit or only the centre of the glass. The frame is part of the installed skylight. A good glass-only figure cannot be compared directly with another product's complete-unit rating.
VELUX's Canadian testing page labels its thermal figures as complete-unit values and states the rating slope. That is the sort of context worth preserving when a contractor copies one figure into a quote.
SHGC: the sunlight that becomes indoor heat
The solar heat gain coefficient is a ratio describing solar energy admitted as heat. Lower means less solar heat gain under the test conditions. It is not a summer electricity-savings percentage.
A room that already overheats may value lower solar gain. Another room may benefit from winter sun, but that does not cancel summer comfort or shading needs. Roof orientation, slope, surrounding shade and how the room is used belong in that choice.
Think about a desk under direct afternoon sun. The owner may be uncomfortable even when the house's thermostat is satisfied. The thermostat has no laptop to read and no sun in its eyes. A product decision based only on whole-house heating cost misses that particular seat.
Read each rating for its own job
Compare the same product size, rating basis and units.
U-factor
Lower means less heat transfer for comparable area and temperature difference.
SHGC
Lower means less solar heat admitted, not a fixed utility-bill saving.
Visible transmittance
Higher means more transmitted visible light, not guaranteed room illuminance.
VT: the light you can see
Visible transmittance, often shortened to VT, describes the fraction of visible light passing through the product. Higher VT generally means more visible light transmitted under comparable conditions. It does not tell you the brightness at the back of a room after light has passed through a deep shaft.
The shape and finish of that shaft, nearby obstructions and the position of the opening still matter. A high-transmission unit placed where its light misses the work surface can disappoint. The one-large-or-two-small comparison starts with that distribution question.
A worked comparison without a pretend savings claim
Imagine two equally sized, hypothetical skylights rated on the same basis. Both have a U-factor of 1.8 W/(m2 K). Option A has SHGC 0.30 and VT 0.60; option B has SHGC 0.25 and VT 0.50. These are arithmetic examples, not advertised products.
The U-factor tie means neither wins that heat-transfer comparison. B admits less solar heat, but also transmits less visible light. Its SHGC is about 17% lower than A's; that does not mean the home's cooling bill falls 17%. Weather, area, orientation, room conditions and the rest of the building are missing from that calculation.
Mike leaves the U-factor tie alone and circles the two remaining differences. The useful next question is which balance suits the room, not which row contains the smallest number. For glare without losing all daylight, also compare suitable shades.
Keep the selected variant attached to the quote
Ask for the model, size, glass code, whole-unit ratings, units and certification reference. If the supplier substitutes a different glazing option, review the numbers again rather than assuming the original figures travel with the brand name.
Confirm the appropriate overhead glass construction separately. A strong energy figure does not settle safety, flashing compatibility or installation quality. The label helps you select a product; a complete roof and shaft detail is what puts it to work.