Passive-solar has been known to work well but certain things about our architecture fail the thermodynamic efficiency test and there are reasons why.

First is heat-transfer through conduction, the wood, drywall, sheathing all having rather high heat-transfer coefficients. Next is convection from a heat-riser which takes the warmth of the floor and moves it to the ceiling.

Until these are dealt with they cost a lot of energy, if one is trying to not require external heating for a building.

A first principle to recognize is that we need to insulate the OUTSIDE of a building to prevent heat-transfer from anything nailed to the studs which act like pathways for heat to be lost to "radiators", the sheathing & siding of the building. Insulation between the studs doesn't do much to prevent this, a direct insulation of exterior heat-transfer surfaces needs to be added, using insulated sheathing is the simple way to do this.

The second is if you don't collect heat you can't create an autonomous building that maintains comfort zone, followed by if you don't have enough thermal mass to heat during the day the building will cool back down before dawn.

Those are the three pieces to use in any situation but standard framing and insulation techniques need to be altered to gain this idea in a design.

The largest single key for comfort is to create and circulate warm air during the day that heats up the thermal mass of the building. If you use insulation on the outside of the structure all of its mass becomes thermal-mass to maintain comfort zone for the interior instead of being able to conduct heat to the outside world.

Then, a greenhouse wall is mandatory in passive solar, so how to get it to function as an air heater requires two panels with an air gap to warm the air well, this is needed to force a flow from floor to ceiling, but that's not good enough, the warm air from the ceiling needs to be drawn all the way down to not lose too much by it all gathering high in the room.

This implies using a slightly different way of building. First the typical plywood sheathing is used on the INSIDE of the walls opposite where the greenhouse wall is with joists between the roof-bearing wall used as ductwork so between studs is open near the ceiling with inlet vents and inside the wall open to the floor joists inside the walls & floor so the air must pass through the floor joists to get to the double greenhouse panels that heat it. If done correctly this air-flow will not require fans. The floor joists are covered by exterior insulated sheathing to open the space to this airflow.

The result is a space heater built into the home that keeps the floor warm on a daily basis, uses all the mass of the building as thermal storage by insulating outside the structure and avoiding easy heat-transfer paths. I'm working on ceiling systems that fit into this scheme, to gather heat or cold for the needs of the season but wanted to put these concepts out to people on the list to consider in their remodels & building from scratch.

Tags: comfort-zone, heat-transfer, passive-solar

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Bud, your glass is half empty so filled from the bottom up, mine is half full so uses expanding a gas as a dynamic process and due to density it's easier for the air to move up into the room than back down.

The only air that can be moved to the bottom of the solar wall is from very near the ceiling, whether the doors are open or not, and normally that will be the warmest air in the room so that's what the cooler air begins as, and the floor won't be much cooler than the ceiling.

If you want to figure this out joists are on 2ft centers, figure a 12ft wide unit now with the high side solar, so not like the drawing, reversed roof. Walls are 2x4, inside plywood sheathing, outside insulated sheathing so the space between joists & studs is wide open as duct.

First Tom, no one has ever mistaken me for a pessimist (glass half empty) :). Now, let me try to explain why expanding gas is not your moving force.

There are three phases for your convective loop, start-up, stable operation, and cool down.

During start-up, yes, the expanding air will be looking for a place to go.  Up is better than down because the air above is probably warmer/lighter than the air below.  There will also need to be air escaping somewhere as this is not a pressurized system.

The cool down phase will result in a contraction and corresponding infiltration of outside air, again, somewhere.

The most important phase of your convective loop cycle is its stable operation.  During this phase, the net expansion will equal the net contraction and the push you are expecting will no longer exist.  Yes, the air in your collector will expand, but yes it will contract an equal amount somewhere around the loop, so that when you look at the bottom of your collector, there is zero net force from expansion.

We know that air circulates due to convection, but what we need to understand and focus on is where the moving force comes from, and again it is the difference in weight between warm and cold air, and that is it.  I know you are resisting this concept and I want you to understand I'm just trying to help.  Once you accept this, there are some additions that can make the air move better, and it would be a lot easier to incorporate them before construction than after.

Currently, once the hot air leaves the collector, there will always be some heat loss, thus the now warm air will flow across and down, being heavier than the hot air in the collector.  This warm air will then be able to do what you want and transfer some heat to your thermal mass before it is again moved up into the collector and around and around it will go, until the entire loop becomes too warm for any significant circulation to continue.  But that is where you want to be.

Re-read my previous link and if you want the same numbers for your system I'll be happy to provide them,  If not, I won't bug you.


Bud, my apology for the analogy, had no clue it would ever be taken as you did, so much for creative writing.

The slower the velocity, the higher the delta-T, so, it'll equalize somewhere is what I expect, and if I got what you're saying, it's what will happen according to density differentials.

There is a cooling part of the loop to back below the floor, it's the only way for air to be replaced in the collectors [this is on a mt. ridge with a lot of cold wind]. The north side has R30, the floor to start R19 foam board as a wall of the duct

Should have new drawings done this week, the design initially used added thermal-mass as the 2x6 T&G instead of a plywood floor to store heat but know that isn't nearly enough thermal-mass to store heat there for a night so adding more using a good thermal fluid strapped to the T&G from below leaving 4" of free air space between joists below them; found bi-metal louvres but their change temps aren't right so may need custom ones for the ceiling.

After more calculations and thoughts I'll be installing 6-inch PVC pipes between floor joists for their length full of thermal-fluid to be able to store enough heat daily to last through a night. With three pipes between each set of floor joists it comes to about 25-cubic feet/192-gallons of fluid with a specific heat of about 2.0, twice that of water.

These are needed, hoping it'll be enough mass to win the Scotch!! [have a bet going].

Back to the top again :)

Here is a rough drawing of how one might view the forces pushing the convective air flow around in your application.  I have assumed a 1/8th Pascal difference per foot between the lighter "hot air" and the heavier "warm air".  Let's see if I can get this to show up?

There, it looks readable.  Not bad for an old dinosaur. Now, I haven't analyzed this as I don't have heat loss and return temperatures, so the estimated 1.5 Pascal differential pressure is simply averaged above and below some theoretical neutral zone.  The starting 100 Pascals is not really important, just a reference point.  Refer to this link for a longer explanation on how the pressure stacks up.


OK Bud, the design flipped so I redid drawings, did more detailed drawings and added more thermal-mass using a couple of 6" pipes filled with water strapped up to the floor between joists. Will apply your formulas to these and see how it comes out.


Hi Tom,


1.  How is the heat in this solar room to be used?  Is there other living space connected?

2.  You state two panes of glass.  Is the heat absorbent surface after those or are you collecting heat from between them.  If between, then why is the second one glass?  Is the glass coated or selective to allow solar energy in and reduce long wave radiant heat loss.

3.  On your third sketch, mark A, B, C, and so on and list the temperatures you would anticipate when the circulation is in progress.

4.  I haven't figured out the benefit of the return air venturi.  Looks like just a restriction in the air flow, loss of pressure.

5.  Just an opinion without any calculations, but I would think you would need a significant increase in thermal storage.  Have you done any reading at places like  Gary has a ton of solar information.


Just saw your questions:

1. No, this is a small cabin, 12'x12'.

2. The glazing pane to absorb heat may be coated but must be rather transparent as the view is the main attraction.

3. Haven't gotten to where I can estimate temp's yet.

4. Well the venturi will reduce turbulence into the slot formed by the panes, there's a damper board that leaves only a 2" gap to feed it, joists are 2x10's so 9-1/2" deep, the upper part will hold the warmer air, the damper and a small board parallel to the joists is trying to keep flow more laminar. So, the restriction is from 2" to 1/2".

5. I've got a possible 192-gallons of thermal-storage but trying 2/3rd's of that at first [two pipes instead of three between joists]. Filled with water that's a decent amount of mass, but everything is screwed together so if needed I can add the third pipe.

Adding that I'll be documenting performance with sensors & data loggers & will publish so people can use the data.

Tom, it's early morning so maybe I'm not thinking correctly, but if the glass is also a window, how will it collect any heat?


: ) ... seems that way but doesn't actually need a coating, the panes heat up OK without anything, but I have to have panes cut to a custom width so was going to see what I can get in anti-glare or something to help it out if it isn't too much more money.

For the really high-tech version having a coating on the top panes that prevents IR from escaping would be nice!!

If I'm looking at this correctly, you are wanting to collect heat from between two panes of glass to circulate and store for after hours use.  I don't think it will work that way.  The amount of energy between the panes of glass will be minimal and if you do circulate air through that space, the temp will quickly drop.  A solar collector needs to be absorbent to heat up and that necessitates it being opaque, not transparent.  High tech glass can be designed to be transparent, or somewhat reflective at certain wavelengths, but I know of none that are designed to absorb heat, unless they are painted black.

Your solar window will certainly warm that room when the sun shines, but you will need some really warm air to transfer heat to your collector and those temperatures would be very uncomfortable inside that cabin.

IMO, your collector needs to be other than a window into the living space.

Not sure what to recommend



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