Design Notes -- Low Thermal Mass Sunspaces for Space Heating & Many Other Purposes


There are some good existing guides on building regular sunspaces, but low thermal mass sunspaces are not well covered.  This set of notes covers some of the unique advantages and characteristics of low thermal mass sunspaces as well as providing some advice for designing and building a good one.

low thermal mass sunspace

If you have a low thermal mass sunspace I'd love to hear about your experiences with it -- email me, or use the Comments section.

Table of Contents

What is a Low Thermal Mass Sunspaces? and, what can it do for me?



Low Thermal Mass  -- The key to high efficiency

Heated Air Distribution -- Getting the hot air where you want it - quietly

Venting -- Prevent Overheating in the warmer seasons

Performance -- Can be excellent

Sunspace Liveability

Examples  -- $22 and up

Potential Improvements


What is a Low Thermal Mass Sunspace?  and, what can it do for me?

The amount of thermal mass included in sunspaces for heat storage depends on what the heat produced by the sunspace is to be used for.  If the heat is used to keep the sunspace itself warm on a 24 hour basis, and to reduce the drop in temperature at night, then a relatively large amount of thermal mass should be included.  This helps to keep the sunspace from overheating during the daytime, and it keeps the temperature of the sunspace from dropping as much at night.  This approach is typically used when the sunspace is used for growing plants that must be kept from freezing at night, or when the sunspace area needs to be maintained at comfortable temperatures into the evening.  These sunspaces typically provide little heat to the attached house. 

The other end of of the spectrum is the low thermal mass sunspace that this guide is about.  In this case, the amount of thermal mass in the sunspace is kept as low as possible, and all of the excess heat is exported to the attached house for space heating.  The sunspace will be comfortable during the daytime, but after sunset, its temperature will drop rapidly, and before too long it will be close to the outside ambient temperature.  The advantage of this approach is that it produces quite a lot of heat for space heating the house, while still providing extra useful space during the day for a variety of activities like: play area, lounging area in the sun, laundry drying, workshop, firewood storage and drying, food drying, ...

Low thermal mass sunspaces are (surprise) sunspaces that keep the amount of thermal mass in the sunspace itself as minimal as possible.   In a nutshell, the advantage of the low mass design is that the sunspace heats up quickly when the sun gets on it, and nearly all of the heat that the sunspace produces can go to the attached house for space heating as opposed to being stored in the sun space mass.    The advantage compared to regular sunspaces is that they provide considerably more space heating for the house.  If well designed, they are just as effective in house heating as similar size, high quality, active solar collectors.

Other advantages (mostly shared with regular sunspaces) are:

I don't know of any other solar project that is so cost effective from an energy saving point of view, and that also provides a whole raft of other benefits.

 In a nutshell, here are the main design features of an LTM sunspace are:

A very good article by Dr. William Shurcliff explaining the basics of low thermal mass sunspaces...


Since low thermal mass sunspaces are intended for space heating in the winter, the glazing should be optimized for winter conditions.

The sunspace guides and also the guides for greenhouses that are intended for growing right through the winter give recommendations for glazing that are good for low thermal mass sunspaces.  In a nutshell, the glazing should face south and use a high tilt angle (60 degrees up to vertical).  Surfaces that don't get a lot of winter sun should be insulated instead of glazed. 

Nick's sunspace Mike's sunspace high tilt angle glazing Kikora Sunspace

Double glazing will reduce heat loss from the sunspace to the outdoors, and improve its heat output and efficiency.  This is particularly true for cold climates.

Insulation and Infiltration

For maximum space heating potential, all of the surfaces that are not glazed and that face an outdoor area should be insulated.  This prevents the heat you collect from being lost out the sunspace walls, ceiling and floors. 

This includes not only walls and ceiling, but also the floor.  Since the sun will likely shine directly on the floor, it will heat up, and, if not insulated a lot of this heat will be lost through the floor. 

As a guideline, I would say that only a small fraction of the heat collected through the glazing should be lost out the other sunspace surfaces.   Like any collector, It is inevitable that a significant amount of heat will be lost out the collector glazing, but, other than using double glazing, there is not much to be done about that.  But, losses out other surfaces can easily be cut to low levels using insulation, and this should be done.

As a rough example, suppose you are building a sunspace that has 200 sqft of good winter glazing, and has 350 sqft of non-glazed wall, ceiling and floor that face the outdoors.   Lets look at a sunny winter day with an outside temperature of 20F, and an average inside temperature of 80F.

Your 200 sqft of good winter glazing will be gaining about 36,000 BTU per hour of heat.

If your 350 sqft of outside facing surfaces has no insulation and is constructed with a single layer of (say) plywood siding, the the effective R value is about 1.  The heat loss for these conditions through these uninsulated surfaces will be about (350 sqft)(80F - 20F) / R1 = 21000 BTU/hr.   This is more than half of your solar gain -- clearly this is unacceptable for an efficient sunspace.

Insulating all of the 350 sqft of surfaces to R20 would reduce the heat loss to about 1000 BTU per hour -- only about 3% of the collected solar heat.  This seems quite acceptable.

Its probably worth noting that floors can be more difficult to insulate and also may not have as much heat loss, so a somewhat lesser R value may be fine.  Vertical insulation around the periphery of the sunspace that extends at least a couple feet downward might also be an option, but I would always include some insulation in the floor itself, so that the mass right under the floor is not cycled each day.

Note that east and west facing walls do have some solar gain -- particularly in the spring and fall.   If such walls are double glazed, they may do about as well as insulating them. 

For the same reasons that insulation is important, infiltration is also important.  The space should be built carefully with attention to not leaving infiltration paths.  This is more a question of taking some extra time during the construction and does not add much cost.

Low Thermal Mass  -- The key to high efficiency

If you want to maximize the heat that the sunspace can transfer to the house, its important to keep the thermal mass in the sunspace as low as possible.   If the sunspace has a lot of thermal mass, then much of the solar heat will go into warming that mass up, and that heat will mostly be lost to the outdoors after the sun sets. 

Taking the same example we used above in the Insulation section, here are a couple examples of how much thermal mass may cost in terms of solar heat that does not get delivered to the house:

- 1000 lbs of wood mass warmed from 20F to 80F absorbs 30,000 BTU of solar heat that could otherwise go to the house.

- 4 inches of dirt over 150 sqft of floor warmed from 30F to an average of 70F would absorb about 50,000 BTU of solar heat.

These are rough estimates, but it is clear that if your sunspace has a lot of thermal mass that a significant fraction of your daily solar heat will be used to heat this mass each sunny day, and that this heat will not get to your house.  Of course, if you want the sunspace to remain warm for a longer period of time after the sun goes down, you may be willing to add thermal mass to accomplish this and take a hit on heat to the house.

Low thermal mass floors seem like a particular challenge in that they have to be tough enough to walk on, but you don't want them to be massive.  Since floors are often exposed to direct sun for an extended period each day, keeping their mass low is important.  Carpeting over a light underlayment with insulation under seems like one approach that might work.  A wood floor with rigid insulation right under the wood might be another option -- for our example sunspace with 150 sqft of floor area, a half inch thick would floor would absorb about 5000 BTU -- a fairly small fraction of the total daily heat gain.   Got any other ideas?

If the sunspace has a deployable netting or shade cloth as suggested in the Sunspace Livability section, then this will absorb some of the solar radiation before it gets to the floor, and make a low mass floor less critical.

The Potential Improvements section discusses some ideas for storing heat in the sunspace that can be used for house heating after sunset.

Heated Air Distribution -- Getting the hot air where you want it - quietly

Low mass sunspaces produce a lot of heat and this heat has to be efficiently transferred to the home living space.  Since no one wants to listen to loud fans, noise is a major design consideration.

In a nutshell, the fan and duct system that distributes heated air to the house wants to:

Each of these areas is covered below in more detail.

Fan Flow Rating:
Based on the tests on our LTM sunspace, I would recommend about 3 cfm of fan capacity for each square foot of glazing to handle the heat produced on a good sunny winter day.  This refers to the actual cfm delivered, not to the fan free air rating, which will be higher than what the fan can deliver under load.  If you can get the fan curve for the fan you plan to use, then, as a rough indication I would use the cfm rating for 0.1 inches of water pressure drop.  The 0.1 inches of water pressure drop allows for the resistance of a carefully designed duct system of moderate length.

For our example sunspace with 200 sqft of glazing the recommended fan capacity would be about (200 sqft)(3 cfm/sqft) = 600 cfm.   

Under sunny winter day conditions, the sunspace can deliver about 33,000 BTU per hour.  So, the fan must be able to move that much heat.

If the air at the peak of the sunspace is 110F and the return air from the house is 60F, and the fan is moving the recommended 600 cfm, then the heat moved is:

Heat Transferred = (Flow)(air density)(Temperature rise)(specific heat)
                              = (600 cf/min)(60 min/hr)(0.075 lb/cf)(110F - 60F)(0.24 BTU/lb-F) = 32,000 BTU per hour
                               So, the 600 cfm fan is well matched to the example sunspaces heat output.

Looking at fans that might fit this, and assuming that the duct system pressure drop is around 0.1 inches of water, a Fantech FG fan in the 10 inch size delivers 597 cfm with a 0.1 inch pressure drop.  Or, two of the 6 inch or 8 inch fans, which would offer more flexible control (see below) might be a better choice.   This is just an example to illustrate fan selection, I'm not particularly recommending this model of fan -- the data on this particular model of fan says nothing about noise levels, so this would be something to investigate. 

sample fan


Our test sunspace used two of these Dayton 10 inch fans.   These fans were able to deliver about 325 cfm each with an about 10 ft duct system.  The noise level was louder than I would like with the fans right in the sunspace, but would probably be acceptable if the fans could be moved outside of the sun and living space and mounted carefully. 
Just as a side note, these two fans use 27 watts each, and when the sunspace is cooking, it delivers about 33,000 BTU per hour, which is equivalent to 10,000 watts.  So, the Coefficient of Performance (COP) is about 185!

sample fan 

Ducting System:
If the area you want to deliver heat to is located on the other side of the wall that the sunspace is on, then a through wall fan (or two) could be used to transfer the heat.  The fact that the fan would be hearable from both spaces puts a premium on low noise level.  Some form of variable speed would also be desirable to cut down the volume of air when sun conditions are not optimal.  And, a closeable damper to keep cold air from entering the living space at night

If the air must be delivered to a space that is a ways away from the sunroom, then a ducting system will be required.  There are ducting sizing calculators that can help in choosing the duct diameter.  The duct pressure losses (including bends etc.) should be kept low by using a relatively larger diameter duct -- this keeps duct noise down and allows the use of a fan that is quieter and uses less power.  I think that keeping the total duct losses under 0.1 inches of water is a good target if possible.

Fan Flow Rate Flexibility:
Since the amount of heat that the sunspace can deliver depends strongly on the how good a sun day it is, and on the time of day, it is a good idea to provide a fan/duct system that can vary its delivery rate.  This might be done using more than one fan and duct, so that under low to moderate sun conditions, only one fan is used, and in full sun, both fans are used.  Or, a variable speed fan would also be a good solution. 

Fan Temperature Rating:
If the fan is blowing the hot air from the peak area of the sunspace into the living space, then the fan should be rated for the kind of temperatures it will see in service -- probably up to around 120F.  If the fan is in the return air flow to the sunspace, then the temperatures will be low.

In some sunspaces, natural ventilation in the form of high and low vents in the wall joining the house and the sunspace is used.  The heated sunspace air flows through the high vent and into the living space, while cool living space air flows back into the sunspace via the low vent.  I do not believe that this kind of ventilation is sufficient for a low thermal mass sunspace -- they produces to much heat to move in this way.  But, natural ventilation can be a supplement to fan forced ventilation as in Mike's sunspace.

If such openings are used, they must be closeable at night to prevent heat loss.

It would be good to hear what people have been using in the way of fans, duct systems, ...

Venting -- Prevent Overheating in the warmer seasons

Low thermal mass sunspaces will need a good system to vent heat to the outdoors when the heat is not desired.   The vent system will be needed during the seasons when you don't want the heat to go to the house, and also for mild winter days when little house heat is needed, but the sunspace is generating a lot of heat.

I think it fair to say that most people underestimate the mount of vent area that is needed -- so go big on the vents.

I'll try to add some more specific recommendations here, but as a start, looking at the venting recommendations for greenhouses  with the same glazed area would be a good place to start.  This is one reference that covers venting requirements for an all season greenhouse...  scaling these by glazed area might be a good starting point.

Performance -- Can be excellent

Low thermal mass sunspaces are amazing producers of heat -- this section puts some numbers to that claim.

Heat Output and Efficiency:
Low thermal mass sunspaces if designed to the guidelines listed above are very efficient heat making machines.  Our test sunspace indicates that they are  just as efficient as high quality commercial active solar collectors having the same glazed area.  This is quite an accomplishment given that you get all the other multi purpose advantages that sunspaces offer in addition to very efficient space heating.

As a rough guide, under full sun conditions, LTM sunspaces will deliver about 160 to 180 BTU per hour per sqft of glazed area depending on season.
In less sunny conditions (600 watt/sm), the delivery rates falls to 80 to 100 BTU per hour per sqft of glazing -- still a very useful amount of heat.

This section on the heat output testing we did on our prototype LTM sunspace gives a great deal more information on performance.  In particular the day 1 test has a lot on sunny day performance.

We also tested our sunspace before certain of the design guidelines listed above were met to get an idea what the sacrifice in heat output would be.  This is covered on this page on heat output for a non-optimized sunspace.  In a nutshell, the heat output for the non-optimized sunspace was less than half of the optimized sunspace under similar conditions.  I think that this large drop in heat output for deviating from the guidelines for low thermal mass sunspaces is something that is not sufficiently covered in most of the literature on building sunspaces.

Depending on the size of the LTM sunspace and the size of your house and how thermally efficient your house is, there may be times when the sunspace can produce enough heat to overheat your house.  In these cases, the sunspace heat can just be shut off, or the excess heat vented.  But, for a lot of us, our large heat leaky homes will take all the heat the sunspace can deliver on a cold winter day. 

During the day, as the sunspace warms the house, the house thermal mass is also warmed, and this allows the sunspace heat to last a ways into the evening.  This will be more effective if the house temperature is allowed to rise somewhat above the normal temperature you set.

The Potential Improvements section provides some thoughts on adding low loss thermal storage to LTM sunspaces for cases where the sunspace routinely produces more heat than can be used.

In any case, the consequences of having more glazing and heat production than the house can routinely use is not as much of a problem as it would be on an over glazed passive heated house in that the sunspace is separate from the house, and excess heat output can just be shut off.

Sunspace Livability

Low thermal mass sunspaces can be quite comfortable during the day when the sun is on them.  The heat distribution system is continually removing heat from the sunspace, so the temperatures in the sunspace remain comfortable.

During the middle of the day on a bright sunny day, and particularly if there is reflective snow in front of the sunspace, the glare can be strong.  To control that, some form of shade cloth screen that lets a fraction of the sun through and that is can be deployed across all or part of the glazing will cut down on the glare.  This is to some degree a matter of personal taste -- some  people like the very bright conditions, and some prefer some screening.  Having a screen that can be easily moved into a position where it provides glare protection, or out of the way for less sunny days makes the sunspace comfortable under a wider range of conditions.

Do you sunspace owners out there have any other advice on making the sunspace more comfortable and useful a wider range of activities?

Examples  -- $22 and up

These are some of the example low thermal mass sunspaces from our website section on LTM Sunspaces.    They show the very large range of designs and builds that are possible.  Low thermlal mass sunspaces can range from a sunspace built in an afternoon with a roll of poly film up through very practical and cost effective sunspaces that can be built for a few hundred to a few thousand dollars worth of materials, up to high end sunspaces with very high level finishes that may cost hundreds of dollars per sqft. 

The example list below gives a brief description of each LTM sunspace -- for more details on each, follow the links. 

Mike's Low Thermal Mass, Multi Purpose Sunspace in the Colorado Mountains

Colorado low mass sunspaceThis is Mike's low thermal mass sunspace in Colorado.  It is used for a variety of purposes including heating the house, drying clothes, kids play room, drying food, and reading the paper.  In addition to all those uses, Mike is currently working on a way to integrate solar water heating into the sunspace.

An exceptional job of blending the added sunspace with the existing house. 
The variety of purposes that sunspaces can be put is what makes them exceptional -- drying clothes alone saves about 1200 KWH and 2000 lbs of CO2 a year.

Mike provides all the details on the design, build, and performance...
Nick Pine's Innovative Three Story Sunspace

All the details on Nick's sunspace...
innovative sunspaceThis is Nick's new and cutting edge three story sunspace. 

It includes a number of innovative features such as custom made DIY double glazing, an experimental mesh system to improve collection efficiency, and a unique heat distribution system.

All the details on Nick's sunspace...

Low Thermal Mass Sunspace in Minnesota,
William Sikora




Architect William Sikora describes his design for a modern and elegant sunspace for this Minnesota home.

The air distribution and control system is very simple, and designed to be maintained by the home owner with only simple tools.

Certainly a beautiful and cost effective addition to the home.

All the details on this modern, elegant sunspace...
Norman Saunders Cliff House

Cliff house by Norman Saunders (pdf)


The Cliff house designed by Norman Saunders uses a large, 2 story, low mass sunspace across the front of the house that provides 100% of the heating demand for the house all year.

The sunspace is coupled with heat storage systems in the attic and crawl space that allow the home to achieve 100% solar heating.
Instant Sunspaces  -- How cheap can a sunspace be?

Laren Corie
Download the Feb, 2005 issue


ESSN Journal, Feb, 2005

A good article by Laren Corie on how to build and experiment with inexpensive sunspaces.  The example used in the article cost a grand total of $22, and went together in a couple hours!

A free download.


Ed's Zero Cost Sunspace ...

Here is another very low cost (actually zero cost) sunspace done by Ed.   Details ...

This shows how inexpensive and easy it is to turn a porch into a sunspace that is pleasant to spend time in, and can supply some heat to the house.

Air based solar-thermal “Hot-Box” collector

Full details on "Hot Box" collector...
solar thermal hot box collectorMitch calls this a "Hot-Box" collector, but I think it could equally be called a mini-sunspace.

Very nicely built -- as Mitch says, its "wife approved".

Some performance estimates provided.

Note the use of the reflector to improve collection -- reflectors are particularly effective in that they increase collection area without increasing heat loss.
I would pass on the idea of adding rocks for heat storage for the reasons discussed above -- unless you have a way of insulating the glazing at night.

Potential Improvements

While the sunspace discussed above as it is does a very efficient job of producing heat, there are some things that might improve it.

Collector Efficiency:
What causes a sunspace to be less than 100% efficient?  Two main reasons:

Part of the solar radiation is absorbed by the glazing itself, and never gets into the sunspace.  For good single glazing about 90% gets through, and for good double glazing a bit over 80%.   So, you lose 10 to 20% off the top to glazing absorbtion.

Heat loss out the glazing from the warm sunspace interior accounts for most of the rest of the reason we don't see 100% efficient collectors.  This loss is partly conduction through the glazing from the warm air next to the glazing, and partly radiation at thermal wavelengths from the absorber toward the glazing.

Some things that might help:

One thing the sunspace already has going for it is that air velocities next to the glazing are quite low -- this reduces the efficiency of heat transfer from the air to the glazing, and that is good.

Deep Mesh:
Nick Pine's sunspace  has a fabric mesh that is fairly close to the glazing.  He introduces the supply air (probably around 65F) between the mesh and the glazing.  The mesh has enough air resistance to spread the supply air over the full surface of the mesh.  So, the sun heats the mesh, and the uniform airflow through the mesh takes that heat away.  The desired end result is that the air next to the glazing and the mesh absorber will both run relatively cool, and that this will reduce heat loss out the glazing, which is what efficiency is all about.  Hopefully as Nick gets further along with his sunspace we will get a reading on how well this works.

If the sunspace is large enough to routinely produce more heat than the home can use during a sunny day, It would be desirable to be able to store some heat in the sunspace for home heating use after sunset.  While its easy to add mass to the sunspace to store heat, the problem is that with the normal sunspace design a lot of this heat will be lost out the sunspace glazing before it can be used by the house.  

Some possibilities would be

Of course, if the sunspace is modest in size and the home is large and heat leaky, then the thermal mass of the home will store some heat during the day without the house overheating, and it will give that heat back after sunset.

Air Pickup:
An air system that picks up heat efficiently from the absorbing surfaces would help to keep the absorbing surfaces cooler, and would reduce heat loss out the glazing.  Sunspaces provide a lot of room to work with, so maybe there is a way to direct airflow so that it picks up heat from the absorber more efficiently?

Absorber Design:
An absorber that absorbs the heat from the sun efficiently and transfers it to the air efficiently will run cooler and lose less heat out the glazing.  For example tests of flow through screen absorbers in DIY solar collectors have proven them to be a good deal more efficient than (for example) a bare black box.

Contrast to an Active Collector:
When used for space heating, the sunspace is being used as an alternative to a conventional solar air heating collector to provide space heating.   Its interesting to think about  some of the contrasts. 

  1. The flow velocities are much lower in the sunspace -- in most places I can't measure them with a good air velocity meter and you really can't feel them, but in a conventional collector, air velocities are fairly high -- up toward 100 fpm and more.
  2. In a conventional collector, there is usually a significant air velocity along the glazing, where as in this sunspace its too small to measure.  
  3. The surfaces that absorb the solar radiation are spaced further from the glazing, are larger, and often run cooler.
  4. The flow pattern in a conventional collector can (it seems) be controlled with channels, baffles, screens.  What would one do in a sunspace to control and direct the flow to the desired areas? 
  5. The sunspace has more outer heat loss area and more potential leakage area.

Do these differences offer some opportunities for greater efficiency?

Got other ideas?  Use the Comments section below or email me. 


Gary January 18, 2013


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