Question

I am interested in adding some skylights to my great room to increase the light and possibly provide some solar gain.

Asked by B. Palmer
Springfield, OR

I recently moved into a new home which was built in the 60's. The house is rather dark and cold due to lots of big trees, both on the property and on neighboring properties which limit the sunshine reaching into the house. I am interested in adding some skylights to my great room to increase the light and possibly provide some solar gain. I have a lovely cedar ceiling with beams which are not structural to the roof. This limits placement of the skylights. To center the skylights in each cedar panel, would require cutting into the joists and restructuring them. I don't think tubular sun tunnels would look good here. How advisable is it to make this kind of structural change to the roof?

Answer

Cynthia Phakos

Answered by Cynthia Phakos

Los Angeles, CA

Koffka Phakos Design

January 11, 2013

Your question brings up a number of issues for discussion. I agree that tubular
skylights would be inappropriate for your ceiling as you described it.

Structural Considerations

If there is an attic space between the suspended ceiling and the roof structure, it might be possible to locate the skylight in between the roof rafters, and then have a larger ceiling opening as close as possible between the cedar beams. You could then frame a lightwell connecting these two cutouts.

  • This lightwell could work as a reflector for the daylight, which would diffuse the light into wider areas of the room.
  • White drywall would work as a good reflector, but of course this is an aesthetic decision.

If the faux beams are right against the roof rafters then you would need to cut the
rafters to install the skylight. This is common in the installation of large skylights but a structural engineer should be consulted to verify that the integrity of the roof is not compromised.

With a structural engineer’s help in either case, adding a skylight to the space would provide daylight to what sounds like a dark space and would certainly contribute to your enjoyment of the room.

Daylighting

Top lighting with skylights is the most effective source for daylighting as it provides the maximum light per unit area and brings light deep into the space.

When installing the skylight, consider the sun’s location and angle and try to avoid direct sunlight coming into the space. This may not be an issue if your roof is mostly shaded.

  • Direct light causes glare, shadows, high contrasts and “momentary blindness” when the eye adjusts to brighter or darker surfaces.
  • Indirect light (reflected or diffused light) is the most comfortable light as it is more evenly distributed throughout the space.

In locating a skylight, also consider what adjacent wall surfaces could reflect the sunlight into the space. Locating a skylight adjacent to a north wall could reflect light into the space for most of the day.

Skylight Performance & Selection

There are a number of glazing options, with some types designed specifically for
daylighting and others designed for thermal performance.

In selecting a skylight the most important considerations are the U-factor, and the Solar Heat Gain Coefficient .

  • U-factor (U-value): The rate of non-solar heat loss measured in Btu/hr-sf-degrees F. The lower the U factor, the greater the resistance to heat flow, the better its insulating value.
  • Solar Heat Gain Coefficient (SHGC): The fraction of incident solar radiation admitted through a window expressed as a number between 0 and 1. The lower the number the less solar heat it transmits.

In the sunny southern climate zones, a low SHGC is more important than a low U-factor. In the cooler northern climate zones, a low U-factor is more important, and higher SHGC can be justified.

Selecting a skylight for your location in Oregon

Oregon is in the Northern Climate Zone, and your located in the northern central zone, with heating the primary concern for your climate.

If you look at the Efficient Windows website (here) you will find recommendations for Oregon’s central zone.

  • A U-value of less than or equal to 0.55 is recommended and is considered a good insulator. This will keep the heat in at night when you need it most.
  • A SHGC of less than or equal to 0.40 is recommended for your area. A product with a higher SHGC rating is more effective at solar heat collection during the winter. 
  • It may be that you could go with a higher factor as your site is primarily in shade. You might contact the local skylight manufacturer’s representative to get an opinion on your particular site.

Given that you are looking to maximize the daylight in your house, another factor to look at is the Visible Transmittance.

  • Visible Transmittance (VT): An optical property referencing the amount of visible light transmitted and includes the impact of the frame.
  • VT varies between 0 and 1 through most values are 0.3 to 0.7 with the higher being the more light transmitted.

Also, when you are working on your skylights, take a look at the insulation in your roof and floors and air seal any cracks or openings in your envelope. These too would certainly contribute the comfort of your home.

Tagged In: daylighting

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