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This article discusses how to ventilate a greenhouse used for solar heating, and how to connect the greenhouse to the house for effective heating. We include discussion of placement of vapor barriers and insulation in solar greenhouses under various conditions. The accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss. Our page top photo shows a small sunspace designed into a New York City apartment. You will notice that the walls to the left of the sunspace had been opened to expose the structural brick wall - a step necessary during removal of mold contamination caused by skylight and exterior wall leaks.
Readers should also see INSULATION for GREENHOUSE or SOLARIUM. Details about solar shades that may be useful in controlling solar glare are found at SOLAR SHADES & SUNSCREENS and SOLAR SHADES, LOW-E EFFECTIVENESS. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
Green links show where you are. © Copyright 2014 InspectApedia.com, All Rights Reserved.
Question about how to best insulate a greenhouse (sunspace) against night time heat losses:
I plan to convert a carport to a greenhouse / dining room. The brick wall between the carport and the house contains a door to the kitchen and a window to the living room.
How should the greenhouse be ventilated and how should the greenhouse be connected to the house for effective solar heating? - Michael Moran, Clemson SC
Answer: direct sunlight defines value of thermal mass in buildings
Sunspace or Greenhouse Air Movement Advice for Solar Heat Design - Getting Warm Air into the Building
To get proper air circulation from the sunspace to the house you will need two vents - one high and one low.
The doorway and window should provide this ventilation, though a high vent can be added if required.
Since natural air convection is relatively week in a one-story building, a thermostatically controlled fan in the wall would give greater control and move even more heated air from the greenhouse sunspace into the rest of the house.
Frequently Asked Questions (FAQs) about Solar Greenhouse Design
Question: What is the best type and location of vapor barrier for solar greenhouse construction in moderate climates such as Kentucky
First, my hardy congratulations to you for an absolutely fabulous website that gives better detailed help than anything else I have found on the Internet. Thank you for this generous public service.
Across our country there is more and more interest in backyard solar greenhouse construction, yet there is very little expert direction on many of the issues involved.
With the high humidity and warm temps inside these structures, and the emphasis on heavy insulation the issue of vapor barrier/retarder is important.
I have explored your excellent website and the principles you give about the use of vapor barriers is thorough. (Thank you.) However, my question involving paint is not specifically answered hence my email question here.
The South wall is glazed. The North, East, and West walls are stud construction with the layers inside to outside as follows: Hardy board vertical siding, 6 mil poly, studs and unfaced fiberglass R-21 insulation bats, 3/4" OSB, 15# felt, Hardy board vertical siding installed per manufacturer's guidelines.
My question involves paint on the outside and paint on the inside. The outside siding is factory primed with acrylic latex. The manufacturer recommendations are for TWO additional finish coats of exterior acrylic latex paint. I am concerned that this paint will form a vapor barrier that will trap moisture in the walls and dampen my insulation. Is this any concern at all??
Similarly, for reflective purposes, I want to put a light-reflecting paint on the inside walls of the solar greenhouse. Do I need to worry about this painting making the inside wall a vapor barrier?
This solar greenhouse is in Central KY. Some other solar greenhouses have been built in this climate zone with NO vapor barrier other than several coats of latex paint on the inside walls. Do you think this is advisable?
Another greenhouse builder is putting 15 mil aluminum coil stock "carefully overlapped" on the inside of his greenhouse and no other vapor barrier. His reasoning: "If there is no air flow here, I won't have any moisture problems." His greenhouse is in Colorado. Again, I find this idea intriguing, but is it a safe way to protect a solar greenhouse from condensation, mold, and rot?
I would appreciate your expert opinion. Warm Regards, R.B., Richmond KY
RE: " Hardy board vertical siding, 6 mil poly, studs and unfaced fiberglass R-21 insulation bats, 3/4" OSB, 15# felt, Hardy board vertical siding installed per manufacturer's guidelines"
It sounds as if you are describing wall construction the inside out. If so the poly is where I'd put it too, since a greenhouse will have higher moisture inside in nearly all climates.
RE: your paint question, while I agree that paints form a moisture resistant barrier, a latex paint is the right coating to avoid adding to a moisture trap.
Moisture moves through walls from the more moist area to the less moist area.
Because you've got a good poly barrier on the inside of the wall (between the more moist greenhouse interior and the less moist wall cavity) it sounds to me as if latex on the wall exterior of the wall should not be a problem any more than it would be on an ordinary building that was not a greenhouse.
When you paint the greenhouse interior wall surfaces, I'd stick with a latex paint for the same reasons. Latex paint is more porous or has a lower perm rating (moisture penetrates more easily) than alkyd (modern oils). That will leave your poly as the most moisture resistant barrier in the wall structure. You could argue for painting the interior surface with an alkyd (more moisture resistant) but I think it's a better design to use a paint that has a lower risk of trapping any moisture that finds its way behind the coating itself - latex.
About Kentucky greenhouses built using no vapor barrier other than paint, if we look at comparative perm ratings, paint has a lower perm rating than poly and foil has a nearly zero perm rating.
A design that relies only on paint on the interior of a KY greenhouse may work just fine provided that moisture can continue to move all the way through the insulation and to outside without ever condensing in the insulation. The risk in the no-vapor barrier design is that if the walls are also insulated, in some of your colder months you may well reach the dew point in the insulation, leading to condensation there, and wet insulation, and ultimately even a rot or mold problem in the wall cavity.
The real answer is that the movement of moisture in and out of greenhouse wall cavities ... well it depends on at least these variables:
- all factors that determine whether or not we reach excessive in-wall-cavity moisture that won't simply dry out when weather changes.
Finally, I agree mostly with your builder who uses aluminum coil stock "carefully overlapped" and I certainly agree with his point about airflow. Moisture movement studies reported at the Boston 1985 Journal of Light Construction building conference confirmed that most moisture movement into and out of building cavities occurs at penetrations where there are air leaks and air movement. Air may move in either direction - in or out of the wall cavity, depending on varying building conditions of temperature, humidity, and even building pressures. No air movement means no consequential moisture movement into the wall cavity in most construction designs.
Further, aluminum coil stock would have a perm rating of zero, except that air could move thorough those overlapped joints unless they were taped with a foil tape.
But in my opinion, for owners who don't want to look at aluminum greenhouse walls, an equally moisture-resistant wall could have been built using foil as the vapor barrier, over which you install a finish paneling or wall covering of your choice. The devil in both cases is in the details: how carefully does the builder seal wall penetrations left for electrical receptacles, switches, and around windows and doors.
But all of the above is my OPINION. By copy of this note I'll invite our solar design expert Steven Bliss for any comments or corrections he may want to offer and I'll post any updates here.
Comments on Greenhouse Moisture Control
In general, as you point out, moisture is much more likely to enter walls via air leakage than diffusion (and most likely to get via exterior leaks due to capillary action, flashing problems, etc.) Air leakage into walls typically occurs around electrical outlets and penetrations for doors and windows, and where the wall finishes meet the floor. Assuming the greenhouse is a fairly moist environment, it would be important to seal these areas well.
If moisture levels are very high and outdoor temperatures low, then diffusion could also play a significant role in the absence of an interior vapor retarder. Using a layer of polyethylene on the interior side of the wall, sealed well at joints and edges with a sheathing tape approved for use with polyethylene (3M Construction Seaming Tape and Tuck Tape are two brands) would work well as both the air barrier and vapor retarder.
With a good seal and effective vapor retarder on the interior, the painted Hardiboard siding should not create any problems.
- Steven Bliss
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