How do we take advantage of passive solar energy in this 1964 raised ranch from only the gable end?
We have a client in a Zone 5 climate. His house does not have any south-facing fenestrations to take advantage of passive solar energy.
He is proposing to remove a gable-end chimney and fireplace, which occupies a good portion of the south facing gable wall, and replace it with low U, high solar heat gain glass to superheat his house interior, hoping the thermal mass of the drywall interior will be enough to store the heat energy his family would need at night. His design includes superinsulated polyiso interior shutters to cover the windows at night.
I feel that with better sealing of house penetrations, increased ceiling insulation and appropriate modifications to the chimney, he would be better served by using its 2-story mass with a glass surround to create a Trombe wall with blower to achieve a better use of his southern exposure without risking the house interior to such temperature swings and related potential problems.
What direction or suggestions should we pursue to properly and safely utilize the passive solar exposure he has to heat his home?
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Replies
Ken,
If the house has no south-facing glazing, then I can certainly understand the homeowner's desire to add windows on the south wall.
A large brick chimney against an exterior wall is usually a bad idea, especially if the chimney is outside the thermal envelope. Such chimneys are often a major thermal bridge. If that's the case here, good riddance to the chimney.
Here are a few suggestions:
1. Try to include a roof overhang that will shade the windows during the summer. This can be tricky on a gable wall, but it's possible, especially if you build sloped exterior shelves over the windows.
2. High solar gain low-U gazing is the right way to go.
3. Follow the usual rules of thumb to be sure that the house doesn't have too much south-facing glazing. Most designers advise that the south facing glazing should be no more than 7% to 12% of the home's floor area. If the house has 12%, you are near the upper edge of what will work without additional thermal mass. More information here: Cost-Effective Passive Solar Design.
4. You won't get me to endorse a Trombe wall. Trombe walls are a terrible idea. I'd rather look out a window than look at the back of a Trombe wall.
Thanks Martin. So what I understand you to say is that if we keep the percentage within the 7% to12% of the floor area the owner will not create an overheating problem. If he wants to increase the thermal mass exposed to the south facing glass, what would you suggest as an approach?
Ken,
Of course it's possible to keep the existing brick chimney if added thermal mass is desired. The house could be extended southward, and the south-facing wall of the new addition could include plenty of windows.
However, I don't know how that would look or whether the new space would be useful.
I'm unwilling to state, flat out, that a house with south-facing glazing equivalent to 12% of the floor area won't overheat. If you are worried about overheating, 7% is safer, and that is a percentage that you see in many passive-solar design guides. One variable is the SHGC of the glazing. Needless to say, if the glazing SHGC is 60%, the house is more likely to overheat than if the glazing SHGC is 39%.
You can use the Passive House Planning Package (PHPP) software to model the performance of the house if you want; other energy modeling programs may also work to predict overheating.
Thanks for the additional information Martin.
Martin is right ......shading glass on a gable wall is tricky
the "sloped exterior shelves" = Architectural Awnings
It is not-too-difficult to make a Sketchup (free software)model of the wall, windows and Awnings
set your local lattitude and play with the design....
watch how the sun moves thru the seasons.
.....adjust window and awning size
Great suggestion John. I am curious to know what would happen over the long haul if the owner insists on trying to superheat the interior during the day in hopes of using the drywall as the thermal mass to store the heat energy accumulated during the day.
the thermal mass (of the drywall) question is beyond my knowledge...
I sorta remember Robert Riversong answering a similar question once....
Robert is usually very helpful ....
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I agree with John that Google sketchup is a simple and easy tool for shadow modelling.
I also agree that Robert Riversong can be very helpful in answering somewhat arcane questions.
Send extra heat to the crawlspace/cellar? As to drywall add two layers of 5/8" for mass. For floors add concrete overlay there as an option. Some ideas.....
Back to your idea, the trombe chimney. Somehow I am thinking your eyes "see" this solution. It may just be the most elegant too as none of us are there with our eyes. I say if trombe chimney is right for the site, then full steam ahead working out the details.
Here is the discussion that I was thinking of
https://www.greenbuildingadvisor.com/community/forum/green-building-techniques/15095/thermal-mass
Ken,
You don't say where you are located, but in Colorado, the drywall would not be enough thermal mass. Try moving some of the heat from the south end of the house to the north end of the house.
Bathrooms are typically in the center of the house and have higher than usual thermal mass. Also few people complain about a bathroom being too warm.
Agree entirely with Martin: ixnay on the Trombe. Really bad idea. Also agree on the chimney removal. Definitely an energy win.
There are lots of ways to increase thermal mass in general but: location, location, location. The floor immediately in front of the windows is really the only place where the mass can store any appreciable heat for diurnal cycling. Tile is a good option. If you're expecting mass in other locations to store useful heat by the indirect warming of room air then the room HAS to overheat in order for it to work.
I'd go high on the fenestration %: you can always add awnings if overheating turns out to be a problem. Consider canvas shades that can be rolled back in winter.
None of us have seen this site. I say enclosing the chimney adds thermal mass for free. If there is nothing worth seeing to the south the trombe chimney would be pretty neat and very one of a kind. If there is a view south then for me I would have to see some sketches of all the different concepts possible.
Ken, instead of a Trombe wall have you considered a simple unheated sunroom addition the width of the gable wall and enclosing the chimney? Vent to the outside in summer, open to the interior on sunny days in winter. Costs a little more, functional advantages are extra living and indoor growing space.