http://www.cibse.org/pdfs/8cimbabi.pdf has an equation for the dynamic
metric U-value of a breathing wall, as corrected:
Ud = VRhoaCa/(e^(VRhoaCaRs)-1) W/m^2K, where
V is the air velocity in meters per second,
Rhoa is air density, 1.2 kg/m^3,
Ca is the air's specific heat, 1000 J/(kg-K), and
Rs is the wall's static thermal resistance in m^2-K/W.
Using V = 1/3600 (1 meter per HOUR :-), and Rs = 5.7 m^2K/W (a US R32 wall),
Ud = 0.058 W/m^2, like a US R98 wall. A more typical V = 10 meters per hour
makes Ud = 1.7x10^-8 W/m^2K, like a US wall with an R-value of 334 million :-)
But counting air heating energy, the total is 1200VdT(1+1/e^(1200VRs)-1),
which increases with airflow. If 30 cfm of 0 C outdoor air flows through
4000 ft^2 of metric R7 (US R40) exterior surface in a typical 40'x60'x8' US
house with no heat exchanger and warms to 20 indoors, V = 3.8x10^-5 m/s, with
0.914 W/m^2 of air heating. The walls and ceiling (eg 8" fiberglass with 9"
TGI joists and studs) would lose 0.914/(e^1200V7-1) = 2.4 W/m^2h, for a total
of 3.31 W/m^2, ie metric R6 or US R34, including fresh air heating as needed,
with (68-32)4000ft^2/R34 = 4235 Btu/h at 30 cfm. This would only work well
in an extremely airtight house.
In which case, maybe it's better to take the R40 with simpler non-breathing
walls losing (68-32)4000ft^2/R40 = 3600 Btu/h and add an 80% air-air heat
exchanger (eg some bidirectional breathing walls) losing about 0.2x30(68-32)
= 376, for a total of 3976 Btu/h.