Low Thermal Mass Attached Sunspaces for Home Solar Heating -- A Performance Test

This section covers some tests done on a low thermal mass sunspace that uses a design optimized to produce solar heat for space heating a home that is connected to the sunspace.

We look at: how efficient it is in producing heat for spaced heating compared to conventional solar collectors, the effect of some design variations on heat output, a survey of the temperatures in various parts of the sunspace, and what things can be done to maximize the use of the sunspace for a variety of purposes (producing heat, pleasant place to spend time, doing laundry, ...).

sunspace for house heating

I think that attached sunspaces are 1) underused as a way to provide both solar heating and very usable space, and 2) often not very well optimized to do the heat production job well.

If you have a sunspace, I'd very much like to hear what your experience has been (good or bad), and what your ideas for improvements are.

I'm doing this work in what will become our all season greenhouse.  This results in some things that are not quite right:

Bottom line is that I think that using the greenhouse as a sunspace simulator gives good results, and there is so little actual measured performance data out there on sunspace performance that its quite useful.

Page Index

- Configurations

cover construction

- Performance Tests (have a brief summary of the tests on this page with separate pages for the details)

- What its like to live in

- Cost

- Conclusions

- Comments



Test Sunspace Description

The sunspace performance tests were actually done in our new all-season greenhouse.  I've had a long term interest in seeing how well an attached, low thermal mass sunspace actually performs compared to (say) active solar collectors, and the greenhouse has a lot of the same features, so we decided to use it to do some sunspace performance tests before it becomes a greenhouse.

The main features include:

The end result is a space that heats up very quickly when the sun gets on it-- it is ready to start making heat for the attached house as soon as the sun is on it. 

At the end of the day, when the sun goes off the sunspace it very quickly cools to the outdoor temperature, because it has no stored heat.

sunspace foundation
Built on footing posts with 1.5 inch
vertical Styrofoam insulation extending
2 ft down.
greenhouse foundation
Built on treated lumber sill plates.
greenhouse frame
Framed with 2 by 6 lumber 24 OC.

greenhouse glazing
Glazed with 10 mm thick twinwall
polycarbonate glazing in 4 by 10 ft sheet.
200 sqft of glazing total.
greenhouse sealing
Pretty carefully sealed.
greenhouse temperature
Gets pretty warm even before
insulation was installed.

should have started earlier    



Tested Configurations

The sunspace was tested in two basic configurations. 

Configuration 1: Non-Optimized

This test was done earlier in the construction before some of the features above were incorporated.  The glazing and shell were in place, but the floor was bare (heat absorbing) dirt, and the walls and ceilings were not insulated. 
Interior surfaces were not painted a dark color.

The idea was to see how much difference it degrades the performance if not all of the features listed above are included. 

<show pictures of config 1>


Configuration 2: Optimized -- vertical screen

This configuration included all of the features listed above.  The walls, ceiling and floor were insulated and nearly all interior surfaces were painted dark. 

In addition, a screen of dark weed fabric was hung vertically in the space.  The fabric was suspended from near the peak and ran vertically down to the floor, and then extended south toward the glazing along the floor.

The idea is to see if this vertical screen is more efficient in getting the heat up to the peak area.  The screen was also helpful in evaluating whether some form of screen is desirable to increase the comfort for people using the sunspace to (say) read or have a cup of coffee.


< pictures>


Configuration 3: Optimized -- screen on floor

This configuration is nearly the same as configuration 2, except that the dark weed fabric was just distributed evenly over the full floor.   So, for this configuration, the sun could shine onto the north wall and north roof.

< picture>


The results of the testing for each of these configurations are given below.

Performance Tests

This section give the details on three days of testing covering the three configurations listed above. 

Each test includes the following:

<give enough of a teaser on each test to encourage people to go to that page

< do optimized tests first

Configuration 1: Optimized Sunspace -- Vertical Screen


Configuration 1: Optimized Sunspace -- Floor Screen


Configuration 1: Non-Optimized Sunspace




How the Performance Measurements Were Made

Getting good performance data on solar heating collectors is challenging, and air heating collectors are even more challenging.  Its right to be a bit skeptical about performance numbers you see around the internet.  So, here is a rundown on the instrumentation I used.

Sun  -- Sun intensity was measured with an Apogee pyranometer that was mounted to the glazing frame and was perpendicular to the plane of the glazing.


In this position, it directly measures the sun intensity on the glazing.

Temperatures -- I used Onset Computer loggers with thermistor sensors to measure the temperatures used for performance calculations.  These sensors specifications call for plus/minus 0.25 C accuracy.  A a check, I generally put all the sensors together in the same location at the start of the test and make sure they read close to the same value. 

Areas -- Glazing area was measured with a tape, and the frames were included  (not just net glazed area) -- this is in the SRCC style.

Flow Rates -- In air collector testing, getting good estimates for flow velocity is the most challenging measurement.  I went through several iterations getting to an arrangement that I feel is giving decently accurate velocities.  The final arrangement includes an about 11 ft long straight length of 10 inch duct on the exit of each of the two fans (13 diameters of straight duct before the measurement station).  This lets the turbulent flow of the fan mix and settle down after leaving the fan.   A 2 ft long flow straightener is used to take most of the swirl out of the flow.  Before the flow straightener was installed, the swirl velocity was about 400 fpm and the velocity in the center of the duct was significantly lower than the velocities around the periphery.  With the flow straighteners, the velocity profile is as expected with higher center velocity, and consistent velocities around the periphery.

I take the velocity measurements with a new Kestrel 1000 turbine style anemometer.  The Kestrels are high quality instruments that all receive an individual factory calibration before going out-- they are certified to be within plus/minus 1.66% for the velocity range I use. 

I take one reading in the center of the duct, and 4 readings around the periphery (up, down, North, and south).  The 5 readings are simply averaged to get an estimate of the average flow.  I take these readings several times during the day -- they tend to be quite consistent.  Typical readings would be 740 fpm center.  Readings around the periphery vary from about 530 fpm to 630 fpm, but the readings at each peripheral point are quite consistent from one reading to the next.  The swirl component of the velocity is less than 100 fpm.

sunspace exit vent
The afterburner on the new sunspace propulsion system.
I'm expecting it to do Mach 0.86 at 32,000 ft.

It is (maybe) interesting to note that before the flow straighteners were installed, and with a lot of swirl velocity, taking the 5 readings in the same places as mentioned above -- each individual reading was markedly different than the after flow straightener readings, but the average of the 5 readings was close.

I may further check these readings with a Pitot tube survey, but the velocities are down in the range that are difficult to measure accurately with a Pitot tube (600 ft/min).  Past experience leads me to believe that the Kestrel is probably the best way to go. 

Air Density -- I use a density of air of 0.061 lb/cf -- this correction from the sea level standard conditions of 0.075 lb/cf accounts for our altitude of 5000 ft. and 80F air temperature.  Based on this calculator.

What is it Like to Live In?

I spent the better part of several days in the sunspace during the testing, so these are some quick impressions of how it is to be in while its in the process of gathering heat for the house.

There were a lot of times when the sunspace was quite pleasant to be in.   But, there were times near midday when the glare from the sun and reflection off the snowfield were a bit uncomfortable for me (this may be a matter of personal taste).  I think that some form of shade cloth like screen that runs parallel to the glazing and can be deployed to filter the sun, or pushed to the side to let in all the available sun would be good.  It would allow you to quickly adapt to full sun, part sun and light overcast conditions and to personal tastes. 

Even though I would call the Grainger fans I used relatively quiet, the fan noise would be objectionable for (say) reading the paper or having a conversation or just having a cup of coffee.    Using fans that are very quiet and/or getting them into locations that noise shield them from the sunspace is important.

As a place to hang clothes for drying, it would seem very good.  The air is warm and is moving, which should reduce drying time, and it could be setup so that the sun shines directly on the clothing to further reduce drying time.

One observation is that it does not take very much sun to warm the sunspace up to where its comfortable to be in.  Even in light overcast conditions where it does not generate enough heat to do much house heating, it can be quite comfortable to be in.

Another thing that is no surprise is that that when sunset rolls around, the temperature in the sunspace falls like a rock.  It very quickly becomes a very chilly place.


This is a very rough idea what an attached sunspace modeled after the freestanding greenhouse in the pictures above might cost.  Its based on what I paid for the materials for the GH, but adjusted down in places because the sunspace would not have a north wall. 

It includes what you see in the pictures -- nothing fancy.

Item Cost 
Glazing $414.00
Foundation Supplies $285.00
Framing and plywood $420.00
Shingled roof $150.00
Insulation $150.00
Door  $157.00
Fans $180.00
ductwork  $77.00
Controls $60.00
Paint, Great Stuff, $100.00
Total $1,993.00

Bear in mind that costs of low thermal mass sunspaces can vary over a huge range.   There are a couple of examples of dirt cheap ones that can be built for $25 ish, all the way up to ones that cost a couple hundred per sqft.  They all heat well  -- the cost is more a matter of the look and feel that you want to achieve.

Just as a very rough idea how other solar heating options might compare:

- 200 sqft worth of commercial air heating panels (you install) -- roughly $10,000 (kind of mind boggling)

- 200 sqft of DIY air heating panels with fans, ducts, ...  $1500?   

- 200 sqft of DIY simple thermosyphon air heating collector like this one  $450

Of course, the sunspace comes with all sorts of functionality beyond the solar heating, whereas the rest of the options just do heating.