N writes:

We have a pot belly pig "Ninja" who has terrible arthritis and we want
to keep his outdoor pen/house warm this winter but we don't have electric
by his house and we don't want to use a space heater because
of the dangers of fire or gas inhalation.

LA is an easy climate for solar heat, with 1300 Btu/ft^2 of sun on a south
wall on an average 56.8 F January day with an average daily min of 47.8.

But how much does Ninja weigh? The ASHRAE Handbook of Fundamentals says
a W-pound animal has a basal metabolic rate of 6.6W^0.75 Btu/h, so a 50
pound pig would make 124 Btu/h.

He might keep his house 70 F on a 47.8 F night with his own heat output
if 70 = 47.8 + 124/G, ie if its thermal conductance G = 5.6 Btu/h-F max.
It might have a raised platform above the top of the entrance opening to
trap warm air in the upper part like an igloo. If the upper part were
2'x2'x1' tall with 12ft^2 of surface, this would work if the walls and
ceiling had a min 12/5.6 = 2.1 R-value... 1" R4 white beadboard should
work fine, with plywood on both sides for Ninja and weather protection.
A ramp from the platform down to the ground could allow Ninja to find
a comfortable thermal level somewhere between them.

With a 1'x2' R1 flat polycarbonate north window and 10 ft^2 of R5 surface
upstairs, G = 2+10/5 = 4, so Ninja might keep the air 70 F on a 70-124/4
= 39 F night. The HOF says winter temps are above 40 F 97.5% of the time
and above 37 99% of the time, and a normally-active critter would generate
2.5 times more than basal heat, so with occasional shivers and rolls, Ninja
might be comfy down to 70-2.5x124/4 = minus 4.4 F. NREL says LA has a 23 F
30-year record minimum temperature.

Nick