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


There are some good existing guides on building sunspaces, and I've included references to them below, 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 design advice for doing a good one.





Table of Contents

What is a Low Thermal Mass Sunspaces?



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

Examples  -- $22 and up

Other Guides On Sunspaces


What is a Low Thermal Mass Sunspace?

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 this 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:

Another difference between low thermal mass sunspaces and regular sunspaces is that the LTM sunspace will cool off quickly when the sun is no longer on it.  It is not a place you (or your plants) will want to spend time on a cold winter night -- it will be nearly as cold in the sunspace as it is outside.  This can be a disadvantage for some sunspace uses, and it needs to be weighed against the greater heat production of the LTM sunspace. 

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


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 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.  

As a rough example, suppose you are building an 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 a 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.

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, every thousand pounds of thermal mass in the sunspace that gets warmed up from 20F in the morning to 80F over the day will

- 1000 lbs of wood warmed from 20F to 80F uses 30,000 BTU of solar heat

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

These are rough estimates, but it 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 (which could be indoor-outdoor) over a light underlayment with insulation under seems like one approach that might work.  Got any other ideas?

If the sunspace has a deployable netting or shade cloth between he glazing and the floor that absorbs some of the solar and reduces the direct solar on the floor, then the mass of the floor would be less critical.



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.

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

Under these 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) = 3200 BTU per hour
                               Just about right.

Looking for a fan to fit this, and assuming that the duct system pressure drop is around 0.1 inches of water,





It is also highly desirable to have


Just to give an idea, a sunspace with 300 sqft of glazing will have to move as much air as a typical full sized house furnace. 





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 -- it 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.




Venting -- Prevent Overheating in the warmer seasons

Performance -- Can be excellent

Examples  -- $22 and up

Low cost ones

Potential Improvements

Other Guides On Sunspaces





Low Thermal Mass Sunspace Guide

This is a guide providing information on Low Thermal Mass Sunspaces for space heating (and other purposes). It supplements the information provided in these more general guides on sunspaces, and provides more detail on low mass sunspaces:
- Guide 1
- Guide 2
While the guides above are fine, they don’t provide much specific information on the advantages of low thermal mass sunspaces – so, this is an attempt to fill in the gaps.

Explain how the LTM SS works – maybe quote some of Shrucliff (or ref)
Hit the multi purpose and high efficiency aspects.

Reference some of the conventional guides – explain how you don’t think they handle LTM spaces well and what the benefits are

Whats important in an LTM SS
- Good extraction system – high volume, low noise, distribution
- Well placed glazing
- Good insulation
- Good absorbing surfaces
- Low thermal mass
- Good venting for summer and spring/fall
What is the penalty for not following these rules – show.
Examples of LTMSS s – vy cheap to expensive.
How does the DIY LTM SS compare to other projects on return? (including pv, …