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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Don't think I need a coating really, I can have 12-ft panels if needed but this is only 12x12 with about 100sf of collector for 1,200-cubic feet of volume so my hunch is it'll work OK as-is & willing to try it since many passive-solar homes I'm familiar with are too hot all year.

From that, this is trying to find a more balanced mix than massive cement thermal-mass walls with glazing which then force one to move the heat to where it's needed.

And, it's an attempt to make the greenhouse the living space so if it's too hot that's no good either as you recognize, the killer on that is having the inner glazing on piano hinges so you can open them up, had to have that ability to clean them & knew it'd be the way to moderate the gain.

So there's the design trade-off in a nutshell. Since it's a rarely visited cabin if it's too hot when you get there it'll be a hang-out!!

My guesstimate is still about 60F for a delta-T over 8-feet of gain with bare glass at the slow flow-rates that are likely ... and if the roof gets hot vents open to recirculate from the peak area back down to the rear wall & below the floor.

Adding that in a normal home this will be very difficult as David pointed out ... for that if you don't go through that much trouble in sealing and all I'd use PV-batteries to power small fans to move heat when it's being made to storage for redistribution from that.

Incident angle affects transmissivity versus absoption in single panes, you're right when the sun is about exactly perpendicular it'll transmit most all wavelengths, but as incident angle changes the glass absorbs more energy and that's converted to heat.

I couldn't remember exactly why panes heat up so didn't want to speculate, found this online of a paper from 1962, go to the end for the graph:

When you live with a few of these buildings I gotten how they work but not always why!! Just posted on the volume change in the slot, 1.2-cf so if it takes 15-seconds for a molecule to rise the 8-ft, not bad flow out into the room.

Double checking, 5.7kw/day seemed low even for a 12x12 and found that I took 400w/hr losses for the glazing as 100 when I added them up, almost triple is correct, should be 13,584w/day, 6.8kw/hr total, 762w/m2 so a big jump from no problem to makeable ... sigh.

Reacting to David has brought out my awareness of using the crawl space soils as thermal-mass by radiating from the floor joists. It brings up that aspect of the design equation that happens over time because in many solar homes after a few years this soil temperature is significantly higher from the "heat-loss" radiated in that direction by where they stored the heat in the passive-solar design [rocks in the basement was popular but you had to circulate the air].

This is all from experience with passive-solar homes over time, they heat up everything and most design equations don't have an aging factor for this, so, consider the crawl space an asset for thermal storage you get from the main priority to store thermal-mass under the floor from the ceiling excess dynamically such that delta-T is reduced significantly, that's a thermal goal always.

Using this online calculator to get approximate losses [], totals about 240w/hr for 0C-20C/32F-68F for diff or 5,760w/day.

For the Pacific NW it's wise to use 2-hours a day of gain for clear-day values for total daily gain, depends on the site of course, so that's 2,880w/hr, collector is 9sm/96sf or about 260w/sm which coincides with charts so in the ballpark.

For this design it'll use plain-glass insulated dbl-panes, not low-E, fast gain at any incident angle is the need for this site. The top row isn't collector so will have moveable insulation to put in when you leave having R30, I'll put in a decent door, R11-ish.

Next is figuring thermal-mass to store it ... haven't tried to figure how much heat is produced either but thought this was interesting.


  You state that "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"  I disagree.  Several years' field experience auditing houses, working with customers, and observing current research has shown that moving air (both through and within the dwelling) is the number one culprit for comfort issues.  The way to improve interior comfort is to minimize interior air flow, minimize other avenues of heat loss (radiation, conduction, etc.) and provide proper (i.e., designed, forced) air exchange via an energy recovery device.

  To this end - if you provide the super insulation on the outside of the building envelope as you suggest, AND you provide an ultra-tight pressure boundary on the interior, AND provide a properly-engineered air exchange system WITH solar gain engineered into the building orientation/glazing design (all parts of the Passive House design criteria) THEN you can use the recirculation/de-stratification effects of the air exchange system to move the warmed air evenly throughout the entire house (even with closed interior doors) without relying on tempermental thermosyphon designs that rarely work well and usually degrade perceived comfort.

Thanks for the thoughts David. The point you make is to circulate warmth to an entire home using a solar-gain room/area is how I take it and can't disagree when trying to use the gain to heat all of it with natural circulation.

For homes, generally by site, to heat them I recommend using a trough collector [can be anywhere on the lot] and store heat using a thermal-fluid in tanks for circulating to heat-exchangers in rooms when needed. This is far more efficient to gather, store & distribute heat over the daily cycle; the fluids have a specific heat of 2.0, are liquid from -45F to 600F, non-toxic and food-contact rated, but it has to be a custom system, you won't find them at any stores.

For this design the site is a mountain ridge with storm-winds that blast it so it'll never be tightly sealed!! So, this is not assumed in the design but it's not needed to be so well done since it's a single room place similar to this one, with sheathing will be on the inside of the wall:

Anyway this is a mountain cabin rarely visited so the main concern is staying above freezing and having air circulation to keep condensation from building up unattended without needing power, I'm using Bud's input to get a feel for how it may work, the volumes of each area are on the drawings so I'm trying to imply the actual gradients I need without buying software!! ... but also point out that the thermal-mass is under the floor, a key issue from my experience building passive-solar over the years.

I'm trying to establish a natural way to pull this off and not alter framing techniques & use standard materials.


I think you missed my point entirely.  If you build a home with an (internal) pressure boundary of <.6 ACH @ 50 Pascals; eliminate thermal bridges going from the inside to the outside; incorporate super insulation (e.g., R100 ceiling, R70 sidewalls, R60 slab for Northern USA - all dependent on your climate); plan an ERV to conserve conditioned air temperature and manage interior air quality/humidity then you will need very little additional energy to condition the interior from what will occur naturally from the occupants, lighting, cooking, other electronics, etc.  Build it according to modern building science findings, and additional solar heating (along with trying to rely on unreliable thermosyphon operations and complex solar collection/distribution mechanisms) becomes a minor issue. See and for more info.

David, what if what you have was built in 1975 and the insulation is crappy, crawl-space doesn't vent well so a source of damp & cold, you're not rich and right now can't afford to build to those standards and the site has the 24'x 42' single-story with a low-pitch roof that gets good solar gain but also winter winds full on?

My method would heat the home no problem on a limited remodel budget where you have to buy new insulation anyway, redo some interior walls but not the full trash-out. It'd be more practical to just add a narrow solar-gain glazing wall on the length of it that's also a wind-barrier.

You don't mention aesthics, R100 in sheating insulation is like 13-1/2" thick unless you use NASA foam, the rafters are 2x10's ... just seems high but then none of the heat in the ceiling is being actively moved back to under the floor in most designs so the priority is to add insulation thickness instead of reducing delta-T and thus needing less.

How are you dealing with the captured heat in rooms, are there any designs moving that heat back down below the floor? It's hard to do without resorting to thermal-fluids so curious.


What if you had a 1931-vintage 2 1/2 story American Four Square with 2 courses of brick with cement on the inside (no space in side walls), windows on every side taking up to 1/2 the wall space (but a next door neighbor's house 10 feet away on the Southern side so very little solar gain) and similar financial situation?  You do what you can.  Whenever I remodelled the interior rooms I added an interior stud wall with insulation and drywall.  I added more attic insulation and a movable foam panel attic access cover.  I foamed any air leakage spots I could find around the basement ceiling (no rim joists, just masonry pockets for the floor joists), and added insulating window treatments to cut down on evening heat loss.  The largest cost of any build, gut-rehab, or improvement project is labor; materials are fairly cheap.

My experience is that it is far more cost effective and better improves comfort to improve the air pressure and insulation aspects of a house than to search for alternative ways to generate conditioned air or come up with elaborate ways of re-distributing air within a house (unless you are starting with a forced-hot air system with easy-to-get-to ductwork).

RE: asthetics - take a look at the Building Science Corp site to see the article on rebuilding the barn office area.  Also look at the high-performance wall documentation to get ideas for your remodel.

Don't vent the crawl space - this concept is built on flawed info.  Seal the stone/dirt area with thick (like 30 mil) polyethelene sheeting (taped if not 1 piece) glued to the inside block walls with epoxy and mechanical fastening (keeps moisture in the ground); cover any existing vents so they are air tight, caulk/spray foam all joints in each rim joist "pocket", insulate each rim joist "pocket", add polyiso panels to the block walls (like R-24) to insulate the space and you're done. Again, mostly labor; cheap materials.

Bottom line - the key is conservation.  Build/remodel to allow the least (practical) loss of air or thermal energy and the other things you are considering will become minor. Look up the reference material.

OK, a brick house example is fine, since the building isn't able to get much direct solar-gain that's the reason I recommend in these cases to use trough collectors anywhere possible on the building or lot to gain heat to use in the home. I can heat hundreds of gallons of thermal-fluid if need be and distribute that to the rooms in insulated piping from insulated tanks located wherever more convenient.

I'm not willing to build or remodel something without solar gain of a significant type designed to the heating needs of the building, why bother, regardless of how much you spend and how efficient it becomes it's always an energy deficit if you didn't install some way to gain from solar input?

In this case by adding a collection-distribution system you may not need to add any insulation for the same cost, at least it will be very close in total costs to the upgrade to the spec's you post.

That's a black-and-white choice to me, solar-gain or not -> If there's no gain it's not passive-solar, energy-efficient isn't the same as putting thermal energy into storage and using it later passive or not.

My point on the crawl space was that it is a source of moisture and coolness, venting is of course no panacea but argueably works in most cases, yet that means you lose the heat along with excess moisture. On the brick home if the floor uses joists it allows removing the batting, replacing solid pressure-blocks with x-braces and adding thermal-mass up under the floor, and also in the walls used for ducting air more pipes can be used to store heat vertically in the wall space. This "installs" expensive ductwork at little cost.

So, on a remodel I'll be doing this, using foam below the floor joists seals the crawl space from affecting the room above directly with fungus spores & bacteria and keeps in the heat being circulated to the floor. But now there's a radiant-heat source in the floor, having this dries out the crawl space and is another reason to build this way.

My other concern on this is that having the soil exposed allows aerobic processes, sealing it over time breeds a lof of fungi and anaerobic bacteria under the plastic, which to me is asking for trouble versus having air circulate over the rather dry exposed soils and dealing with mainly fungi due to the low light, ymmv.

Bottom-line for me is to gain more than you lose on the daily cycle or more, moving heat to under-the-floor & air-return wall thermal-mass for storage in this built-in design by using air ciculation to move it around.

For your example, this deals with where fungi & rot usually occur at the corners and ends of joist runs under the wall by having it open to warm, interior air flow yet closed & insulated from the crawl space. So that's two important jobs being done by the design and I can work it into the brick home using the thermal-fluid collection system and heat-exchangers instead of a glazing wall to gain with, and assuming the need for pumps and fans to make it work in that context to heat the floors.

To me buildings don't qualify for "passive-solar" if they're not designed to gain it. I agree working with insulation and all is important, but you have to gain heat first & foremost and if that's not available due to an urban context you put the collectors on the roof or wherever and figure out how big to make the tanks to handle heating needs.

Also, this will be able to heat water, dry clothes & so on if we had some simple thermal elements to work with those appliances since you're storing 550F in the tanks whenever you get clear sunlight for a while.

Doing this first for the brick house then figuring out how much of an sealing-insulation upgrade is needed seems best since you'd be making decisions from knowing exactly where & how much is needed.

For sure with all the insulation you recommend this will take the building off-the-grid for space heating, but my point is that it may not need any insulation upgrade if it has enough heat put into the correct places in the building to use it conservatively from a thermal-fluid system.

By heating below the floor and letting that rise throughout the room, keeps your feet warm, and there's no cold, stratified layer of air possible in the room anymore, it has to mix upward ... that's a feature of this technique most people in cold climates can appreciate as worth a ton o' money honey!!


Again, you are missing my point.  The ROI on doing air sealing and insulating is FAR greater than any solar project. Less cost; more savings; longer life; larger ROI  In fact it is very easy to provide too much solar gain if you do proper air sealing and insulating first.  Heat moves from warmer to colder area (regardless of dimension or direction) so if you eliminate the thermal losses you will have less/no change in air temperature from one section of the building to another (reduction/elimination of stratification).  Most Passive Homes have only minimal space heating devices only for use in extreme cases (no in-floor heating as it becomes excessive).  Look at the sites I gave you previously.  Look at the projects done by Go*Logic in Maine.

Sealing the floor of a crawlspace without sealing the source of moisture infiltration will not solve your moisture problem and insulating the floor instead of the crawlspace side walls will (usually) be more expensive as there is more square feet involved.  Research the sites I gave you.

Gaining more heat than needed via solar will only show a moderate return if you are only talking about domestic hot water (needed year 'round) - not space heating.

You really need to do more research into current building science as some of your statements show flawed underlying logic. If you just want to experiment with solar collection, then have at it. But understand that experiments by their nature do not usually lend themselves to cost efficiencies. 

Well David it's my turn to tell you that you've really missed what I'm doing entirely.

Doesn't matter how much I lose, I'll gain more than I need, the thermal-mass under the floor dries out that soil and it becomes part of the thermal-mass of the home without too much further ado.

This place will not need one watt from an external source to stay comfortable, if your upgrade didn't include some kind of solar-gain of significance it will still cost energy to keep it warm, that's the point.

Do you get the point that if you don't include solar gain you don't have anything but an energy loser regardless of how good the insulation is?


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