Passive solar: is it better to cover the windows in the summer?

Hi all,

I recently purchased a new home that was built to maximize passive solar heating gains. Its built to face due south, and has a significant amount of glazing on the south face including 4 large skylights. The floor of the southern room is a cement slab with slate tile surface.

In the summer there is a huge amount of sun streaming in from the skylights and hitting this cement floor as you can see in these pictures. I'm torn on what makes the most sense to maximize the passive solar effect. The room gets quite hot with that much sun, leading us to want to run the AC to be comfortable, but if we pull shades over the skylights then we, in theory, are not maximizing the heat gain of the summer months into the mass of the floor. I'd love to hear input form anyone who has more experience with passive solar designs on what we should do here.

Attached are photos all taken on June 19th, they each have the time they were taken as the caption. These show the path of the sunlight as is passes over the floor.

I live in Southern New Hampshire (USA).

There is probably a more simple approach than this, but I do know there are shades on the market using remote controls for the skylights.  

I didn't realize that heating the floor mass with summer sun was intended store heat until the fall/winter. Without any knowledge or experience on the topic, I'm skeptical that it could work that way.

I thought passive solar homes were designed to block direct summer sun. So if you're asking if it's ok to do that when your home gets too hot, my answer is definitely yes.

The heat loss in winter through those skylights will be an order of magnitude higher then any residual heat that might be left from warmer time... skylight aesthetics are nice but they totally mess up heating and cooling...

Jackie,

Your room looks beautiful by the way, and it actually somewhat resembles my great room in my house which we built specifically for southern exposure and western protection.  Like yours, my great room is a double height or two story ceiling that both helps to visually connect the upstairs and downstairs, but also helps circulate natural convection throughout the house.  

I guess the part that would concern me would be those skylights.  My great room has lots of windows, especially on the south side, but some on the east side as well.  In my case, this means that in winter, the low sun pours through the southern windows all day and in summer, as the sun rises much higher, those southern windows are essentially shaded, limiting the accumulated heat.

But in your case, the skylights are going to defeat any potential shading.  They look beautiful and let in lots of natural light, a feature I built into my house, but understandably problematic with summer heating.  I would think that a potential simple solution would be to put some type of shade under the skylights.  Would it be possible to attach a thin sheet of white fabric to the internal frame of those skylights?  My thought is that a thin fabric, almost like cheesecloth, would reflect a lot of heat right back out of the skylight, but would still be translucent and let some indirect light through.  Potentially you could still get lots of natural sunlight (and personally I love natural sunlight), but not so much heat and I would think that the floor would not heat up nearly so much.

Just as a thought, I really do like the idea of passive solar homes as they are beautiful, comfortable, inviting and make good use of solar energy.  However, I doubt that the heat stored in the summer will last till winter.  Rather I think that the heat stored in the day will last into the night, but this is but a minor point.

Other than shading the underside, I guess you could tint the skylight, but then that defeats the purpose of the skylight in the first place.  I strategically planted trees on the east side of my house to shade the eastern windows in the heat of summer, but the south side is in the clear and I don't know how you could plant trees to block the sun at noon in the summer what with the sun being more of less directly overhead.  I guess you could simply replace the skylights and cover with traditional roof, but this is both expensive and cuts into the architectural  charm of your house.  But my best guess is to prevent direct sunlight from striking that (beautiful) floor in the first place.

Your room has such a similar layout to mine that I almost want to attach a picture for comparison, but for the moment I don't have one handy.

I wish I had better ideas, but the shade fabric is the idea that stands out to me the most.

Good luck and I hope this helps.  Please let me know what you think.

Eric

You want to keep the sun out for the summer.  Passive solar is mostly about your daily to weekly storage and NOT much your annualized storage.  So in summer your goal is to keep the heat out and store cool at night by opening the house up at night and closing up during the heat of the day.  Late summer, early fall you start changing to storing heat.  You make yourselves miserable without real gains if you start too early.   I have lived with passive solar in northern WY for more than 35 years.  With the house on pure passive I do about 80% of my heating needs for the winter.  But the house still gets too warm so heat is vented.

I wish I had added the fans pulling cold air off the basement floor and blowing it up to mix with the hot air of the upstairs sooner.  Thereby sucking the warm air down storing that heat in the basement walls.

Looking at your pictures I am going to guess it does great for sunny day winter heating but is poorly designed for much of the rest of the year.  A good design automatically excludes the summer sun thru the use of eves and overhangs etc with the people taking no action.  Since it looks like the skylights open guessing you will want them screened to you can open them at night during the summer to chimney heat out.  And you most definitely want them shaded so they do not let summer sun in.   This time of year it becomes more questionable.  Since roughly 82 degrees is the max comfort level for me I try to hold the house near that as much of the time as possible for max heat storage during the fall.  Likely during the winter you will find to great a heat loss thru the windows at night.  The best answer is automated insulated shutters on the outside of the house.  But simply some sort of well sealed insulated drapes to cover stuff at night will help.

I've lived and worked in passive solar heated buildings for about 25 years, and before that I was in New England. Yes, good thermal mass does hold some heat (or coolth) from one season to the next, up to a point. Here in the Himalayas, we do that with thick earthen walls. There, you are planning to do it with a massive floor, presumably well insulated.

The issue is that south facing windows do that magical "keep cool in summer, admit heat in winter" trick, whereas skylights (and E, W, and N windows) do the opposite job, "admit heat in summer and lose heat in winter," because of the angles of the sun. Heating up your mass in June when the sun is highest is problematic, because July and August are the season of uncomfortable heat in New England. Overheating can be a problem. And then in winter, even if the skylights have good double or triple glazing to prevent conductive losses, transparent glazing tends to allow a lot of heat to radiate out to the cold depths of the sky. In the solar heated school I lived at for over 20 years, the buildings with about 50% of the south wall as windows kept reasonably warm in winter and cool in summer. But the dining hall, which had skylight for more than 50% of the roof instead, was extremely cold in winter, especially evenings, and uncomfortably hot in summer, especially days. For midwinter 6 or 8 weeks, we'd actually shift dinners into a smaller room. And in summer, people tended to always take their lunch outside.

So if you can, I think it would help keep a more stable temperature if you can operate shade curtains and insulated curtains up under the skylights.

Thank you all for the responses. It seems the consensus is that the heat gain from the summer months is not the goal here.

I am planning to get some insulated shades for the skylights that we can open at night in the summers and in the day for winter. We are debating if the cost of the motorized ones is worth it over the ones that use a pole to open/close. I think we will probably end up just getting 2 of each.  That way if we’re feeling lazy that day we can at least manage to get half of the skylights used properly.

I certainly am experiencing what most of you have described about skylights not being all that effective for passive solar use.

I’m glad that a previous owner planted some maple trees on the east side so we will get the benefit of their shade in years to come.

I’m grateful that the community here was so helpful and quick to respond!