Laren Corie

Back in my passive solar designer days, I designed quite a few (dozens) underfloor sand thermal storage systems for houses, to serve as relatively short term (days) thermal storage to mainly even out temperature swings, from very large south facing window areas, usually using solar sunspaces These heat sinks used a foot deep sand bed, under a fairly standard 4" concrete slab. For the air tubes I used 4" or 6" corrugated polyethylene septic drain tile tubing, 12"oc, which has a relatively large heat transfer surface area, and tends to tumble: the air through to increase turbulence and relative air speed, thereby increasing heat transfer efficiency. The corrugated tubing walls, while plenty strong enough, are also thin, so the heat transfer is far superior to a thick smooth tubing like PVC. It also costs a lot less, and is flexible to align with the holes into the insulated supply and return plenums along two of the the foundation walls. The separation between the plenums and sand is maintained by a line of standard concrete blocks, laid on their side, and glued to the lower rigid foam insulation. Another line of rigid foams stands against the blocks, to hold the sand back. The tubes run through block cavities with a tight round hole through the foam. Now that cans of spray foam are available, they might be used to seal any potential sand leaks. The tops of the plenums are covered by metal roofing (or similar) before the slab is poured. It became a pretty easy system, after working out the little tricks.

Here is a picture of one of the systems ready for the slab, with EPDM rubber tubing for the radiant backup heating system, for when the sand bed was not putting out quite enough heat to fully maintain the house temperature. This one was built in a very cold, very cloudy, Northern Michigan climate, with full exposure to the north winds over the lake.. I did another one in Ontario, and later a different (water based) system in Nova Scotia.

For another option that still allows a view, this is a full time, partially insulated window.  This (image) is a double-hung, double-glazed, low-E (≈R 3.3) with an additional storm window, so the upper sash is a real good ≈R4.3  But, the lower sash has an additional 1 1/2" of ISO insulation plus the wooden shutters, for  roughly R14, better than most walls.  That does not produce a net Rvalue of (R4.3 + R14.3)/2 = R9.3   But, it does bring the whole window up to: ≈  1/ ( (1/R4.3 + 1/R14.3) /2) = R 6.6  which brings a real good but not extreme window up to the insulation level of very expensive super-insulating windows, and it still meets bedroom egress requirements..  We bought this clad wood window, with storm, unused but second hand/recycled, at a Habitat for Humanity ReStore, for only $65. The shutters, also unused/recycled, from a Restore were $5, and the insulation (similarly sourced, cost less than a dollar. This window lets in more light than most windows with curtains, while also providing adequate privacy, while we can awake to birds at the feeder, and looking out at the trees.  Point being that it is not necessary to completely insulate windows, or to totally block the views, in order to significantly reduce heat loss.

This is a simple formula, originally from an old solar greenhouse book by Bill Yanda:

     cfm = 16.6 Av sqrt((Tu-Tl)h)

Tu is the temperature at the top (outlet).
TI is the temperature at the bottom (inlet)
h is the height between, in feet
Av is the smallest cross sectional area of the vent, in square feet.
16.6 is a constant

It could also be written as:   CFM = 16.6A sqrt(HdT)
with A as Area,
H as the Height,
dT as deltaT (the temperature difference, top(outlet) minus bottom(inlet))

It is a bit different from some other formulas, but simple and seem to work well enough.

Phil wrote:  "To avoid the problem of leaks, how about filling with sand?"

Hi Phil;

Though standing bottles upright, with the sealed opening  on top, should solve the leak problem, there are generally more problems with sand than with water, due to effectiveness.

Sand has less than a third the thermal mass of water, per volume.

Sand also weighs 60% more by volume, therefore about 5 times (or more) as much weight per thermal mass, so it generally needs to be located low.

Dry sand has an Rvalue of about 0.58 per inch, while water thermally circulates, so has virtually 0 Rvalue, though it tends to stratify, which for small containers with large surface areas, such as bottles, is generally a non-factor. ..and in big containers can be used as an advantage.

Concerns with water might be 1) freezing, 2) leakage, and 3) corrosion.   All three of these might be overcome by using recycled plastic containers, which can literally freeze solid without bursting. . However, that introduces an issue of evaporation through the surfaces of the containers, after several years. That could be reduced by a low permeability, vapor barrier paint, and/or a partial covering (around the middle) of aluminum tap, which while it would be difficult to fully cover a bottle, has a permeability that is virtually zero, and also reduces the radiant heat losses, which might be an advantage in an air convective heat storage.

As I have stated before, my recommended location of a thermal mass using small containers would be above the heat source, where air can naturally/passively convect up to it. This configuration also provides very good control of heat coming out of the heat storage to the space that is to be heated. An active fan (electric, even supplied with green electricity) can be used with a thermostat for passive solar heating with virtually full control, while avoiding the labor and/or cost and thermal uncontrollability of a heavy masonry thermal mass.  It also opens up the choices for the floor, including wood frame structures. and second story solar gain.

- Retired designer of passive solar and highly energy efficient homes -

  Just to expand on Anne's wise suggestion to "heat the person not the space"..........   While more insulative blankets and clothing can work well, it can be bulky, and it can also be nice to have the bed already warmed when you first get into it.   For that, electric bed pads or electric blankets can work well, and can usually be turned off after getting into bed.  

  Back in my young hippy days I had an electric blanket develop a short in the middle of the night, and woke up with the room filled with smoke.  It was a very dangerous situation, though it ended alright after we dragged the smoldering blanket outside where it literally burst into flames on the fence.....an image that I will never forget.  The new ones are fireproofed, but that might not be all that healthy either, so I avoid them.

  As someone who spent a couple of decades in a leaky old, woodstove heated, log home in the north woods.....my favorite bedwarmer (by far) ended up being a recycled two-liter plastic drink bottle, filled with water and heated in the microwave.  The round water bottle can be adjusted closer or further away, for how much, or where, the heat needs to go.  They really work exceptionally well, with enough heat for all night.  Years back, I ran some big Yahoo Groups on sustainable living, and a lot of those people ended up using two-liter bottles as bedwarmers too, and even lowering their thermostats at night.  Try it!  It really works well.   Just carefully monitor at first, to figure out how long the bottle needs to be in the microwave.  Now, they even sell stuffed animals with water inside, to warm in the microwave.

 As a retired designer of passive solar and highly energy efficient homes, remodelings, and additions, I have to commend the posts so far.  This is a smart group ;O).  What i can add is maybe a bit of prioritization.   We don't know what climate you are in......only that it gets cold.  In the long run, insulation on the outside of the block, and in the attic, would be ideal, but interior insulation would be good as well, since thermal mass in that location doesn't really do much that is effective, and might even extend summer overheating, from west wall sun exposure, which might be mitigated with plantings/shading or even sun reflective paint..  For now, you need to make the walls not fell so cold. So, as was already recommended, do what they used to do in stone castles, and hang cloth or blankets over the walls, to present a warmer surface and reduce radiant heat losses from the people and objects in the room.  Raising the average temperatures of the exposed surfaces, is roughly 40% more effective than raising the temperature of the room air.  Of course, seal any noticeable air leaks, too. Then leave the door open and use a box/floor fan to draw cold air out of the room, so that warm air will flow in through the top of the doorway. Having the fan blow out of the room (besides not creating a cooling breeze in the room) will work with the natural air flow instead of again it.  Of course, warmer bedding is always a good thing.  Get the kids involved in figuring out and solving their own problem, too.

 Instead of putting the bottled (or other moderate sized water containers) in the floor, place them up high, in a loft, supported by a bearing wall on a foundation, like the exterior wall between a sunspace or solar greenhouse, and the rest of the house.  such a wall can easily support the weight.  Warm air (from a sunspace, solar greenhouse, solar air heater, or even dark blinds at windows, that will intercept the sunlight and serve as a solar absorber) will then rise (passively) to warm the bottles, creating a thermal syphon loop.  With the heat storage located above the living space it can get quite warm without overheating the living space.  Then, when the house needs heat either a duct and fan, or even a ceiling fan can bring warm air down to living space level, so that you effectively have passive solar without needing the big heat loss through direct gain windows, and can even have thermostatic control, to avoid temperature swings.

- Retired designer of passive solar homes -

Jay Angler wrote:

>>>>  I have found in homes with little or no thermal mass (think stick built), the furnace in winter and the air conditioner in summer, cycles between too cold and too hot, at least partly because of the air flow. So my experience is that adding thermal mass helps to dampen those cycles, rather than it just storing excess heat. Having thermal mass radiating even a little heat, seems more pleasant than vents "blowing" heat.

 Yes, forced air furnaces (and others) can be obnoxious.  However, the temperature swings, and the blasting too, are often the result of over-sizing, which is the general practice in the fossil fuel HVAC installation industry.  Thermal mass for overcoming that needs to have a very large surface area spread throughout the house, preferably at a relatively high height, and still will not  
really make much difference.  The much green alternative is mini-split heat pumps, which include concealed duct units. They can be powered by solar electricity, even from community solar farms, via the grid.

>>>>>In my stick-built home the season matters a lot. Around the summer solstice, the sun is so high that even though it's warm outside, the sun doesn't shine into the windows that much. However, come late August and early September, we get a lot of solar gain in the living room with no good way to spread that gain from daytime to evening.

The vast majority of people do not realize that the sun's path is identical on the Vernal Equinox (≈ March 21st) and the Autumnal Equinox (≈September 21st) when it can be 40°F or more warmer.  This means that fixed overhangs are not (alone) a satisfactory solution for avoiding passive solar overheating. Direct gain windows also represent very large heat loss. So, while code compliant and view  windows should be well solar oriented, the way to get significant passive solar (or hybrid) is to use isolated gain, in a space that can be allowed to experience much wider temperature swings (sunspace/solar greenhouse)

>>>>In winter, we get too much cloud for solar gain to be useful.

 It is not uncommon, in the east US and Great Lakes area, to get too much cloud cover for direct gain to be comfortable for part of the year, but isolated gain can still works very well. And, there are always good sunny shoulder seasons.

The purpose of thermal mass in the living space is to stabilize room temperatures by storing excess heat (and only during the heating season) that is causing uncomfortable overheating, so that the heat is then released later (hours) when the temperature in the space drops to below the temperature of the thermal mass.  It is also not as simple to implement as it may seem.  So.......if the space is not overheating during typical winter/heating season days, then there is no reason to add thermal mass (or even to more simply just circulate the warm room air to heat other rooms) and it would be a waste of time and resources to add thermal mass.  Besides, you are likely to create a "Three Bears" scenario for your cat.  Stone might be too cold (especially in the morning) and can literally suck heat from your cat as well as from sunlight, and black metal would absorb more light instead of reflecting it away to be absorbed by other surfaces in the room, so it could get too hot.  Your cat seem think that right now it is just right, and loves it the way it is.......don't spoil that. ;O)

- Retired designer of passive solar homes -  

I am a retired designer of passive solar homes, specializing in sunspaces, but in cold/cloudy climates.  A lot of the advice you have already received here is very good, so let me summarize the priorities. First, solar heat is a good thing during the heating season, so don't waste it by venting it to outdoor and then heating your home with costly fossil fuels.   The vent to the house was a good thing, so define and solve the problem you have with it, instead of throwing away the venting system including its advantages. The old adage is "Don't throw the baby out with the bath water." In other words, vent the excess sunroom heat (during the heating season) to heat the house in some other way. That may be as simple as leaving the door open during the day, or moving the fan opening to vent the warm air into a different room. Or....is it really a problem to vent the hot air through your daughter's room in the afternoon, during the heating season, when she is probably in school?  Is the issue the noise, or the excess heat...both? or neither? To avoid sunroom apparent overheating in winter, and still use it to heat the house, you might use an "interceptor" strategy such as the interior window screens suggested already. What that does is absorb the sunlight, which heats up the dark screen material, which in turn warm air that rises through it, to the ceiling height and can be vented into the house via the circulation fan through the wall. This works like a typical solar air heater, but with a room behind it instead of a wall.  During summer, you may use a shade sail or other outdoor strategy to reduce sunlight getting to the windows, and/or also vent the sunspace to outdoors through the east or west walls., (windows or vents).  Since you are in new Mexico there are a lot people who understand how passive solar heating works, and have dealt with many of its idiosyncrasies. You can ask around and invite one (or more) over for lunch and a look at your situation.  New Mexico is the big passive solar state.