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Design Notes -- Low Thermal Mass Sunspaces
for Space Heating & Many Other Purposes
Introduction
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.
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Table of Contents
What is a Low Thermal Mass Sunspaces?
Glazing
Insulation
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:
- They are true multipurpose family spaces that can be used for lounging
in the sun (even when its 10F outside), reading the newspaper, kids (or
adults) play area, workshop, ...
- They provide a good year round area for drying laundry, which can add
substantially (1200 KWH a year) to the energy savings.
- They can be aesthetically very pleasing and add value to the house.
While I personally think that conventional solar collectors look great on
any house, not everyone has come around to that viewpoint. Sunspaces
can be a solution that everyone (including your HOA) agrees looks great.
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:
- Large glazing that is optimized for winter sun collection.
- Low thermal mass in the sunspace (including floors)
- Insulation for all non-glazed surfaces.
- Finishes that absorb sun well for all surfaces.
- A high capacity and quiet system to transfer heat from the sunspace to
the house.
- A good vent system to avoid overheating.
Glazing
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
Comments
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 dont provide much specific information on
the advantages of low thermal mass sunspaces so, this is an attempt to fill in
the gaps.
Contents:
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 dont 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,