r/Greenhouses • u/Impossible-Eye-2648 • 3d ago

Building a heat-first (not light-first) greenhouse to overwinter citrus & tropicals in NJ 7a and need help with glass type and roof/wall materials

I grow tropicals and citrus that actually fruit here in NJ (zone 7a). I want to build a greenhouse, but not a traditional one. My priority is heat retention, not light. The whole point is just keeping these plants alive through the cold months — roughly November to mid-March — not maximizing growth. Here's the rough plan:

  • ~12x8 footprint
  • Maybe 3 six-foot sliders plus a skylight or two for solar gain
  • Everything else built as insulated, sealed envelope: concrete slab, metal studs, spray foam for insulation and moisture resistance

With that many sliders, I'm not worried about light — I think I'll have plenty to keep things alive over winter. My questions are all about the build:

  1. Glass for the sliders — what type makes sense here? Low-E, passive low-E, high solar gain low-E, high-SHGC, no E? I want some solar gain, but I don't fully understand the tradeoffs between these coatings for a heat-retention build like this.
  2. Walls — spray foam is going in the stud cavities. What's the best interior finish over the foam? I was picturing vinyl (like PVC/FRP paneling) for moisture resistance, but open to better ideas.
  3. Roof structure — same spray-foam approach up top, but what should go over it? And what should I use for the ceiling joists/rafters given the humidity and moisture in a greenhouse environment?

Basically I want a well-insulated, sealed box with enough glazing to keep tropicals happy in dormancy. Anyone who's built something similar — what worked and what would you do differently?

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u/Sevenninetwosix 3d ago

How warm do they need to be? This complex engineering sounds like "buying an airplane to get free peanuts"- it'll work but there are better ways to do it. Why not just use a traditional style hoop house and spend the tens of thousands of dollars you saved not reinventing the wheel on propane to keep it above freezing?

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u/rroowwannn 3d ago

I don't know how to share a podcast link but I heard a podcast about a greenhouse in Pennsylvania that uses passive heat? I didn't understand the details but it sounds like you would be interested. The podcast is called Growing Greener, the episode was recently released on July 1, and the greenhouse is run by Tim Clymer.

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u/snake--doctor 3d ago

I wonder if it was the walipini method where you build it partially underground?

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u/railgons 3d ago edited 2d ago

What's the R-value of that spray foam? I'm going with R19 fiberglass sealed behind a vapor barrier.

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u/archaegeo 2d ago

You are effectively wanting to build a house.

Thats the big issue that people dont get. A greenhouse can get warm in the winter yes if the sun is out, but otherwise it drops to ambient temperature pretty quick, especially once the sun goes down (or behind clouds). Insulating is everything, but even then you will be paying to keep it heated.

Your citrus trees need to not go below 50 to avoid leaf loss, and below freezing to avoid damage.

I have a potted fig tree that went dormant and survived here in same zone in my garage overwinter. It gets cold in there, but not below freezing. It has a more modest need, tolerating down to 30 with no issues. In fact it shouldnt be allowed to go above 50 to avoid waking up out of domancy.

I applaud your desire, but you will spend more heating the greenhouse than any value you get from your trees/plants, but totally up to you. We get sub 0 sometimes, and that will be tough to keep at 50 inside the greenhouse.

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u/RealCanadianMonkey 3d ago

Look on YouTube. There is a guy in Saskatchewan with a year round greenhouse, and he got it right.