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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.

© Copyright 2012 InspectAPedia.com, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use page top links to major topics or use links at the left of each page to navigate within topics and documents at this website. Green links show where you are in a document series or at this website.

Advice for Insulating a Greenhouse or Solarium Against Night Time Heat Loss

• Q&A: Converting a Carport to a Greenhouse: Ventilation & Trombe Walls for Greenhouses or Solariums - PDF version; use your browser's back button to return to this page

The question-and-answer article about greenhouses or solariums, quotes-from, updates, and comments an original article, (see links just above) from Solar Age Magazine and written by Steven Bliss.

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 Trombe Wall Retrofit Advice for Solar Heating The brick wall between house and the new greenhouse has the makings of a Trombe wall, with one minor hitch - if it does not receive direct sunlight, its value as a thermal mass that absorbs, stores, and later returns heat is greatly reduced. Cutting skylights into the carport roof would help the thermal mass wall work better by allowing sunlight to strike it directly. Our photo (left) shows a large sunspace constructed atop a converted factory building in Poughkeepsie, NY.

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.

Greenhouse or Sunspace Ventilation - Summer Needs

Green House or Solarium Roof Leak Points to Watch For

As Carson Dunlop Associates point out in their sketch (left), greenhouses and solariums can be leaky, especially where an add-on solarium abuts the original structure. Often we can spot these leaks as stains down the building wall below the points of contact between the solarium roof and the building walls. Watch Out: a solarium or greenhouse leak that enters the wall cavity may not show up immediately as a stain on the building interior, but it can lead to rot, insect damage, or a mold problem. Also see GREENHOUSE / SUNSPACE GLARE.

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

Reply:

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:

• the amount of moisture in the greenhouse interior air
• the use of exhaust fans that may at times place the greenhouse interior under negative pressure with respect to outdoor conditions
• the cool weather conditions that the greenhouse will experience
• the amount or R-value of insulation in the greenhouse walls
• details of wall construction and the care with which all wall penetrations are sealed against air movement in and out of the wall cavity
• the exterior wall cladding material and its permeability and contribution to the total wall R-value

- 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

Questions & Answers regarding this article

Questions & answers about greenhouse and passive solar greenhouse design, insulation, and ventilation

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• Additional technical contributors & reference sources for this article are listed below.

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

GREEN BUILDING CONSTRUCTION CODES GUIDES
GREENHOUSE DESIGN for SOLAR HEATING
GREENHOUSE / SUNSPACE GLARE
  Insulating a Greenhouse or Solarium
  Sunspace or Greenhouse Air Movement
  Greenhouse or Sunspace Ventilation
  Green House or Solarium Roof Leaks

SOLAR ENERGY SYSTEMS
  BLOCKBED RADIANT FLOORS - SOLAR DESIGN
  GREENHOUSE DESIGN for SOLAR HEATING
  GREENHOUSE / SUNSPACE GLARE
  PASSIVE SOLAR DESIGN KEY ELEMENTS
  PASSIVE SOLAR DESIGN METHOD
  PASSIVE SOLAR ENERGY MONITORING
  PASSIVE SOLAR FLOOR TILES, PHASE CHANGE
  PASSIVE SOLAR HEAT PERFORMANCE
  PASSIVE SOLAR HOME, LOW COST
  PASSIVE SOLAR PERFORMANCE PROBE
  PASSIVE SOLAR Roof & Window Overhangs
  PHOTOVOLTAIC POWER SYSTEMS
  POLYCARBONATE GLAZING
  REMOTE ELECTRIC POWER, PHOTOVOLTAIC
  ROCK-BED SOLAR HEAT STORAGE DESIGN
  SLAB INSULATION, PASSIVE SOLAR
  SLATE THERMAL MASS for SOLAR HEAT STORAGE
  SOLAR COLLECTOR AIR or GAS COLLECTION
  SOLAR COLLECTOR EFFICIENCY COMPARISONS
  SOLAR COLLECTOR FILMS
  SOLAR COLLECTOR OUTGASSING
  SOLAR COLLECTOR WOOD HOUSINGS
  SOLAR GAIN CALCULATION
  SOLAR HEATING SYSTEM DESIGNS
  SOLAR HOT WATER HEATERS
  SOLAR HOUSE EVALUATION
  SOLAR MODULE MANUFACTURERS
  SOLAR SHADES & SUNSCREENS
  SOLAR SHADES, LOW-E EFFECTIVENESS
  SOLAR WATER DISINFECTION
  SOLAR HOT WATER HEATERS
  SUNSPACE DESIGN for SOLAR HEATING
  SUNSPACE GLAZING for SUNTANNING
  STORM WINDOW INTERIOR
  STORM WINDOW PLASTIC CHOICES
  STORM WINDOW WEEP HOLES
  SUNGAIN, FILMS, LOW-E GLASS
  SUNSPACE GLAZING for SUNTANNING
  SWIMMING POOL SOLAR HEAT, INDOOR
  SWIMMING POOL SOLAR HEAT, OUTDOOR DIAGNOSIS
  THERMAL MASS in buildings

• Solar Age Magazine was the official publication of the American Solar Energy Society. The contemporary solar energy magazine associated with the Society is Solar Today. "Established in 1954, the nonprofit American Solar Energy Society (ASES) is the nation's leading association of solar professionals & advocates. Our mission is to inspire an era of energy innovation and speed the transition to a sustainable energy economy. We advance education, research and policy. Leading for more than 50 years. ASES leads national efforts to increase the use of solar energy, energy efficiency and other sustainable technologies in the U.S. We publish the award-winning SOLAR TODAY magazine, organize and present the ASES National Solar Conference and lead the ASES National Solar Tour – the largest grassroots solar event in the world."
  
• Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education. Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
  Excerpts with updates and annotations expanding the original Best Practices Guide text can be found in the online review and book summary at BEST CONSTRUCTION PRACTICES GUIDE and also at DECK & PORCH CONSTRUCTION, at INDOOR AIR QUALITY IMPROVEMENT GUIDE, and in other articles found at InspectAPedia.com such as HOUSEWRAP AIR & VAPOR BARRIERS, SOUND CONTROL in buildings, and other topics.
• Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
• Passive Solar Design Handbook Volume I, the Passive Solar Handbook Introduction to Passive Solar Concepts, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v1.pdf
• Passive Solar Design Handbook Volume II, the Passive Solar Handbook Comprehensive Planning Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v2.pdf [This is a large PDF file that can take a while to load]
• Passive Solar Handbook Volume III, the Passive Solar Handbook Programming Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v3.pdf
• The Passive Solar Design and Construction Handbook, Steven Winter Associates (Author), Michael J. Crosbie (Editor), Wiley & Sons, ISBN 978-047118382 or 0471183083 is available at Amazon.com and via the The Passive Solar Design and Construction Handbook, Steven Winter Associates (Author), Michael J. Crosbie (Editor), Wiley & Sons, ISBN 978-047118382 or 0471183083 is available at Amazon.com and via the InspectAPedia Bookstore
• "Passive Solar Home Design", U.S. Department of Energy, describes using a home's windows, walls, and floors to collect and store solar energy for winter heating and also rejecting solar heat in warm weather.
• "Solar Water Heaters", U.S. Department of Energy article on solar domestic water heaters to generate domestic hot water in buildings, explains how solar water heaters work. Solar heat for swimming pools is also discussed.
• "Heat Exchangers for Solar Water Heating Systems", U.S. DOE describes the types of solar water heater heat exchange methods between the sun and the building's hot water supply
• "Heat-Transfer Fluids for Solar Water Heating Systems", U.S. DOE, describes the types of fluids selected to transfer heat between the solar collector and the hot water in storage tanks in a building. These include air, water, water with glycol antifreeze mixtures (needed when using solar hot water systems in freezing climates), hydrocarbon oils, and refrigerants or silicones for heat transfer.
• "Solar Water Heating System Maintenance and Repair", U.S. DOE
• "Solar Water Heating System Freeze Protection", U.S. DOE,using antifreeze mixture in solar water heaters (or other freeze-resistant heat transfer fluids), as well as piping to permit draining the solar collector and piping system.
• "Scaling and Corrosion in Solar Water Heating Systems", U.S. DOE
• www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=12850 is the base U.S. DOE website for these articles
• "Active Solar Heating Systems", U.S. Department of Energy, including
• "Radiant Heating Systems" U.S. DOE
• "Absorption Heat Pumps & Coolers", U.S. DOE
• "Solar Air Heating" U.S. DOE also referred to as "Ventilation Preheating" in which solar systems use air for absorbing and transferring solar energy or heat to a building
• "Solar Liquid Heating" U.S. DOE, systems using liquid (typically water) in flat plate solar collectors to collect solar energy in the form of heat for transfer into a building for space heating or hot water heating. The term "solar liquid" is used for accuracy, rather than "solar water" because the water may contain an antifreeze or other chemicals.

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
  
• Best Practices Guide to Residential Construction, by Steven Bliss. John Wiley & Sons, 2006. ISBN-10: 0471648361, ISBN-13: 978-0471648369, Hardcover: 320 pages, available from Amazon.com and also Wiley.com. See our book review of this publication.
• Decks and Porches, the JLC Guide to, Best Practices for Outdoor Spaces, Steve Bliss (Editor), The Journal of Light Construction, Williston VT, 2010 ISBN 10: 1-928580-42-4, ISBN 13: 978-1-928580-42-3, available from Amazon.com
• "Energy Savers: Whole-House Supply Ventilation Systems [copy on file as /interiors/Energy_Savers_Whole-House_Supply_Vent.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11880?print
• "Energy Savers: Whole-House Exhaust Ventilation Systems [copy on file as /interiors/Energy_Savers_Whole-House_Exhaust.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11870
• "Energy Savers: Ventilation [copy on file as /interiors/Energy_Savers_Ventilation.pdf ] - ", U.S. Department of Energy
• "Energy Savers: Natural Ventilation [copy on file as /interiors/Energy_Savers_Natural_Ventilation.pdf ] - ", U.S. Department of Energy
• "Energy Savers: Energy Recovery Ventilation Systems [copy on file as /interiors/Energy_Savers_Energy_Recovery_Venting.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11900
• "Energy Savers: Detecting Air Leaks [copy on file as /interiors/Energy_Savers_Detect_Air_Leaks.pdf ] - ", U.S. Department of Energy
• "Energy Savers: Air Sealing [copy on file as /interiors/Energy_Savers_Air_Sealing_1.pdf ] - ", U.S. Department of Energy
• Appliances and Home Electronics, - energy savings, U.S. Department of Energy
• Basement Moisture Control, U.S. Department of Energy
• Building Pathology, Deterioration, Diagnostics, and Intervention, Samuel Y. Harris, P.E., AIA, Esq., ISBN 0-471-33172-4, John Wiley & Sons, 2001 [General building science-DF] ISBN-10: 0471331724 ISBN-13: 978-0471331728
• Building Pathology: Principles and Practice, David Watt, Wiley-Blackwell; 2 edition (March 7, 2008) ISBN-10: 1405161035 ISBN-13: 978-1405161039
• Historic Preservation Technology: A Primer, Robert A. Young, Wiley (March 21, 2008) ISBN-10: 0471788368 ISBN-13: 978-0471788362
• Building Pathology, Deterioration, Diagnostics, and Intervention, Samuel Y. Harris, P.E., AIA, Esq., ISBN 0-471-33172-4, John Wiley & Sons, 2001 [General building science-DF] ISBN-10: 0471331724 ISBN-13: 978-0471331728
• Crawl Space Moisture Control, U.S. Department of Energy
• Energy Recover Ventilation Systems for buildings, U.S. Department of Energy
• Energy Savings Methods: Whole House Systems Approach, U.S. Department of Energy
• Historic Preservation Technology: A Primer, Robert A. Young, Wiley (March 21, 2008) ISBN-10: 0471788368 ISBN-13: 978-0471788362
• Log Homes: Minimizing Air Leakage in Log Homes, U.S. Department of Energy
• Log Homes: Controlling Moisture in Log Homes, U.S. Department of Energy
• Moisture Control in buildings, U.S. Department of Energy
• Moisture Control in Walls, U.S. Department of Energy
• Quality Standards for the Professional Remodeling Industry, National Association of Home Builders Remodelers Council, NAHB Research Foundation, 1987.
• Quality Standards for the Professional Remodeler, N.U. Ahmed, # Home Builder Pr (February 1991), ISBN-10: 0867183594, ISBN-13: 978-0867183597
• Natural Ventilation for buildings, U.S. Department of Energy
• R-Value of Wood, U.S. Department of Energy
• Spot Ventilation for houses, U.S. Department of Energy
• Slab on Grade Foundation Moisture and Air Leakage, U.S. Department of Energy
• Straw Bale Home Design, U.S. Department of Energy
• "Vapor Barriers or Vapor Diffusion Retarders", U.S. DOE: how vapor barriers work, types of vapor diffusion barriers, installing vapor barrier
• Ventilation for energy efficient buildings, Purpose, Strategies, etc.,
• Weather-Resistive Barriers, how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
• Whole House Ventilation Systems, U.S. Department of Energy
• Whole-House Balanced Ventilation Systems, U.S. Department of Energy
• Whole-House Exhaust Ventilation Systems, U.S. Department of Energy
• Whole-House Supply Ventilation Systems, U.S. Department of Energy