Building engineer Jim Costigan has spent decades constructing high-rises in Manhattan for a living. He's also spent a signifiant chunk of his spare time over the past six years putting his building experience to use realizing a passion project: a green-roofed energy-efficient hobbit home in New York State.

We've reported on a lot of self-built hobbit homes over the years, but this one, named Hobbit Hollow, is definitely one of the better examples and really looks the part. It's located in Pawling, New York, near a stream and waterfall, on 1.7 acres (0.68 hectare) of land.

"When I saw the Fellowship of the Ring and saw Bilbo Baggin's house (Bag End) I thought that that was one of the most original and unique pieces of architecture that I had ever seen," Costigan tells us. "Like this was completely off the charts, I thought that that would be one of the coolest things ever to live in. I went on the web trying to find one that was built but couldn't. So I built the original Hobbit Shed in my backyard for my lawn tractor (that's why my website is called My Hobbit Shed).

"Anyway I put it on the web and got a tremendous response to it. So I started to think about how to build a full scale one that you could live in. This is what I came up with. It's sort of for the modern day Bilbo Baggins who is concerned with climate change and all that other good stuff."

Hobbit Hollow was primarily built by Costigan himself during free weekends, with help from his children, wife, and friends. Its quaint appearance belies a considerable structure hidden beneath that green roof. This consists of two parallel 16 inch (40.64 cm)-thick concrete walls running the length of the house that support a reinforced concrete roof that's 16 inches thick over the walls and 9 inches (22.96 cm)-thick at it's peak.

The interior comprises a total floorspace of 1,500 sq ft (139 sq m), which is spread over two bedrooms, two bathrooms, a living room, dining room, hallway, kitchen, and utility room. The home's interior isn't finished yet, but does have some nice touches already, including decorative patterned skylights and a replica sword hanging over the electric fireplace.

It's important to stress that Hobbit Hollow isn't an officially-certified Passive House, however Costigan built it to conform to the famously stringent green building standard. The home sports triple-glazed windows and its insulation is rated as R-60 in the roof and R-50 on the sidewalls.

Its air-tightness – which is a crucial measure of a Passive House and goes a long way to ensuring its energy-efficiency – is rated at 0.36 ACH (air changes per hour) at 50 Pascals following the standard air-tightness test. This will be a meaningless figure to many, but suffice to say it's a good level of air-tightness, and well within the Passive House requirement of 0.6 ACH.

The house is heated and cooled using a Mitsubishi Hyper heat pump. Costigan has kept the house at a temperature of 67 degrees Fahrenheit (19.4° C) constantly and the electricity bills for the last year average out at roughly US$45 per month. The home is hooked up to grid power, though it could also be outfitted to solar panels with relatively little fuss.

Hobbit Hollow is now up for sale, though no pricing has been revealed. As for the future, Costigan won't completely rule out building another hobbit home but indicated to us that it's unlikely.

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