The sentence needs a verb that can directly take "further heat loss" as its object, without needing an extra preposition. "Prevent" works this way, "prevent heat loss" means to stop heat loss from happening, and it reads naturally without any preposition in between.
"Guard" normally needs the preposition "against," as in "guard against heat loss," so "guard further heat loss" is not correct on its own. "Protect" usually needs "from" or "against" too, as in "protect the building from heat loss," so "protect further heat loss" is also incomplete. "Save" collocates with resources like energy or money, as in "save energy," but "save heat loss" does not make sense because you are not trying to preserve the loss.
Only "prevent" fits both the grammar and the meaning of the sentence, so insulation is fitted to prevent further heat loss.