Choosing the skylight shape

Comparison between barrel vault, ridge and half-round skylights

• 

  Barrel vault skylight:

  for the proportion: h = l / 5
  length of flat polycarbonate sheet = l + 13%l
• 

  Ridge skylight:

  for the conditions of 90° isosceles triangle
  length of flat polycarbonate sheet = l + 42%l
• 

  Half-round skylight:

  for the condition: l=2r,
  length of flat polycarbonate sheet = l + 57%l

As can be seen from the illustrations above, it is the barrel vault skylight which proves to be the most economical solution. After a careful calculation we arrive at the conclusion that, for the same area of glazing, the barrel vault skylight uses 29% less polycarbonate than ridge skylight and 44% less than half-round skylight. One important issue taken into account in choosing the right skylight shape is its cost, related to the amount of materials used. But the main criteria are the illumination and heat losses. Every skylight designer tries to achieve the lowest heat transfer coefficient (U). There is a big difference between the U=2,4 for a skylight and the U=0,5 of a rooftop for an industrial building, so it is inevitable that the construction of a skylight generates heat losses. Barrel vault skylights allow to reduce these losses because their surface of cooling is 29% and 44% smaller than that of ridge and half-round skylights respectively. The arched shape also minimizes snow retention during winter.

Our long-term experience tells us that skylights are the best solution for an optimal connection of low costs of exploitation and fully functional illumination of the interior.

Roof skylights - influence of shape on heat parameters

There is a common, though misleading opinion that it is only the thickness of a polycarbonate sheet and the number of cells that influences the amount of heat losses generated by a skylight. In fact, the shape of a skylight is equally important. In order to illustrate the comparison between the three most common skylight shapes, we assumed that the roof area they cover is 3m x 10m in dimension and the material used for their construction is 10mm-thick three layer cellular polycarbonate for which the U=2,5 W/m2K. The results of our analysis are shown the table below

Type of skylight	Top area of the skylight	Side frames area	Skylight area in total	Actual U parameter for 1 m2 of skylight in graphical projection
Barrel vault skylight	33,90 m2	1,20 m2	35,10 m2	2,92 W/m2K
Ridge skylight	42,60 m2	4,50 m2	47,10 m2	3,92 W/m2K
Half-round skylight	47,10 m2	7,06 m2	54,16 m2	4,51 W/m2K

The barrel vault skylight turns out to be the cheapest solution for a number of reasons:

• for high skylights, wind is an additional cooling factor;
• ridge skylights and half-round skylights require more materials - not only PC sheets but also longer, thicker, heavier and more expensive aluminum profiles.
• in the case of ridge skylights, the material for the construction of the ridge frame is an additional cost