"An energy efficient mortgage (EEM) (or "green mortgage") is a loan product that allows borrowers to reduce their utility bill costs by allowing them to finance the cost of incorporating energy-efficient features into a new housing purchase or the refinancing of existing housing."

"Energy-efficient" would be the better option because they've already said "green" earlier in the sentence.

It sounds like you're having trouble with the whole sentence. I suggest the following:

"Energy was embodied in the original construction, but this will not be expended again, as the building has largely been preserved."

Embodied energy, sometimes known as grey/gray energy, is defined as follows:

"The energy required to produce any goods or services, considered as if that energy was incorporated or 'embodied' in the product itself. The concept can be useful in determining the effectiveness of energy-producing or energy-saving devices, or the "real" replacement cost of a building, and, because energy-inputs usually entail greenhouse gas emissions, in deciding whether a product contributes to or mitigates global warming. One fundamental purpose for measuring this quantity is to compare the amount of energy produced or saved by the product in question to the amount of energy consumed in producing it."

In other words, the property already has embodied energy from when it was built, and it won't consume much new energy to convert it.

no ecological ballast will be incurred

...and no ecological mortgage is being created or generated

Seems to fit the bill

--------------------------------------------------
Note added at 1 day19 hrs (2015-03-01 17:05:59 GMT)
--------------------------------------------------

No ecological legacy has been inherited

--------------------------------------------------
Note added at 1 day20 hrs (2015-03-01 17:23:10 GMT)
--------------------------------------------------

Here's an example of the use of the term:

From 1850 to 1950, the Tri-State Mining District of southwestern Missouri, southeastern Kansas, and northeastern Oklahoma produced 50 percent of the zinc and 10 percent of the lead in the United States. The last active mine closed in 1970, but mining’s ecological legacy remains throughout the region—lead-contaminated acidic soils, toxic smelter sites, large quantities of mine tailings called “chat,” and thousands of acres of land with little or no vegetation.

Also described as environmental legacy, as here: This paper examines the environmental legacy of the Dylife lead/zinc mine in the central Wales mining district.
This would, for example, also be the term used in the case of industrial sites being preserved as museums.
For instance, heritage railways and waterways (canals) and industrial monuments like lead mines, collieries, ironworks, pumping houses, winding engines, textile factories (spinning & weaving), pottery and glass kilns etc.

As a mine researcher, I have been involved in various projects of this type.

Trivia: I am currently compiling a map of all shafts, adits, drifts and dayholes for the extraction of coal, ganister, ironstone and fireclay for the county of Yorkshire from 1800 to the present day, parts of which have already been shown at lectures at the National Coal Mining Museum in the UK - so, if anyone lives in the area, I would be glad to let them know if their house is built on a mineshaft.