Cooling with Night Air — Steve Baer (SUNPAPER, 1984)

Steve Baer — Zomeworks, 1221 Edith NE, Albuquerque, NM 87102 — SUNPAPER, January 1984

Source file: raw-sources/archives/1984-01-01-cooling-with-night-air-baer.pdf

Core argument

Opening windows at night and closing them during the day is the traditional strategy for summer cooling in New Mexico adobe houses. The problem: a well-insulated house runs 10°F above average ambient — desirable in winter, not in summer. The house needs two modes:

Daytime: Low U value (thermally isolated); thermal mass absorbs internal heat
Nighttime: High U₀ (open ventilation coefficient); cool outside air flushes heat from thermal mass

Harold Hay’s movable insulation concept is cited as the paradigm: “The secret for summer comfort is to abandon the well insulated condition of the house as soon as it cools off during the night. This was Harold Hay’s revolutionary solution to house comfort — moveable insulation.”

The U₀ concept

U₀ = the open (ventilated) thermal resistance value of a building per square foot of floor, in BTU/°F·hr.

An element of thermal mass has two resistances in series: 1/h (surface conductance to inside air) and 1/hv (ventilation carrying capacity: hv in CFM per sq ft floor, multiplied by 1.1 gives BTU/°F·hr·ft²). Knowing h/hv allows approximation of U₀.

Baer House measurements (5:30 AM, August 23, 1983)

Room	Drum temp (Tm)	Air temp (Tx)	Ambient (Ta)	h/hv	U₀ approx
Guest room	76.1°F	77°F	62°F	25	1/26 = .2 × J
Living room	74.5°F	74°F	62°F	5.2	4/6 = .7 × J

Both rooms have 25 55-gallon drums; floor area of exposed thermal mass: J ≈ 6 (guest room), J ≈ 4 (living room). Guest room: ½ sq ft vent at top (miserably underventilated). Living room: 10 sq ft vent, h/hv still only 5.2. Both are underventilated on still nights.

Conclusion: The ratio h/hv must be large (ventilation carries heat away fast) for effective cooling. Natural convection alone is insufficient for most rooms.

Natural ventilation sizing

ASHRAE formula for ventilation velocity: V = 9.4 √(ti − ta) × E (ft/min)

Where: ti = average indoor temperature, ta = ambient temperature, E = elevation difference between inlet and exit vents (feet).

Design example: 300 sq ft room, 3 sq ft thermal mass per floor sq ft, target 1.5 cfm/sq ft thermal mass → 1350 cfm needed. At 94 ft/min natural velocity (√10 × √10 = 10°F differential, 9’ elevation), vents must be 14.4 sq ft inlet and exit.

Design target: 1–2 cfm/sq ft of exposed thermal mass.

Rate-of-cooling graph

Governing equation for natural ventilative cooling:

(√(Tx-Ta))³ + Jh(√(Tx-Ta))² - hJ(Tm-Ta) = 0 10.3×RE 10.3×√RE

Room properties: vent area/floor = 0.04, shape factor R = 0.7, elevation E = 9 ft, thermal mass area/floor J = 3, ambient Ta = variable, Tm = 75°F, surface conductance h = 1.

Graph 3 shows: even small Tm - Ta differences create substantial heat flux. The natural ventilation scheme works best precisely when it’s needed most (large Tm - Ta).

Santa Fe vs Albuquerque comparison: At Tm = 75°F and Ta = 60°F: Santa Fe loses 25 BTU/sq ft/hr; Albuquerque at Ta = 70°F loses only 5 BTU/sq ft/hr. The 10°F ambient difference has a dramatic effect.

DOE/LASL critique

“I understand that the DOE is contracting LASL to do a study of cooling with night air. I expect we will receive the familiar computer generated curves and confident assertions about performance from the LASL group… My own theory is that the DOE continues to spend money for solar research at the weapons labs because it uses the solar work as a deodorant. A little of the especially fragrant ‘Passive Solar Program’ sprinkled on LASL makes the lab more acceptable to the public who is horrified by nuclear bombs.”

Characteristic Baer: empirical building data paired with institutional critique.

Other sources cited

“Concentrating Skylights, Insulation and Ventilation” — Steve Baer, Home Remedies, Mid Atlantic Solar Energy Association, 1980 (not in archive; contains additional Baer House data)
Computer calculations: Michael Zeiler

Significance for the archive

Earliest Baer House temperature data in the archive: August 23, 1983 measurements predating all the 2002 Cool Cell engineering work
Zomeworks address at this time: 1221 Edith NE, not the later 1011 Sawmill Rd NW — the company had a different location in 1984
First explicit engagement with Harold Hay’s movable insulation as a conceptual framework
The U₀ ventilation analysis prefigures later emphasis on thermal coupling between storage and occupied space (Cool Cell brochure, 2002)