There has been a lot things happening behind the scenes the last week and a half and very little by me.

Kara has been designing the entire greenhouse in Google Sketchup and I must say, WOW!  At first I wasn’t sure how it would look and if it would compliment the new exterior of the house that we did last summer.  The talk of a slanted south wall was a little discerning to me and in the back of my mind I just wasn’t sure if the CB South Property Owners Association (POA) would accept it. After seeing the complete perspectives with the matching siding and barnwood accents my concerns were put to rest quickly.  Having good 3D drawings really helps with telling the story of how a building will look and I even think I am pretty good at being able to see things in 3D from a set of plans as I used to draw home plans back in Texas before I move to Colorado.  I think the new design turned out perfect!

Passive Solar Greenhouse SE Perspective

Passive Solar Greenhouse SE Perspective

A few major changes that happened during the redesign are:

  • The size increased from 12′ x 10′ to 12′ x 20′.  So we doubled the size!!  Mine as well go big and grow enough fresh vegetables for all your friends and neighbors.
  • There are now no windows on the East and West walls.  You will just lose more heat then you would gain on both of these walls so we are just going to insulate them heavily and maybe have some vents in these walls to create cross ventilation.
  •  The north wall height is now 7’6. The north wall height at first was 6’3 but it was decided to go higher to make the greenhouse feel more spacious.  One of the reasons to have a year round greenhouse is to have a sanctuary during the winter and you wouldn’t want the greenhouse to feel cramped. That would just defeat the purpose.
  • The roof overhang above the south wall was extended.  This extension is to prevent the greenhouse from overheating during the summer months when the sun is at its most intense.  The larger overhang will give some shading on the south wall during the middle of the day.

With these updated plans and changes I was able to talk to Dom at the CB South POA and much to my relief he seemed very open to the idea and got me on the agenda for the next weeks Design Review Board Meeting.  What is great, is CB South is trying to work with homeowners to become greener, which means them being more willing to work with homeowners on greenhouses and solar panel installation.  Plus CB South doesn’t have much in the design guidelines for greenhouses so it is handled on a case by case bases.

Plans are submitted and fees are paid and just crossing my fingers that all goes well at the meeting.  You and I will both find out next post how the meeting goes as the meeting is tomorrow evening.

Sorting out the Details

So Kara and her friend Ben had come up with a lots of information, facts, ideas and suggestions on designs items and items to include in our Passive Solar Greenhouse.  Let’s explore each item and I will give you some of my thoughts on each and why it will or won’t work in our particular situation.


Normally, it is best to orient the building slightly to the southeast. This is so that the greenhouse can heat up in the morning and avoid overheating in the afternoon. Special consideration must be made for the horizon. If there are mountains to the west, it is best to absorb as much of the hot afternoon sun as possible before the sun disappears and no additional radiation can be absorbed. Have it within 45 degrees of south.

I really can’t angle it to the southeast because per the CB South POA it needs to be aligned to the existing structure which means the greenhouse we be slightly orientated to the southwest.  I would guess only 5 to 10 degrees off of straight south.

The greenhouse is going to be on the west side of the house so hopefully there isn’t too much shading from the house but we will be dealing with the underground power lines that come into the house and with the 25′ setback that is part of CB South. We will know more of exact location once the snow melts and we are able to locate the underground power lines.

Glazing Angle

Glazing angle for winter growth is latitude plus 15 degrees – Crested Butte is located at 38.9 degrees N which makes the ideal glazing angle 53.9 degrees for optimal winter growing. Glazing for season extension can be more. In a small greenhouse footprint, a steeper glazing pitch is more ergonomic, so with both variables considered, in our initial design we’ll start with a 60 degree pitch.

For a standard greenhouse, glazing shouldn’t be more than 20% of the south wall if thermal mass is included. However, greenhouses in comparable climates such as Calgary have approached 80% coverage with ample heat retention and heat rejection strategies.

All good here.  As long as CB South POA is fine then I am fine.

Heat Retention Strategies

Climate Battery

Circulate the entire volume of air in the greenhouse into a loop of perforated piping buried in an insulated area of soil under the footprint. Do so roughly 5 times per hour, ideally at 3 mph. This allows a thermostat to turn on a fan above a specified temperature  to force the hot moist air of midday into the ground where it condensates, further heats, and stores significant BTU’s for later use. At night, the system can be turned on once the temperature drops below a certain point so the stored heat can be recirculated into the cooling environment of the greenhouse to keep it warmer.


The North, West, and East walls will be highly insulated with additional insulation on the South knee wall. Heat loss will be slowed wherever possible.

Thermal Mass

The North wall will hold the majority of the thermal mass as this is where the sun’s radiation can be stored most effectively. Stacked 55 gallon drums filled with water is the method of choice. This is for two significant reasons. First, water has 4 times the thermal mass storage capacity of cob, stone, concrete, and other dense solid materials. Second, plastic 55 gallon drums are cheap and easy to install. The downfall of water, especially in Crested Butte, is that you must employ additional measures or redundancy to assure the water does not freeze.

There are still some details to be worked out in this section. Still need to figure out the climate battery layout and what type of fans and thermostats to use.  How deep we need to dig.  I am just excited to rent a mini excavator and dig a big hole in the ground!  Also if you were like me and don’t exactly know what a climate battery, this link has some great info and diagrams on climate batteries. CRESGS Ground to Air Heat Transfer. Same thing as a climate battery just a different name.

We also still aren’t sure what the exterior walls will be.  Are we going to do clay/straw? Maybe a double 2×6 wall? Maybe just a 2×8 wall or something? I don’t know.

Heat Rejection Strategies

Vents on the top and bottom of the south wall can create a thermal siphon. These can operate with powerless wax driven arms. They can also be covered with an insulated door for the coldest parts of the winter to allow for greater heat retention. Vent sizing should be 25-30 percent of glazing surface. Top vents must be larger than bottom vents to account for air expansion when it heats, otherwise the siphon effect would not be as effective.

Shade cloths under front eave to block summertime sun radiation.

There must be additional venting near the ceiling on the West and East walls. This may need the aid of fans to help expel summertime heat. Some sort of fan is also advisable to point towards the plants to give them “exercise.” Plants must have air movement.

I know nothing here.  Don’t ask me.  I don’t even have shades on my living room windows and now I need shades for the greenhouse?!?  The vents and shade cloths will all be determined in the future.  There isn’t a rush on any of these items.

Auxiliary Heating

Pellet Stoves are easy to use, have thermostats so that you can assure temperatures do not drop below a specified point, and are enjoyable to sit around.

Rocket Mass Heaters can produce ample heat, can provide a place to sit that will be heated, are enjoyable to sit around, and use small sticks for fuel. The downside is that fuel wood would have to be small which might actually be inconvenient in Crested Butte.

We had talked about a Pellet Stove but after looking into it I am not sure we can find something small enough.  Everything I found online could heat a 2,500 square foot house.  Seems a little excessive.  Research here we come! We will be looking for some sort of secondary heat but we aren’t even sure what the fuel source will be yet.

Material Considerations


Clay/Cob/Clay Straw would provide some thermal mass and a great aesthetic, but there is uncertainty whether they would hold up to the high humidity condition of the greenhouse.

SIP Panels would come together very quickly and are moisture and mold resistant. Rob Avis of Verge Permaculture used SIP panels on his passive solar greenhouse that performs well in the cold of Calgary.

Plywood Ceiling/Interior Sheathing painted white/light color with antifungal paint would allow for light to be reflected and prevent mold growth

Oh, here is the wall section.  Still not sure what we are actually going to do for walls.  Clay Straw would be a cool, neat, green way to build a wall with natural insulation but it might not work out.  Probably a double wall would give us the highest R value as we can reach around R40 while a clay straw wall would only give you around a R20 for the same thickness.


Possibly polycarbonate, there are half inch, three layer polycarbonate panels that have 78% transmissivity and provide an R value of 2.

Definitely polycarbonate! I don’t have double pane window money.  Glad we could agree on that.  Now how do we attach this stuff?  Ahh, we will figure it out when the time comes.

Thermal Mass

Water – use 8-12 L of water/ sq. ft. of glazing, with our current drawings we have 160 sq. ft. of glazing, using 12 liters of water per sq ft it requires 1920 liters of water. That converts to 507 gallons, so roughly 10, 55 gallon drums.

Wow!  We are going to be storing a lot of water!!  If there is a drought I guess we can sell water and make a killing.

Stone, Concrete, Cob – use 40 – 60 kg per sq. ft. of glazing. If concrete was used for thermal mass and we use 60 kg per sq ft of glazing, we would need 141 cubic feet to accommodate the thermal mass from 160 sq ft of glazing.

Just like I don’t have double pane window money, I don’t have concrete type money.  It is insane how expensive concrete is here.  I am used to 10 years ago in College Station, Texas where you could get a yard for about $80.  I do have some left over cultured stone from my fireplace that is going to be in the sitting area of the greenhouse.  Going to give it that rustic modern feel every greenhouse is striving for.  Heck maybe we will have some leftover barnwood that we can do some inside accents. Maybe a little reclaimed wood floor in a section of it.  Now I am just dreaming.  We shall see once the final product is done.

This is where a lot of ideas and information came from.  Again we have been using the internet as a resource so we just aren’t starting from scratch.  Thank you Rob Avis on a great blog post.

Coming up in the next few weeks

Well we all will know how the Design Review Board Meeting plays out tomorrow.  Hopefully it is smooth sailing! I am also going to talk to the Gunnison County building department to start figuring out what is needed for a building permit. Which also means we are probably going to have to start making some decisions on foundation design and type, wall type and anything else we are going to have to spec on the plans for the county.  It would be nice to have all of our permits and approvals in place in the next couple of weeks so we can start in the beginning of May.