Question

We are debating the merit of a solar tube fan vent for an interior bath vs. a high-efficiency fan only.

Asked by Lisa Carey-Moore
Baldwinsville, NY

We are seeking a LEED Silver rating for our new home. The debate centers around whether or not to use a light tube in an interior shower/toilet room that will not have natural light. I understand these tubes can be fitted with exhaust fans, but haven't found information regarding the products' efficiency relative to a high-efficiency straight exhaust. We would also need to exhaust a small bathroom adjacent to this one, but that has a window. I can't figure out what the trade-off is between using the tube/fan combo and saving on some lighting costs vs. just exhausting. The builder argues we can put the lights on an OC and that would reduce the electrical use. He has never used solar tubes, and argues that putting another "hole in the roof" would impact our energy savings. We are doing a HERS model, so we might get some more definitive information from that. Any thoughts?

Answer

Elizabeth DiSalvo

Answered by Elizabeth DiSalvo

Norwalk, CT

Trillium Architects

January 15, 2011

I agree with your contractor.

  • Although there are benefits to a solar-powered fan -- and of course the light coming in is very appealing -- I would guess that the hole through the roof is a big negative.
  • I would also guess that the efficiency of the solar fan will be quite low.

(And I say these things as a huge "fan" of both natural light and solar power!)

I have heard many complaints about leaking solar tubes, and my own experience with solar "gadgets" would lead me to think that the fan may work but it won't be powerful, consistent or reliable.

Therefore, you would have to have some other fan system in your house anyway. (I am sure you already have one in the specifications to install, because if you are building a LEED home and going for the tight envelope this implies, you will surely need to introduce fresh air and change the air in the house in a regular and reliable way.) I do not believe you could depend on the solar fan alone to do this. You don't want to spend several cloudy days in a well-sealed house with no fan operating to exchange the air.

Other mechanisms for fresh air

We usually would use an ERV (Energy Recovery Ventilator) or an HRV (Heat Recovery Ventilator) to take care of the air exchange in a well-sealed, well-insulated house.

  • These are very efficient in that they recycle heat or "energy" from the outgoing air.
  • The ERV or HRV would definitely be much more efficient and reliable than the solar fan.
  • So assuming it is a given that you will have an HRV/ ERV, having the solar fan would be redundant.

And though the solar tube does bring light into the room -- saving on lighting costs -- you would most likely lose more energy through the roof than you would spend on operating one very efficient light. You are only using the light once in a while, but a hole in the envelope operates 24/7!

Lighting for the bathroom

We specify LED can lights when we are installing recessed lighting. These are extremely efficient, long-lasting bulbs or fixtures. They also operate at a very cool temperature and are able to be placed in wet areas.

The only care that needs to be taken with LED cans is if you are placing them in an exterior roof/ceiling location. As with any can fixture, you have to be very careful putting insulation around them. With normal cans, you have to keep the insulation away to prevent fire or burnout in the fixture because these lights operate at a high temperature.

  • With LEDs, you don't have the fire worry (as they operate at cool temperatures), but to make the fixture last a long, long time -- as is expected of these fixtures -- you want to keep the insulation away.
  • With the LEDs, the slightest temperature raise over a period of time can reduce the light's life expectancy.
  • (In other words, LEDs, too, are temperature sensitive. We are just talking about a much smaller temperature differential, and the light will usually still work. You won't know that it is slowly dying faster than it should.)

In a home such as yours, I would recommend not to put any can fixture against the roof because you cannot put sufficient insulation there, and again, you are basically creating a hole in the envelope.

Forgo the solar tube unless..

I guess the bottom line is that unless you are extremely attached to the idea of bringing natural light into that bathroom, I would forgo the solar tube.

  • But do the HERS model to see what it says. I would be interested in the result. I have just been onsite for too many blower-door tests and felt the air rushing out through "holes" like this to trust it.
  • Plus it can be really, really well sealed upon installation but over time, as the house moves and shifts, that seal will break and at the very least, air will leak out.

I love natural light, and I myself would be torn on this decision! I hope I have helped.

Best of luck with your project!

 

For more information:

Read Harris Woodward's Q&A "Are skylights a good idea? My partner says it will admit too much heat in the summer."

Tagged In: home air quality, daylighting, energy efficiency

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