A plastered straw bale wall gives the insulation a passive solar house needs but almost no thermal mass, so add mass inside it, such as a 10 cm earth or stone floor, and put the glass near the outer face because a bale-thick reveal shades it. With the carrier’s R-1.5 per inch, only a bale about 51 cm thick or more reaches R-30.
Part of Passive Solar Building Design
Combining straw bale walls with sun-facing glazing, interior thermal mass and overhangs: what the bale wall provides and what the design must add.
The Bale Wall Is the Insulation, So the Mass Goes Inside It
The carrier’s pairing is straw bales for superb insulation (R-30+) with a dark stone or adobe floor for thermal mass and south-facing windows to complete the system. The rule it states is general: mass inside, insulation outside. The bale wall is the insulation layer; the interior plaster and the floor are the mass.
How Thick Must the Wall Be for R-30?
The carrier gives straw at about R-1.5 per inch. R-30 needs 30 / 1.5 = 20 inches = 51 cm. The straw bale construction page gives standard two-string bales as about 90 x 45 x 35 cm and three-string bales as about 100 x 60 x 40 cm. At R-1.5 per inch:
| Wall thickness | Inches | R (US units) | m2 K/W |
|---|---|---|---|
| 35 cm | 13.8 | 20.7 | 3.64 |
| 45 cm | 17.7 | 26.6 | 4.68 |
| 60 cm | 23.6 | 35.4 | 6.24 |
So a two-string bale laid flat (wall thickness equal to its 45 cm width) gives about R-27, a three-string bale laid flat (60 cm) gives more than R-30, and the plaster adds a little. “R-30+” in a wall requires the thicker bale or a different count of R per inch than the carrier’s 1.5.
How Much Mass Does the Bale House Have?
Interior earth plaster is thin. The straw bale page gives a scratch coat of 15-20 mm, a brown coat of 10-15 mm and a finish coat of 3-5 mm, 28 to 40 mm in all. Taking 34 mm of earth plaster at 1.53 MJ/m3 K on 50 m2 of interior wall:
- Heat capacity: 0.034 x 1.53 x 50 = 2.6 MJ per degree.
- Over a 5 K swing: 13 MJ = 3.6 kWh.
A floor of earth or stone 10 cm (0.10 m) thick on 50 m2 (5 m3) at the same capacity: 5 x 1.53 x 5 = 38 MJ = 10.6 kWh. So the floor holds nearly three times the plaster, and in the sun path it works best. Compare with the 30 kWh sample in Mass Placement & Sizing: both together are still well short, so add interior cob or adobe partitions or water containers where the sun reaches. See Mass Materials: Stone, Earth, Water.
Windows in Thick Walls: The Reveal Is an Overhang
A window set at the inner face of a 45 cm wall sits at the bottom of a 0.45 m deep reveal, and the head of the reveal shades the glass. At 40 N at noon (Northern Hemisphere wording; swap hemispheres south of the equator):
- 21 December, altitude 26.6 degrees: shade = 0.457 x tan 26.6 = 0.23 m, 15% of a 1.5 m window.
- Equinox, altitude 50 degrees: shade = 0.457 x tan 50 = 0.54 m, 36% of the window.
Winter sun is lost in the reveal. Put the glass in the outer plane of the wall (with sill and head details that shed water) or splay the reveals to the room. Splaying the sides also spreads the light.
Eaves as the Overhang
The straw bale page asks for roof overhangs of at least 45 cm, preferably 60 cm or more, to keep rain off the walls. At 40 N, with an eave projecting 0.6 m at 2.4 m above the floor and a window from 0.9 m to 2.1 m (1.2 m high):
- Winter noon: shadow below the eave = 0.6 x tan 26.6 = 0.30 m, down to 2.10 m, just clear of the window head: fully sunlit.
- Equinox noon: 0.6 x tan 50 = 0.72 m, down to 1.69 m: the top 0.41 m of the window (35%) is shaded.
- Summer solstice noon (altitude 73.4 degrees): 0.6 x 3.35 = 2.0 m, down to 0.39 m: below the sill, so the window is fully shaded.
The rain-shedding eave is also the solar overhang at this latitude. At other latitudes recompute using Overhang Design for Seasonal Control.
Common Mistakes
- Counting the bales as mass — they insulate; the heat never gets in.
- Glass at the inner face of a deep wall — the reveal blocks winter sun.
- Unplastered or wet bales — see Straw Bale Construction. Moisture must stay out.
- Dark mass wall with insulation inside it — mass must stay on the inside.
Safety
Bale walls rot when wet and burn when bare
Keep bales dry from the foundation up, keep eaves wide and plaster promptly: wet straw rots and bare straw burns fast. Keep stoves and flues well clear of bale surfaces and fully plastered. Straw dust and mould spores harm lungs; wear a cloth mask while handling bales.
What’s Next
- Learn next: Straw Bale Construction — building the wall
- Combine with: Window Placement & Sizing — how much glass