Monday, August 24, 2009

My greenhouse cometh

Recent growth in my plant collection has highlighted the inevitable - I'm going to need a greenhouse sometime in the future.  And that future seems to be quickly approaching.  For quite a while I have thought that a greenhouse might be possible sometime in the future, but we recently decided to go ahead and do it now.

Plants are almost literally growing out of our ears!  We need someplace to put them.  While my wife and I love our little house, it is lacking in the number of windows needed for happy houseplants.  During the winter months, our house stays at 60F most of the day and we only turn it up to 62-63F when we are going to be at home for a while and get cold.  The plants would prefer 80F and 80% humidity.

A greenhouse can provide that kind of growing environment on winter days, by simply taking advantage of the sun's awesome power.  I intend to take advantage.  On winter nights, a heater is required.

In the next month, we're going to build a small greenhouse on our back porch.  The area we have to work with is about 8'x12', so the inner dimensions will likely be about 7'x11'.  This area is pretty heavily shaded during the growing season by our huge Sycamore tree.  When Fall arrives and the temperatures drops, so do the leaves.  At this critical time of the year my greenhouse will receive the most light.

One wall of the greenhouse will be shared with the exterior of our house.  The roof of the greenhouse will adjoin our house roof.

My back patio where the greenhouse is to be built

I plan to lay 2-3 layers of cinder blocks and then build a simple wooden frame out of 2"x4"s on top of that.  I have experience laying cinder block from house-building mission trips to Mexico.  My father-in-law has built several houses and will be helping with the framing.  The walls and ceiling will be covered with 8mm triple-wall polycarbonate sheets. Twin-walls are a little cheaper and more common, but the cost differential is quickly paid for with reduced heating costs and size of heater needed.

Initial sketch of my greenhouse plans
The polycarbonate sheets can be attached very easily by nailing/screwing them into the wood frame.  Additionally, you have to seal all joints to avoid leaking the warm, humid air of the greenhouse.

To cover the screws or nails, you cover the polycarbonate joints with trim wood.  I plan on cutting my trim pieces to fit and then staining and sealing my trim pieces before attaching them to the greenhouse.


I will buy a small space heater that will run in the greenhouse during the winter months over night.  During the day, it should stay pretty warm, even when the temperatures outside are cold.  I have used a calculator online to determine the BTU output my heater will need.  Assuming the temperature falls to about 20 F outside and I want my greenhouse to stay at or above 60 F, I will need about 500 BTUs to heat the greenhouse.


I ran some rough numbers and have an estimated cost of the main materials.  Those materials are the lumber, cinder blocks and polycarbonate sheets.  They are the most costly and also the easiest to figure.  For instance, I know almost exactly how many cinder blocks I'll need, but have no idea how many nails.

My initial estimate doesn't include all of the fasteners (nails, bolts, etc.), sealers, stain, or bags of mortar and cement.  Other considerations are any extra tools for building (beyond what I already own), the exhaust fan, and a simple fluorescent light fixture.  I plan to find some cheap shelves and build the remaining ones to fill the space inside.

All in all, I figure the total cost won't be much more than 50% greater than my initial estimate for the base materials.

Room for improvement

Over the years the greenhouse will probably undergo a number of changes.  I've already thought a couple of them through.  We would like to add a room onto our house one day.  At that time, we will be ordering bricks to match our house.  I would like to have a professional mason cover over the cinder blocks with the matching red brick, so I will be leaving room on the back porch pad when I lay the cinder blocks.

Another upgrade I foresee is incorporating irrigation in some way.  I'm not sure how I want to do this and I think I will probably have a better idea after the greenhouse is built.  For now I'm going to just drag the hose in through the door or use a watering can.  One potential watering system would simply be to collect rainwater runoff from the roof and route it into a container in the greenhouse.  There are a couple of areas around the house that would benefit from gutters diverting heavy rains to other locations.

Other potential upgrades include improved circulation, ventilation or heating.

There is a good discussion of hobby greenhouses at Thyme for Herbs.

Stay tuned for more posts soon with pictures of the greenhouse in progress and complete with occupants!

See other phases of the project here: