This is explained in detail in annex I.5 of the standard EN ISO 13790. The main difference is that the degree day method contains an implicit assumption about the effect of internal and solar gains on the heat balance (independent of the specific building), while this effect is explicitly taken into account in the monthly method (as a function of the building parameters).

Question submitted by: Dick van Dijk (TNO, The Netherlands)
Anwered by: Dick van Dijk (TNO, The Netherlands)
Date: 21/10/2009