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

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

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?

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Answers

Cynthia Phakos's picture

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 bepossible to locate the skylight in between the roof rafters, and then have a larger ceilingopening as close as possible between the cedar beams. You could then frame a lightwellconnecting 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 asa 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 buta structural engineer should be consulted to verify that the integrity of the roof is notcompromised.

With a structural engineer's help in either case, adding a skylight to the space wouldprovide daylight to what sounds like a dark space and would certainly contribute to yourenjoyment of the room.

Daylighting

Top lighting with skylights is the most effective source for daylighting as it provides themaximum 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 directsunlight coming into the space. This may not be an issue if your roof is mostlyshaded.

  • 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 ismore evenly distributed throughout the space.

In locating a skylight, also consider what adjacent wall surfaces could reflect the sunlightinto the space. Locating a skylight adjacent to a north wall could reflect light into thespace 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 mostimportant 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-degreesF. The lower the U factor, the greater the resistance to heat flow, the better itsinsulating value.
  • Solar Heat Gain Coefficient (SHGC): The fraction of incident solar radiationadmitted through a window expressed as a number between 0 and 1. The lowerthe 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 SHGCcan be justified.

Selecting a skylight for your location in Oregon

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

If you look at the Efficient Windows website (here) you will findrecommendations for Oregon's central zone.

  • A U-value of less than or equal to 0.55 is recommended and is considered a goodinsulator. 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 productwith a higher SHGC rating is more effective at solar heat collection during thewinter.
  • 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 anopinion on your particular site.

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

  • Visible Transmittance (VT): An optical property referencing the amount ofvisible light transmitted and includes the impact of the frame.
  • VT varies between0 and 1 through most values are 0.3 to 0.7 with the higher being the more lighttransmitted.

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