why infiltration in summer and winter are different?

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Generally, the winter winds are stronger, thus making infiltration higher. Infiltration, in my opinion, is difficult to model accurately. I like the treatment described by Lisa Ng, Steven Emmerich and Andrew Persily in NIST Technical Note 1829. (http://dx.doi.org/10.6028/NIST.TN.1829)

It provides coefficients which vary infiltration by wind speed, though you still need to assign a base rate for the infiltration.