Lately I'm thinking a natural convection air-air heat exchanger might be nice,
with more airflow than a sensible heat exchanger, since humidity adds bouyancy
and the outgoing air would be warmer, with condensation.

Something like this sensible calc, with poly film plates...

20 H=4'hx height (feet)
30 W=2'hx width (feet)
35 NP=8'number of hx plates
40 A=2*NP*H*W'hx area (ft^2)
50 U=1.5'hx conductance (Btu/h-F-ft^2)
60 CFM=37'hx airflow
70 NTU=A*U/CFM'Number of heat Transfer Units
80 E=NTU/(NTU+1)'hx effectiveness
90 TCI=31.8'cold inlet temp (F)
100 THI=65'hot inlet temp (F)
110 TCO=TCI+E*(THI-TCI)'cold outlet temp (F)
120 TCA=(TCI+TCO)/2'average cold temp (F)
130 THO=THI-(TCO-TCI)'hot outlet temp (F)
140 THA=(THI+THO)/2'average hot temp (F)
150 DT=THA-TCA'average temp diff (F)
160 AV=CFM/(16.6*SQR(H*DT))'vent area (ft^2)
170 S=12*AV/((NP-1)*W)'min plate spacing (inches)
180 DS=4/25.4'Coroplast spacer depth (inches)
190 NS=INT(S/DS+.5)'# Coroplast spacers
200 D=(2+(NP+1)*NS)*DS'hx depth (inches)
220 PRINT E,D,S,NS
230 PRINT THI,THO,TCI,TCO

effectiveness depth (in) spacing (in) # spacers

0.838428 4.566929 0.4124437 3

Thi Tho Tci Tco

65 37.16419 31.8 59.63581

To avoid freezing, a $13 TC-3 Thermocube might turn on a small
room air exhaust fan when the room air outlet temp drops to 35 F.

Nick