First Pass Modelling of Sunspace, Design and Performance.

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A spreadsheet goes with this file :: Click Here

The spreadsheet models a 2 story sunspace attached to the south side of a house. The sunspace is about 26' long and about 3.5' deep. It has 5' tall windows along its entire length except the downstairs omits a window and has a door instead. The exterior of the sunspace faces solar south. The north side of the sunspace is attached to the house and has a multitude of doors and windows. These are described in the spreadsheet.

In general terms, the sunspace is a large solar collector. Thermal mass in the form of a water wall is added to the inside of the sunspace to help temper conditions. The spreadsheet shows how much solar energy is collected, how much is lost through walls, ceilings and floors. With respect to window energy losses, the windows facing south, east and west are loosing or gaining energy from the outside or ambient conditions. These are referred to as the exterior part of the structure in the spreadsheet. Windows, doors and walls on the north side are exchanging energy with the interior of the house. The sunspace comes with an overhang on the south, both floors. This shades the windows to prevent overheating during the cooling dominated months.

The model is done for 42 deg. Latitude north. Specifically, values for Detroit, Mi. are used. The overhang style is horizontal. Shading is computed for south facing windows.

This is a very simple model using very basic conductive heat transfer equations and thermal mass storage capacity of a water wall. The intent is to get a feel for the effects of various designs without having to deal with a complex program such as Energy Plus until the design is nearly finalized. The spreadshet can be used to solve the basic design issues and find basic design parameters.

In order to define the dimensions of the sunspace, we have to start with some physical constraints. If the sunspace is attached to the house and has a shed roof, then the depth of the sunspace is dictated by the view out of the windows. The roof of the sunspace has to end a a point that is higher than the top of the windows in the house in order to maintain a view outdoors. The first pass definition of the sunspace takes this into account. If the house has a 5 pitch roof and the sunspace has a 4 pitch roof, then the sunspace cannot be deeper than 4'4” with a 1 foot overhang before the sunspace roof obstructs views out of the windows.Since the sunspace roof is fairly light, the greatest loads are going to be due to snow and wind. The drawing shows windows 5' tall in the sunspace and in the south wall of the house that interfaces the sunspace. According to code windows must be at least 18” off the finished floor height. A header over the windows of only 6” is shown as the roof load is on the lighter side. To compute the actual size of the header http://bct.nrc.umass.edu/index.php/publications/by-title/calculating-loads-on-headers-and-beams/ For purposes of computing shading, the sunspace is shown with a 1' overhang. This is the overhang depth. On the second floor, the distance between the overhang and the top of the window is shown to be only 6” on the sunspace. This is the overhang spacing. On the house wall, which is the north side of the sunspace or the south side of the house, the overhang depth is 5'4” and the overhang spacing is 1.5'. On the first floor, the second floor of the sunspace serves as an overhang. There will also have to be an overhang added to the exterior of the sunspace inbetween the first and the second floor. We know from the windows spreadsheet that a window doesn't start to gain more than it loses until it has at least 2 panes of glass with an air space. The first pass analysis of the sunspace uses double strength glass in the sunspace exterior walls as well as the party wall in between the sunspace and the house. The 1' overhang of the sunspace must include the depth of the gutter, which is assumed to be about 6” in width.

The next physical constraint pops up when designing the overhang for the sunspace itself. Our goal is to have 100% direct solar gain to satisfy our heating needs in the middle of winter and 100% shade in the middle of the summer. The transition months according to Redbook are May and September.

We use Suntools overhang analysis to find out what the shading factor is on the windows and when, given the details of an overhang design. To access these tools Click Here Inputs to suntools overhang annual analysis are 1' overhang, 1.5' overhang spacing and 5' windows. Here are the results.

We can see that we do not achieve 100% shading in the hot summer months, June, July and August. We must make the sunspace overhang larger. We want to try to keep the depth of the sunspace itself no less than 3.5' at this time. That will allow 6” for a water wall and 3' of walking space – a standard hallway. Even with a 2' overhang, we obtain significant sunlight in the sunspace in August on the first floor. It is not possible to achieve shading every month of the year to be in alignment with heating and cooling needs for that month. In this case, the amount of solar gain into the sunspace in August is still 50% of the total, which is significant. On the other hand, May and September are transition months so we do not want 100% shading during those months either. These are the results with a 2' overhang, 1.5' overhang spacing and 5' windows, the first floor:

On the second floor, moving the outer sunspace wall toward the house did not make any difference in the overhang spacing. Here are the results for the second floor sunspace. It is likely acceptable to have the second floor of the sunspace more shaded than the first floor as extra hot air from the first floor is likely to rise up there anyway. It is therefore convenient that we end up with such a small overhang spacing on the second floor. The following results are for a 2' overhang depth, 0.5' overhang spacing and 5' window height. Second Floor:

It is time to examine the sunspace itself and find out what heating and cooling needs it has. If those needs are met and the temperature inside the sunspace can be regulated to be between 40 deg F and 80 deg F then that is how the ambient air will appear to the entire south side of the house.

There will be another problem to address. There is no direct way to insulate the sunspace ceiling to 9” without creating a thermal bridge near the south end of the roof. The insulation shown in the following drawing is only 8”

For the south side of the house, which is the north side of the sunspace, we need to determine how much direct sunlight gets by the sunspace overhang and passes right through the house windows. For an overhang depth of 4.5', overhang spacing of 1.5' (which applies to both floors) and a window height of 5.25', we can use the same chart for both floors.

Modelling the sunspace.

Details of the spreadsheet.

The fluctuation between daily low and daily high temperatures can be envisioned like a sin curve. The difference between these 2 temperatures is taken and divided by 12 to get the incremental temperature difference per hour.

The sunspace spreadsheet models energy transfer using the simple conduction formula for every hour of the 24 hour day, for 1 day of each month.

The daily low temperature typically occurs just before sunrise. The solar time of sunrise differs on a month by month basis. To keep things more simple, the daily low temperature is modelled to occur at 5 am and the daily high temperature at 5 pm, year round.

If the sunspace roof is deep such that it does not allow direct solar gain into the south rooms of the house, then those rooms will be dark, year round. This may not matter much for a bedroom but can be uncomfortable and cause the use of more artificial lighting for most other rooms.

Sheet 2 of the spreadsheet has the information for the windows used in the model on Sheet 1. This data has the solar gain for windows oriented N, S, E and W, for every month of the year. This data has been adjusted from its original sources and a shading factor has been applied to the south facing windows. To understand how and why this adjustment is done, refer to the windows file available from the main page.

The sunspace is assumed to be 26' long. On the first floor it has 4 windows, each about 5' high and 5' wide. It also has a door 3' wide and 80” tall. The second floor of the sunspace has 5 windows, each about 5' high and 5' wide. The east and west sides each have a window 3' wide and 5' tall.

On the south side of the house, which is a wall that interfaces to the sunspace, we assume on the first floor 2 large windows, each 5' high and 5' tall on the first floor. We also assume a glass patio door at 6' wide and 80” tall.

On the second floor, we assume 2 french doors, each 5' high and 5' tall and a third window about 3' wide and about 4' tall. French doors don't stop at 18” off the ground. This detail is omitted from the model. The actual solar gain into the house will be higher. To model that, the amount of shade for glazing of that height has to be taken into account and the shading factor of the french doors and windows has to be dealt with separately.

A sunspace can be designed to satisfy a long list of goals, not all necessarily the same. One of the goals is to make the south side of the house appear as if its actually in San Francisco, where the temperature may fluctuate between 40 deg F and 70 deg F at all times. It is not necessarily the goal of the sunspace to behave like a solar cooker. The higher the temperature difference between the sunspace and ambient conditions, the more drive there is to lose energy quickly. If the sunspace temperature can be maintained with zero energy within a certain range, which may vary from month to month, then one of the greatest impacts is how it makes the ambient air appear from the point of view of the south side of the house with respect to energy usage for climate control, as well as lighting, glare and other issues.

For every month we start off defining our initial conditions. In January, we start off by setting the interior sunspace temperature the same as the interior of the living space. We then run through all hours of the clock to see what happens. We look at the heat lost through external wall, ceiling and floor assemblies. We look at the windows for each floor separately. For every glazing orientation the energy lost or gained will be different, even if they are all the same kind of glass with the same number of layers of glazing materials.

North windows of the sunspace may have a negative heat transfer number associated with them. That means that the sunspace is taking energy from the interior of the house. The south facing windows of the house would lose energy to the outdoor air if the sunspace wasn't there. The presence of the sunspace means at least some of the energy lost from the house is going to stabilize the temperature of the sunspace instead of being carried off in the winds. The same is true for energy lost through the south walls of the house. Instead of making it outside to be carried off, once it gets through the wall of the house, it encounters the thermal mass water wall in the sunspace, where the energy will be transferred to the water wall.

At the end of every hour, we are left with a new temperature for the sunspace dependent on how much energy was lost, gained or stored. We can now make adjustments to parameters such as the number of layers of glazing, the insulation in the walls, ceilings and floors, the amount of glazing and the size of the water store, until we see the behavior that we are looking for.

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