Renovation work at the library in Windsor Road is set to reduce CO2 emissions by eight tonnes per year.
Lights will also be upgraded to LED bulbs, an air source heat pump will be installed to replace dated electric heating, and solar panels will be fitted to the roof.
Lancashire County Council said the library would not be reopening until "early January 2022," but books can be returned to any other Lancashire Library, renewed in person, by phone, or online while it is closed.
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Book-lovers will need to travel to either Poulton library in Blackpool Old Road or Lancaster library in Market Street if they want to check out books in person.
Fines for overdue loans from Garstang library will be waived while the buildings are shut, and all books are being renewed automatically until January 17.
County Coun Peter Buckley, cabinet member for community and cultural services, said: "We're sorry for any inconvenience this may cause, but these are major improvements and the work can only be done safely while the buildings are closed to the public.
"People can use our excellent online library services, or a nearby alternative, while the work is completed. This Government-funded scheme will help us to improve Garstang and other libraries and make them more environmentally friendly. "
County Coun Shaun Turner, cabinet member for environment and climate change, added: "Reducing our carbon footprint is absolutely crucial. We're working hard to do all we can to reduce energy usage in our buildings and projects like the Garstang Library improvements are a key part of our plans.
"Making investments like this in our existing buildings and ensuring any future ones are greener will really have a positive impact on climate change as well as helping us to achieve carbon net zero."
How to renew your books
- Online at www.lancashire.gov.uk/libraries
- In person at any Lancashire County Council library
- By phone via a 24-hour automated service 0300 123 6704 (library card number and PIN required).
- By calling the Customer Contact Centre on 0300 123 6703.