The Energy Saving Ordinance as a regulation with mostly technical issues is supported to a large extent by technical standards. These are especially referenced concerning the calculation methods to prove the compliance with requirements.
The references are nearly all of the "dated" type, where the application of a specifically named version of the standard is compulsory even if a more recent version is available. As far as these standards contain undated references to further standards, according to § 23 section 5 EnEV 2013 these standards have to be applied using the version that was in use at the issuing date of the standard that is reference by the ordinance.
In Germany, technical standards are developed by the German Institute for Standardisation (DIN) with participation of all relevant stakeholders and issued by the Beuth-Verlag. The texts of standards cannot be obtained free of charge, but are accessible for free in many libraries and in official "standards-repositories" throughout Germany.
With respect to the practise of "dated" references, Beuth-Verlag provides (not free of charge) a special portal for the EnEV. Subscribers have access to a rich package of standards connected with the EnEV.