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archdiver

Pros & Cons of Adding a Heating Cable to a Cold Frame

Steven Laurin & Company
November 30, 2009

Last weekend I constructed a cold frame with the intent to get an early start with seedlings next Spring - which will be germinated indoors under lights. Ultimately, I'm planning on building a custom greenhouse - but decided to take baby steps with this cold frame first.

My wife and I are just getting back into vegetable gardening, after having other diversions for the past 20 years - or so. I suppose the renewed interest is partly due to empty-nest syndrome, but we did sell our marina-kept big boat, so will have more time available for gardening during warm weather.

After doing a bit of research into some cold frame basics, I designed a unit to receive the sash from an old triple Pella wood casement I had removed from the house and replaced with a new unit several years ago. It's still in good shape and has insulating glass.

Following are a few photos taken yesterday of the partially completed unit - in position over a brick base in my back yard.

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Aside from purchased 1x8 PT pine boards, galvanized hinges w/loose brass pins and foil faced rigid insulation, I re-used surplus materials I had on hand. For example, the brackets for the prop-bars were cut from some aluminum angle stock and secured with stainless steel bolts + nuts. The support pegs for the prop bars are simply sections of 1/2" copper pipe driven though holes drilled through the sash mullions

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I sketched some quick details on an 8-1/2" x 11" sheet as a guide - but of course, ending up improving on things during construction.

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With this much done now - I won't need to deal with building it in the Spring and can focus on getting indoor plants going in preparation for the transfer.

Part of my plan was to have the option of making this a Hot Bed, by installing a heating cable under the cold frame bed - which I understand must be set over 4" of sand. I've read that 1/2" wire mesh (hardware cloth) should then be placed over the cable, followed by 4" of soil, or planting mix. I'll most likely simply place seedling trays and pots directly over the bed - instead of growing directly in the soil.

I have not yet done much research into which heating cables may be best and how long of a cable is required for a 3'-4" x 6'-0" frame. Since this is new to me, I'm hoping to get some recommendations here from a few gardeners who have had success building hot beds, along with any helpful tips and advice.

I'd also like to hear any constructive criticism of this design, so I may make necessary changes before next season, how best to run a 40 foot long UG electrical cable from my garage service to the frame, what thermostats are good choices, the feasibility of automatic vents (12 lb lifting load) and anything else that I may be missing.

All best - Steve

Comments (30)

  • calliope

    Cold frames are so underused, and I wish more people could/would explore their potential. My father did a lot of his vegetable starts under a cold frame and they can give you a real boost on the season. I prefer them to g'house starts for some vegetables, because with proper ventilation you can harden your starts off in situ.

    That is the most beautiful cold frame I have ever seen. You did a great job. The brick base should provide a good heat sink, too. I have one question, however, and that is why you chose to place it in such an exposed situation where it gets no respite from sun/wind/temperature extremes.

    I sometimes set up temporary cold frames but usually erected them against the south side of an existing structure, like my chicken coop. That way, when a really freaky cold snap hit, I could bale them over with foam sheets or hay more easily and they'd get some protection and ambient heat from the structure and have one less side where wind was a factor. Also, if you had power in that structure, a shorter distance to run electricals if you should so desire.

  • PRO
    Steven Laurin & Company

    Thanks for your kind comments calliope - but in my opinion, it's really very basic. My wife however, has always said when I plan on building anything, I typically pull out all the stops. ;-)

    I suppose it is in an exposed location - but since the back of my house faces north, I wanted the cold frame to get full sun w/o getting shaded by the house - was this logic misguided?.

    The frame is also alongside the rear row of our first-phase 20 x 28 veggie garden. When (or if) we expand the garden area, it will extend to the front of the cold frame. If I need to relocate the frame, the expanded garden can cover the excavated area . . . not difficult to move.

    We have some raised planting beds as well, adjacent to the garage wing - where I suppose the frame can be moved to easily enough if that location would be best. I don't see the electrical cable distance as a problem, since I will need to install electrical conduit to the planned greenhouse anyway - which is currently planned to be sited near the cold frame, along the rear property line.

    Another consideration I had was to re-install the Pella micro-blinds between the 2 panes of glass. They came with the windows and can be opened or closed with the remote turn-knob - enabling shading of the interior during warmer weather . . . just a thought of course.

  • dcarch7 d c f l a s h 7 @ y a h o o . c o m

    You obviously have some architectural design background, if you are the one who drew those scaled drawings.

    Very impressive work.

    My suggestions:

    1. You may want to make the windows higer. They seem too low to me at the low point.

    2. You may want to consider spring load or a counter balance the windows. This will make operating the windows much easier.

    3. Electric heating the soil may be not practical. You are heating all the earth below.

    4. Treated wood can warp if subjected to different moisture conditions on each side. Your foil-faced insulation is a very good detail as a moisture barrier. Still, you may want to paint both sides first.

    5. I can't see nails or screws. How did you frame the whole thing?

    dcarch

  • PRO
    Steven Laurin & Company

    darch,
    Thanks - yes, I've been in the profession no-stop since 1974, started a practice in 1990. Use AutoCad but still prefer the old-school method of hand drawings.

    Great suggestions - most of which have been considered. I didn't want to make the frame too high but did desire a somewhat steep glazing angle. You're right - the front inside clearance is about 9-10". I could simply add brick courses to provide more interior height . . . have a ton of old bricks.

    The sash aren't heavy - but I will consider counter balances, or interior springs attached to the prop bars to assist lifting. The prop bars are actually designed to be self-activating, which lock onto the peg stops by gravity.

    You may be right about the heating cable - thought it might allow for even further extending the growing season.

    I was allowing the PT wood to age a few months before staining - perhaps a Hunter green color; foil facing is just friction fit for now, so both sides can be finished for stability. I banded all exposed foam edges with 1 x 1 trim.

    I ripped PT 2 x 4s into 1-1/2 x 1-1/2 stock and used that for internal corner posts. The horizontal boards are screwed to each post from the exterior with coated deck screws - as seen in this pic:

    {{gwi:308285}}

    I used 3/4" x 2" PT trim strips to cover the boards end grain, using galv. finish nails for the trim. The perimeter 2 x 3 along the base, serves as a compression support.

  • dcarch7 d c f l a s h 7 @ y a h o o . c o m

    Ha! You can't fool me :-).

    Yes Autocad has advantages. Too expensive. TurboCAD will give you everything, less than a 10th of the cost.

    Hand drawing is good, except you can't e. mail hand work. pdf files from CADD are great to scale up or down and send and print.

    Regarding counter balancing: I actually knew your cleaver prop bar design intend. I was thinking more for your thermally operated mechanisms, the lighter the easier for them to operate.

    Again, impressive work.

    dcarch

  • PRO
    Steven Laurin & Company

    "Hand drawing is good, except you can't e. mail hand work. pdf files from CADD are great to scale up or down and send and print."

    But dc, you've heard of Epson and Acrobat, haven't you? ;-)

    BTW - your work is also very impressive :^)

  • calliope

    I suppose it is in an exposed location - but since the back of my house faces north, I wanted the cold frame to get full sun w/o getting shaded by the house - was this logic misguided?.

    No your logic isn't misguided, you don't want a shady situation. I have outbuildings, and that made it easier for me to find an appropriate southern exposure with a building as a windbreak. The chicken coop just happens to abut my vegetable garden, so it was handy in all sorts of ways, including putting the poo close to the garden to compost in winter. LOL.

    Some people earth mound around the back of their frames, but even a bale(s) of straw can serve as a good insulator if need be. There shall be spells when you might find it'll make all the difference if you get freakishly cold snaps after your plants are already up and growing. Are you planning on sowing directly in the soil or in trays/pots? Some sort of insulation will really help to retain the heat you trap in your frame through the nights. Even if it's bagged up leaves.

  • PRO
    Steven Laurin & Company

    The interior face does have foil-faced rigid insulation and the sash is double glazed. The unit is temporarily set over the brick base, but will eventually have a neoprene gasket. Also, just out of sight in the photos, is a stand of incense cedars, which will help in blocking the prevailing northwest winter - early spring winds.

    By early Spring - when I plan on simply placing trays/pots over the bed (not directly in the ground) I could stack bales of straw against the sides . . . it certainly won't hurt. If the forecast is for extended freezing conditions, I could always bring the trays into the garage until conditions improve.

    Thanks for the comments - I truly appreciate them.

  • Dan _Staley (5b Sunset 2B AHS 7)

    Am I missing something? I see no pix. I'm almost done with my third incarnation of a cold frame that goes all winter in Z5 with only solar heating & I might be able to offer comments if I could see it...

    Dan

  • PRO
    Steven Laurin & Company

    Don't understand why you can't see any pics Dan. Do you get a red-X in a box, or just blank white spaces? I could email you a direct link to the photos if you'd like.

  • Dan _Staley (5b Sunset 2B AHS 7)

    Nothing. Not an X, nothing. No indication a foto is there, not even the HTML tag. Using Firefox, but hasn't stopped me at this site before...you can e-m or post the link here, arch.

    Dan

  • PRO
    Steven Laurin & Company

    Dan,
    Click on the link below for a separate album of just these pics.

    Here is a link that might be useful: Cold Frame pics

  • Dan _Staley (5b Sunset 2B AHS 7)

    Got 'em. Very nice work. I'm an urban planner from UW and have collaborated many times with your discipline. Nice.

    First, the front wall IME should always be as low as possible Why? The solar angle from Nov 15 - Feb 1 is too low for a high wall. You want sun to strike the soil to heat it, and a high front wall makes a long shadow. Short wall, plant maché. Don't re-do with a higher wall. That is fine and your angle for the glass is fine and if anything I'd make a little steeper to cut down the angle of incidence. My latest I was looking at tonight and looking at my scraps of polygal and thinking I could have done a two-layer window (% ~R-5) on the front wall for ~33-50% of the face to let in more light

    Second, bricks are fine but you'll get, IME, better performance with something going farther down into the soil for insulation if you want to go all year. In that zone you don't need supplemental heat.

    Third, I like how you lift the windows. Nice nod to history and improving upon it. When you do another, you'll want to think about an auto vent opener in there so no one has to go out twice a day. I'll post some pix to PhotoBucket soon to show what I did with my latest.

    Fourth, a major modification for me in the wind was lips to stop the wind from hitting the joints/weatherstrip directly. I'll post a detail to elaborate. Even tho you have a clean joint, the wind likes to steal. Your wish for better and more salad throughout winter will reveal this issue.

    Last, clean and well done. The sills allowing for rain to collect and not run off is a function of the discipline, IMHO, and I get to see Liebeskind's DAM leak all the time and the poor crews try to fix. 6º on the sills and you're good. Please think about how you can scale this up for folks in the area and how your community can make a few of these for those who need greens in the winter. San Diego is ahead of me in my scaling up but we all share and many among us can utilize such a box...

    TTYS

    Dan

  • dcarch7 d c f l a s h 7 @ y a h o o . c o m

    A heliodon is very important tool to have access to when designing anything that has to do with the sun.

    dcarch

  • PRO
    Steven Laurin & Company

    Thanks dan - I appreciate your comments and will reply to each later. I've got a few bigger plans to focus on in the office today. Hm-mm - reminds me of one of your fellow planners philosophies: "Make no little plans. They have no magic to stir men's blood . . . " D.H. Burnham

    dc, I constructed a very simple heliodon a while back for a model of a passive solar house - simply a gyroscopic base and movable light source. It's definitely more intuitive than computer programs.

  • PRO
    Steven Laurin & Company

    Dan,
    I've got a couple minutes to respond to your items while I eat my lunch . . .
    The chosen front wall height was actually based upon a dimensional 1x8 board = 7-1/4", rear wall 3 x 7-1/4" minus the angled top board for the head + sill. I adjusted the angle to 22.5º (clean detent on the compound mitre saw). So, determining the angle of incidence was unscientific :)

    Re: thermal protection below grade, I intend to use some left-over rigid insul. to apply to the inside face of brick, 3 courses down or 8" deep - and overlapping the upper insul board where it'll be sealed with panel adhesive.

    I'm ahead of you with the vent opener - ordered this unit last night:

    {{gwi:308286}}

    It automatically opens at a user-set temp of 55º - 80º , I believe. The lift load is claimed to be 15 lbs, or a 30 lb sash weight. I think it should easily lift the center sash. Otherwise, a counterweight can be devised somehow.

    I'd like to see your wind-seal detail, although I do have some neoprene bulb weather stripping left from another project. I plan on stapling that with Monel staples all around the top frame joints - protecting all 3 units (I'm a life-long boat owner with tons of boating gear and tools).

    You've lost me somewhat on your last item referring to the sills not allowing for rain run-off. The entire top edge is sloped 22.5º - allowing for moisture to migrate around ea. sash. The 2 mid-mullions will have a neoprene gasket, providing a channel to funnel water to the lower front panel, where it will drip off the 1 x 2 trim.

    Good idea about spreading the cold frame joy to the community - although, you'd be amazed at the level of organic growers on my island community . . . I consider my meager efforts falling somewhat behind that mainstream. Although, I'm catching up :-).

    Thanks again for all your thoughtful time.

  • dcarch7 d c f l a s h 7 @ y a h o o . c o m

    For my small removable greenhouse, I just use a small fan (250 CFM) and a thermostat.

    dcarch
    {{gwi:294794}}

  • PRO
    Steven Laurin & Company

    dc,
    Interesting setup with the reuse of a standard wall thermostat and computer fan - now that's thinking green and sustainable! Being borderline anally retentive though, I'd devise a way to enclose those wires and seal the space around the fan . . . ;^)

    One question though - how are you supplying DC power to the fan?

  • dcarch7 d c f l a s h 7 @ y a h o o . c o m

    You ask too many questions. You want all my secretes? LOL.

    Yes, that's a brushless computer "muffin" fan, quiet and efficient with a 50,000 hour rated life.

    I don't need to enclose the wiring because it's low valtage.

    DC power is from a old computer 12V power supply.

    Thermosat is a cheap($5.00) bi-metal room thermostat with one contact for cold and one for hot.

    When it gets hot, the fan goes on, when it gets cold it kicks on a heater (with a relay).

    dcarch

  • PRO
    Steven Laurin & Company

    I realized it was DC based upon the muffin fan and thermostat . . . simply curious to know what you used for a power source, not after your secrets :^).

    Regardless of the wires being low voltage, the very high humidity could short out the connections (didn't use marine grade connectors and some bare wires are showing ;-). Looks messy as well - I would at least bundle the wires in a plastic cable chase.

  • dcarch7 d c f l a s h 7 @ y a h o o . c o m

    HaHa, it's worst than you think. :-)

    You noticed that everything is mounted using just Velcro tapes?

    The whole thing is just a quick setup for about a month then removed. My greenhouse is removable and to be reinstall quickly every season.
    Check this out:
    http://www.youtube.com/watch?v=MzDqDBw_F54

    dcarch

  • PRO
    Steven Laurin & Company

    Pretty cool dc - enjoyed your "soda" can lantern too! :)

    Seems as though you have a passion for metal arts and tomatoes . . . idle diversions from architecture?

  • deborah512

    have you considered digging a bit further below and laying in a piece of rigid insulation? we've just finished a tool shed and did that for the concrete block piers. we were told that it would help stop freezing/heaving. you could also do some a bit away from the perimeter so you 'confined' the heated area, rather than it just dissapating at will. obviously, don't make an insulation "box" as you do need drainage. and make sure that you have several inches of sand or soil (sand is better heat conductor), so the cable doesn't contact the insulation and melt it!
    lovely job..i'll be showing it to my husband with the header of 'christmas list'!!!!

  • dcarch7 d c f l a s h 7 @ y a h o o . c o m

    "Posted by archdiver
    Pretty cool dc - enjoyed your "soda" can lantern too! :)
    Seems as though you have a passion for metal arts and tomatoes . . . idle diversions from architecture?"


    I enjoy everything that has to do with beauty and science. Architecture is one of them.
    Check out my posts in the " Cooking Forum" and the "Junk forum" here in GW.

    dcarch

  • PRO
    Steven Laurin & Company

    Some other time, perhaps . . . getting ready to attend a festive Christmas party :~)

  • Dan _Staley (5b Sunset 2B AHS 7)

    Huh. I had a brilliant and insightful reply disappear.

    Basically, I wrote that I suggest the next incarnation has a steeper angle - mine is 30º - to have less reflected light and a better angle of incidence for the low winter sun. Last winter I didn't heat my coldframe in Denver area and had greens all winter. No electricity required.

    Dan

  • Sarah5614

    ArchDiver,
    I work for University of Nebraska- Lincoln Extension in Lincoln, Nebraska, and would like to ask if you would allow me to reuse your pictures of your coldframe. Each month we publish a newspaper-type publication called the NebLine, that goes out to over 12,000 households in the Lincoln area. You can check out the NebLine at http://lancaster.unl.edu/nebline/.

    In September, I will be contributing an article on the benefits of coldframes and would really like to use a picture of your coldframe as an illustration. It's very well done. Thank you for your consideration!

    Sarah Browning
    University of Nebraska- Lincoln Extension
    (402) 441-7180
    sbrowning2@unl.edu

  • PRO
    Steven Laurin & Company

    Sarah, Sure, why not . . . you're free to use the images for your article.

    Send me an IM if you need additional info.

  • dwyerkg

    Hi, can you tell me how this worked out as far as full season growing? Thanks

  • hudson___wy

    Hi Steve,

    To address your original questions about soil heat cables.......we live in a very cold climate here in Wyoming at over 6,000' elevation with winter temps as low as -40 degrees F. We have not used soil heat cables in an outside grow box - but did decide to experiment with a grow box inside our unheated GH. We bought the cable length we needed based on the area we wanted to cover - 30" X 96" and according to the spacing the cable manufacturer recommended. We followed instructions and mounted the cable to 1/2" hardware cloth and then mounted the hardware cloth to cattle panel for extra stability and ease of moving/storing. We purchased a thermostat and set the temperature at 55 degrees F and placed the cable buried 4"s in a raised bed within our GH and made a polycarbonate cover to create a box with lids.

    We tried to grow salad veggies during the winter months - November thru March. The daytime temps in the GH on a sunny day were between 30-70 degrees F even when OS temps were 0 degrees F so I would raise or remove the lid during the day and cover the box at night when OS temps were as low as -30 degrees F and IS GH temps as low as around 0 degrees F. The soil heat cables were usually only running during the night so I did not notice much difference in our electricity bill. The photo below was taken on January 31st. We are in Zone 3 where danger of frost remains through June 1st and there is snow on the ground usually through April but we have had snow and frost in every month of the year during cold spells.

    I think most would agree the experiment was a success! The cons in my opinion is the cost of the cables, wire and thermostat and the daily covering/uncovering of the lid (although the polycarbonate only needed to be removed on sunny days because of the heat build-up). When I retire I may do this every winter but for now - it is nice to have a winter break from the GH. We also use the Soil Heat Cable as a germination mat on top of the soil in the GH in February - March as an overflow box from our inside grow lights - that works well too. We are building grow boxes this year OS for our squash and other warm weather plants and appreciate your design!

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