Applied Passive-solar Structural Design

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.

I'll be building a prototype on a mountain ridge with no utilities so will have stats on this concept in spring.

Update to plans in knowing there's a need to add more thermal-mass for storing heat below the floor, and, using 6" PVC pipes filled with a thermal fluid can do that fairly well with 3 pipes between joists that are on 2ft-centers, about 190-gallons [i.e. XCELTHERM 500, specific heat nearly 2.0, food-contact rated, non-toxic, fluid from -60C to 260C]. It wouldn't be too hard to add this idea to the walls but for this hut that seems overkill.

Initial sketch depicting nailing the studs directly to the floor joists without using a bottom-plate to open the space:

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

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