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Water Storage Building Design & Ventilation

PLDCAL | Posted in Energy Efficiency and Durability on

I am constructing an off-grid residence in phases about 30 miles SE of Atlanta, GA (Zone 3).

I’m currently constructing a 16X24 structure to house two (2) 1500 gallon water tanks, as well as, provide storage space for garden tools and equipment. The tanks are filled by a Grundfos SQFLex DC pump powered by a 1500 watt PV array. I intend to use a single 215 watt PV panel to provide DC power for lights and ventilation.

The walls will be 2×4 insulated by an exterior layer of 1″ XPS and batts between the studs. The roof is a 2×8 rafter configuration to permit the tanks (5′-4″ X 10′-8″) to fit inside the structure framed with 8′ high walls. I plan to insulate the roof with a layer of 1″ XPS and between the studs with R-19 batts. Due to design and other considerations, the walls need to be only 8′. Comments on this system are invited.

The exterior will be a combination of cement fiberboard siding and polar bark siding . I plan to use 1″ X 2-1/2″ vertical furring (1′-0″ O.C.) behind both the bark siding and fiberboard.to provide a continuous air space between and siding and the XPS (foiled). The air space will be screened at the bottom and will discharge into a screened vented soffit. The 1″ air space will be continued across the roof from the soffit to a vent ridge. The roof is metal.

The building will not be used for any thing other than water storage and tool/equipment storage, thus there are no windows and only a 6′ wide double door at one end that would rarely be used. Water will be flowing through the 1-1/2″ fill line during the day (when the sun is shining). I do not plan to heat this building, but rely upon the building envelope/insulation system and any thermal benefits the 1500 gallon water tanks may contribute (?). I would like to hear from the experts on this plan.

My ventilation plan is use a D.C. powered Zephyr in-line fan (http://zephyrvent.com/ )designed to vent battery boxes. This unit consumes <3 watts, thus it will run continuously. It is rated at 6 CFM and based upon my calculations, I should get about 2 air changes per day. I would like to hear from the experts on this. What are the experts thoughts about installing the fan to “draw” the outside air at the end away from the tanks and exhaust near floor level at the tank end. Obviously, this would create positive inside pressure. The tanks would be 12-13 feet from the intake vent. I am in-and-out and do not have ready access to my computer, so I will be slow responding to any questions and responses. Thank You Paul DeMent

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Paul,
    It will be interesting to hear from Georgia readers -- do you think the water will freeze? It certainly would in Vermont. Up north, we would bury the tanks for freeze protection.

    Why waste electricity to ventilate a storage shed? The ventilation is unnecessary.

  2. Riversong | | #2

    You don't indicate whether there will be any inside wall surface. I wouldn't leave the batts exposed to high interior humidity, and I would use the fan to create negative pressure (exhaust) to avoid pushing moisture into the thermal envelope.

    If you're creating condensation on the water tanks by ventilating with humid outside air, then you might be better off either with no ventilation and moisture-resistant wallboard painted with vapor retarder paint or with a fan controlled by a humidistat which operates only when the outside RH is low.

  3. PLDCAL | | #3

    Martin and Robert-
    Thanks for such quick responding! We'll be thinking about all this. By the way, the interior will be painted plywood; exterior is Poplar bark siding (not polar).

    Something we've been thinking: With the water coming into the tanks at 60+ degrees, so that there are two 1500 gal tanks within a an insulated building of 16x24 square feet., We're thinking (hoping?) the water will not freeze. Comments for GA readers?

    Paul was wondering: If we decide to ventilate, what type of wall vent would you recommend?

    Thanks, Cheryl (Paul's wife)

  4. Riversong | | #4

    Paul/Cheryl,

    Why do you think the building would need ventilation? Are the tanks open?

    Why are you worried about freezing in the Atlanta climate? Will the supply pipes be protected from sub-freezing temperatures?

    If you feel you need to insulate this building, I would suggest using the foam board and NOT the batts (I assume you mean fiberglass) so as to reduce the opportunity for mold.

    What kind of vents are you asking about? Natural convection vents, like in a crawlspace, or a supply/return vent to balance the fan? And why would you both insulate and ventilate an outbuilding? The two approaches are at cross-purposes.

  5. Michael Chandler | | #5

    We have a couple of 1,550 gallon tanks (black ABS from Agri-Supply, closed at the top) sitting out behind a house we recently built here in central NC with no insulation or protection at all and they didn't freeze at all last winter. It takes a lot of cold to freeze 1,550 gallons of water. Likewise, even though the tanks are black the hot plastic on the top doesn't transfer heat to the water in the summer across the air very effectively.

    Assuming that your pipes are insulated or buried and that you vent the air space at the top of the tanks with an overflow pipe to the exterior you shouldn't have to insulate the building at all and a simple passive vent opening at the roof and one at the base should be more than adequate to mitigate humidity in terms of keeping tools from rusting so long as you don't have exposed wet soil in there.

    You will, however, want to look carefully at the installation recommendations for that poplar bark siding. We install that with ring-shank non-galvanized 8D nails 3" on center, I cannot imagine any way to install it on vertical furring strips 12" O.C. We clamp the pallets at night with trailer straps and sleepers and keep the sun and rain off them with tarps until they are permanently fastened to the building to resist curling and that's the precautions we take with premium Bark House siding which is heated to take the temper and curl out of it and kill any insects and mold that might be in it. I'd hate to think what might happen if you tried to use un processed bark siding without proper fastening. It would likely look like potato chip siding in a few weeks.

  6. Jeremy Bollman | | #6

    Great discussion here!

    Michael is correct in expressing concern for installing poplar bark over furring strips, the outcome will surely be "potato chip siding." We at Highland Craftsman would be happy to assist in the details of bark installation. Please feel free to call us at 828-765-9010. And for more information visit us at http://www.barkhouse.com .

  7. Paul DeMent | | #7

    Robert and Michael,
    My apologies for the delayed response. The concerns for ventilation of the structure is for mold/mildew problems and hydrogen gas created by the PV array battery bank (when charging). The tanks are closed, thus moisture from the water storage would be condensation. I was not concerned about the water tanks freezing, only the supply lines.
    My original thinking for both insulating and ventilating (cross purposes) would be to ensure that freezing of the supply lines would not ever be a problem with a tightly insulated structure, and the mechanical ventilation was a means to control the air flow to alleviate both the mold/mildew and hydrogen gas issues and still maintain the benefits fromthe insulation.
    This utility structure came into existence meet several needs. First, the solar controls for the pump required some kind of enclosure; the PV Array required a support structure, our drip irrigated garden will be about 1/2 acre requiring tool storage and equipment (tractor) sheltering. Also, I believe the service life of the water storage tanks will be much longer if sheltered from the elements. Rather than have multiple structures, we thought it made more sense to put it all into one.
    After receiving all your responses, we've changed the design. The only insulation on the structure is 1/2" R-Max insulation board with the foil side out (for the summertime heat). It will also serve as a substrate for any uses for the building that could evolve in future years. The the water supply lines will be insulated. This is far less expensive and makes much better ecological sense than the original design.
    Michael, for the bark siding, we're installing the furring horizontally to facilitate the nailing. Thanks for the heads-up. Also, wood furring will be installed on the roof to create an air space between the insulation board and the metal roof.
    Thank-you all very much for your considered responses, we will be posting more questions in the future as we finalize the design on our residence.

  8. Riversong | | #8

    Paul,

    I would recommend not insulating the building at all, or at least not with anything impermeable to water vapor. If the roof is vented to avoid condensation under the metal roofing (make sure there is a roof membrane with a clear drainage path), and the structure is built entirely with wood or cellulosic materials, then it will be hydroscopic and breatheable and interior condensation should not be a problem as long as there is a floor drain to evacuate liquid water.

    I would weatherstrip the battery box and vent it separately to the outside (as is typically done), perhaps using a small DC fan if passive ventilation is not sufficient. Then no mechanical ventilation of the building should be necessary.

    Insulate the water pipes and you should be good to go.

  9. GBA Editor
    Martin Holladay | | #9

    Paul,
    In winter, your battery bank will be cold, affecting the batteries' performance. If you can find a location to install the battery bank within the heated envelope of your house, you'll get better winter performance.

    Of course, it's still important to build an airtight enclosure around the battery bank, and to make provisions to handle any spills of battery acid and to provide ventilation for hydrogen gas. (Passive ventilation is usually sufficient.)

  10. Jeremy Bollman | | #10

    Paul,
    Weather the furring strips are vertical or horizontal we strongly recommend a plywood substrate behind bark shingle siding. If your objective is to achieve a rain screen and/or a thermal break the best strategy would be to apply at least 1/2"x4x8 plywood to the furring strips and then the bark siding. If this strategy is not suitable to you, then my bare minimum recommendation would be 3/4" plywood furring strips 5" wide 9" o.c. horizontally. The reason being, the nailing pattern requires 2-1/2" ring shank nails 3" o.c. along bottom edge of that bark and staggered pattern 9" from the edge. At 3" o.c. this will affect the integrity of 2x4 material and the nail will not properly hold. If the bark is not properly fastened it will curl up and not lay flat. We've seen this mistake too many times.

    Good luck with your project. Call with any questions at 828-765-9010. And for more information visit us at http://www.barkhouse.com .

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