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Building a Tight Storage Shed

Canada_Deck | Posted in General Questions on
Hi All,
I’m in Canada and I’m building a heated tool shed.  I’d like to keep the inside of the shed from getting below freezing in the winter so that I can store some items like latex paints that are susceptible to damage from melt/freeze cycles.  I will also be storing expensive tools and toys and I’d rather not expose them to extreme temperatures.  At the same time, I don’t want this project to kill my energy bill.  The climate where I live is similar to Seattle but a little colder and further north.  Very wet winters with driving rain.  It’s not uncommon for mushrooms to start growing on the lawn and on any untreated/unpainted wood.  Temperatures during cold snaps will reach 20 F on a regular basis and I might see 5 F once/decade.  I expect that I won’t need to turn the heat on for eight months of the year but will need to use some heat from November to February.
The size of the shed is limited to 100 square feet by my local building code so a large consideration is that I’d like to minimize the number of different types of products being used or I will end up with a lot of leftovers due to minimum purchase sizes.
There will be no plumbing in the shed and it will never be occupied (e.g. no workbench.)  It is purely a storage shed.  That said, I may occasionally introduce moisture (e.g. putting away wet bikes or wet yard tools) but that will happen almost exclusively during the warmer months.
My current thinking:
– The building will sit on blocks which sit on a cement pad.
– There will be no windows and a single door
– I will use a shed/lean-to/mono-pitch/skillion roof with a sloped ceiling.  One wall will be taller than the other.
Option 1:
– The walls, roof, and floor will all be similar.  2*8 walls.  2*8 rafters for the roof.  2*8 joists in the floor.
– The walls, roof and floor will all use 5/8″ plywood as the interior sheathing (I like being able to put screws anywhere into a wall in a shed).
– There will be no vapor barrier on the inside of the shed.
– I will insulate all cavities with Rockwool batts.
– Exterior sheathing will be 5/8″ plywood.
– I will add EPS or XPS foam on the exterior of the walls, roof and floor.
    – Floor: Nothing after the EPS/XPS.  This will be the bottom layer.  It will sit 4-6″ off the ground and there will be an air-space below.
    – Wall: Rain screen furring strips on top of the EPS/XPS and then Hardiplank siding
    – Roof: Vented airspace on top of the EPS/XPS and then sheathing with a single roof

 A variation of this option is to replace all of the EPS/XPS with Rockwool Comfortboard and install a poly vapor barrier on the interior side of the walls, roof, and ceiling.

Option 2:
Similar to the above, 2*8 walls, rafters and joists.
However, with this option there will be no external insulation.  Instead, I will install 2*4 strips perpendicular to the studs, rafters, and joists on the interior side and then another layer of Rockwool batts.
On all six sides of the building, the core assembly will be (from inside to outside):
– 5/8″ plywood
– 2*4 with cavities insulated by Rockwool
– Structural 2*8s perpendicular to the 2*4s insulated by Rockwool batts
– 5/8″ plywood

Roof: I would then just put tar paper and shingles
Walls: Furring strips and siding
Floor: Nothing additional

With this option, I would not install a vapor barrier.
Because there is no vapor barrier on the inside, the roof can still dry into the shed if needed.

A nice thing about this option is that I only need to buy a few packages of the 3.5″ thick Rockwool batts. I can use two layers in the 2*8 section and one layer on top in the 2*4 section.

It also greatly simplifies the exterior details and that is important to me because those take a lot of time and I need to think about making the building pest proof.

I’d appreciate any and all feedback.

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Replies

  1. Jonny_H | | #1

    Here's my stream-of-consciousness:
    Starting from the goal, let's say you want a 10' x 10' x 10' cube that stays at 50F when it's 20F outside, using no more than 300W continuous. That's 600 square feet of area, and a heat flux of half a watt per square foot. Converting the units, that's 1.7BTU/hr per square foot. Divide by your 30F temperature delta, you get a U-value of 0.056666; the inverse of that is R-17.6 -- so you'd need a whole-assembly R-value of about R-18

    7.25" rockwool batts are rated R-30; with 16" OC framing that gets you an effective R-value of about R-22 (https://www.appliedbuildingtech.com/fsc/woodcalculator). Either of your assemblies with either exterior foam or interior strapping will substantially increase that by reducing thermal bridging. Congratulations, you're efficient just by virtue of being small -- I'd imagine either of your assemblies could be kept warm-ish just by a 100W light bulb in there.

    For dealing with moisture, I'd get a little portable dehumidifier that can connect to a hose draining the condensate outside. I actually feel like just the heat generated by a dehumidifier might keep the place warm on most days, but you can put the 100W light bulb on a thermostat just to be sure.

    In any case, pay attention to air sealing -- in a space this small, little air leaks will turn into a lot of air exchanges, and could easily dominate the heating load -- but you have one door, no windows, and a simple shape, so it should be relatively easy to get pretty airtight.

    I wouldn't personally go with exterior foam here -- seems pretty overkill and isn't cheap -- I kind of like the cross-strapped interior with rockwool for eliminating thermal bridging. Note that your unvented roof assembly filled with air-permeable insulation doesn't meet code, but this also isn't a house heated to 70F with interior moisture loads. Someone else with more knowledge on the hygrothermal aspects can chime in on the safety of this -- I'd just say make sure your interior air sealing is good, and it'd probably be safer to go with a vented assembly -- or somehow get a good air barrier in the middle (ie between the layers of insulation).

    Finally, in your first assembly, you'd definitely want something below the foam on the floor -- exposed foam on the underside would just become a nice nest for a bunch of mice and bugs.

    1. Canada_Deck | | #17

      Thanks for this. In particular, I didn't think about needing to protect foam from critters but that is good to know. Also a big fan of the idea of a humidifier.

  2. Kyle Bentley | | #2

    I'm going to list a few ideas that you may want to reconsider, that may shape the frame of mind for the suggestions I'd have below.

    1) It's important for both cost and constructability to logically separate insulation and structure. Build the structural components to sufficiently handle design loads, and insulate for the design heat loads. Otherwise you'll make a structure that costs more and performs worse when the two start to overlap.

    2) The goal of insulation is (obv I know) to prevent the flow of energy. If your shed is well insulated, but without sources of heat or cooling, it will eventually reach a freezing temperature inside, no matter how much R value you throw at it. You'll need a source of heat to keep the temperatures elevated during the long cold winters, else it might still actually freeze on the inside.

    With those, my suggestions would be these:

    1. a 10' x 10' structure can be built with 2x4 walls and 2x6 rafters (16" o.c.) to handle 60 lb/sq.ft. snow loads, if you need more, 2x8 rafters will handle up to 100 lb/sq.ft. 19/32 on the roof is fine, but 7/16" is sufficient for the walls. Building walls that are structurally twice as thick doesn't provide anything more, than a convenient bridge for cold to get from outside, to the inside through the studs.

    1a. 2x8 walls will quickly eat up a lot of space, leaving you with a smaller than intended area. You'd end up with only about 75 sq.ft. of finished space, not a lot of room for tools, especially when bikes / mowers are introduced.

    2. Put your insulation on the outside, and think of the insulation in the 2x4 wall cavities as a nice bonus. A 2x4 wall with R13 insulation probably provides somewhere around an R9 total wall value. Add whatever you want to the outside and attach with furring strips for siding.

    3. Same goes for the roof. Fill the rafter cavities with insulation, you'll get about R17 out of that assembly. Use XPS or polyiso or EPS on top, in how every many layers it takes to achieve your desired R value, and use 2x4's on the flat to create a ventilation channel. Then proceed to roof however you want.

    4. Run an electrical outlet, for lights and a small wall heater, such as one of these. It'll practically never come on with the amount of insulation you claim to add, and when it does, it will not run a long time, especially if you set the thermostat to something like 50 degrees F.

    https://www.homedepot.com/p/Cadet-Com-Pak-1-000-Watt-120-Volt-Fan-Forced-In-Wall-Electric-Heater-in-White-with-Wall-Thermostat-20014/314730249

    Besides that, 2x4 walls are easy to lift solo, 2x6 rafters are easy to move solo, 7/16" osb is easy to move solo. Polyiso / XPS / EPS are light, and mineral wool is actually kind of heavy when you lug it around all day. It's also irritating, in my experience. I'd wear at minimum some kind of dust mask, preferably a respirator when agitating it. It's been shown not to be cancerous, but it'll give you an itchy throat for a few days.

    Hope that provides some thoughts!

    1. Canada_Deck | | #18

      Thanks for this. On the roof, I'm trying to avoid having three layers of sheathing. Would you do something like this?
      OSB on interior (the ceiling)
      Rafters (filled with batt insulation)
      EPS/XPS/Polyiso attached directly to the top of the rafters
      2*4s on the flat running up-slope (parallel to the rafters, not perpendicular)
      1/2" plywood
      Tar paper
      Shingles

      1. Kyle Bentley | | #29

        I wouldn't put the insulation directly on top of the rafters. The bending moment in the fasteners becomes large, since they're essentially cantilevered out by the thickness of the insulation. Having two layers of sheathing may seem excessive, but for such a small building it might only add up to $150 all in for the extra materials. Otherwise, the stackup looks fine.

  3. DCContrarian | | #3

    I suspect your soil temperature is around 50F even in winter. A well-insulated building that is open to the ground at the bottom should get enough heat from the ground to stay above freezing.

    I also like the idea of running a dehumidifier. They are efficient space heaters with a COP that rivals heat pumps.

    1. Canada_Deck | | #19

      That's an interesting idea. The ground where the shed will sit is a sloped concrete slab. I was planning on putting down blocks (of varying heights to get to level) and then building on top. The airspace below the shed would be at air temperature. I may consider putting it somewhere else so that I could leverage the heat of the ground more effectively.

  4. Walter Ahlgrim | | #4

    You may want to reread your local rules generally when the rules limit you to 100 SQF they also disallow electricity in the shed.

    Walta

    1. Expert Member
      Malcolm Taylor | | #7

      The BC Building Code doesn't require permits for storage sheds under 10 sq m. They can be supplied with power as long as they obtain an electrical permit. If they only have 2 circuits or less they can be supplied from the main panel. For larger loads a sub-panel is required.

      1. fast_cheap_mediocre | | #15

        Are you sure it's 2 circuits and not one multiwire branch circuit (2 hots with shared neutral)? Here (Maine, USA) you can only supply a MWBC from the house panel to a shed, otherwise you have to install a sub panel.

        1. Expert Member
          Malcolm Taylor | | #16

          It's two 15 amp circuits for plugs and lights. No idea of the reasoning behind it.

          Neither situation (two circuits or sub-panel) triggers the need for a building permit.

          1. Canada_Deck | | #20

            Yeah it's a little funny in BC. Local jurisdictions use the BC Building code and don't require a permit if it is under 10 sq m. Electric is done through a different provincial agency and most local jurisdictions don't get involved. Malcolm - I have two circuits I could use for this. One is a single 15 amp 120 circuit. The other is a 15 amp multiwire branch circuit. Do you think I could use either of these (but probably not both)?

          2. Expert Member
            Malcolm Taylor | | #30

            CD,

            I confess I don't know.

  5. Expert Member
    Malcolm Taylor | | #5

    Canada_deck,

    I'm going to suggest a variation on the good advice both Jonny and Kyle have given you - and like the other posters I don't think there is an appreciable advantage to the levels of insulation you are proposing for such a small largely unoccupied space.

    I would build everything from 2"x6".
    - The floor much as you suggest, but cover the foam board on the bottom of the joists with plywood to protect it against damage.
    - The walls get filled with batts and no exterior insulation. Tape the sheathing and provide a rain-screen gap.
    - For the roof I would leave the rafters empty, and put 3" of EPS on top of the (taped) sheathing, followed by either another layer of plywood, or strapping.

    1. Canada_Deck | | #21

      Thanks Malcolm,
      So the roof would be:
      - No interior sheathing/ceiling
      - Open rafters
      - Sheathing
      - 3" EPS
      - Strapping (for metal roof) or plywood (for shingles)

      Is that right?

      1. Expert Member
        Malcolm Taylor | | #31

        CD,

        Yes that's it.

  6. Johngfc | | #6

    Canada Deck - I believe you're WAY overthinking this. Our hot tub is 103F in winter, in Colorado. With our 5" foam cover the (light, fluffy, powder) snow does not melt on the cover. So 5 inches of foam (maybe R30 if you're optimistic) seems to easily maintain a 70-100 deg temperature differential. You're not cooking or showering in the shed -- any sort of breathable wall is going to work. Do something simple, with an air-tight but vapor-open construction and you'll be fine. For the most part you only need to maintain on the order of a 20 deg temperature differential to keep things from freezing - you'll be able to do this with a light bulb.

    1. Bas Sommer | | #8

      2 cents from me - if you have the osb inside you do not need that one outside anymore. From a raking strength pov one layer is enough. A ventilated rain screen does not need the sheathing - furrings will do. A layer of tyvek on the studs outside as wrb - it is simpler/cheaper, if you go 6" studs then maybe a bit of external insulation.

      1. Canada_Deck | | #22

        Thanks. That is an interesting simplification.

  7. Robert Opaluch | | #9

    Maybe consider having that concrete slab as your shed floor. It may simplify your floor construction and reduce your heat losses. Your floor would only be dealing with above freezing ground temps of about 50F (?) vs. your outdoor temp of 20/5F with a raised floor. You might have the ground temp keep the shed above freezing. However you would have to insulated the perimeter of the concrete floor with EPS to prevent heat loss to the outdoors. Buried wing insulation would help reduce your heat losses and depth of perimeter insulation.

    Exterior insulation would help align the shed exterior with the exterior of the proposed concrete floor. But not sure if this pushes your building dimensions beyond the maximum allowable 100 SQFT. You might have to reduce the size of the shed's walls.

    What exactly determines the legal "size" of the shed to keep it under 100 SQFT? Exterior dimensions of the building including cladding? Exterior size considering wall stud locations?

    1. Expert Member
      Malcolm Taylor | | #12

      Robert,

      Where to measure from is a grey area in the BC code. Typically inspectors use the dimensions on the plans, which are to face of concrete 0r stud, but it would be interesting to see how a building with thick exterior foam was looked at.

      1. Canada_Deck | | #23

        Thanks Robert and Malcolm,
        This concrete slab has a large slope (part of a driveway) so I can not use it as my floor. I could pour additional concrete but I'd rather avoid that because I like the idea of being able to bring in a machine to move the shed if I ever need to do that. I agree that the code is very ambiguous regarding external insulation and how that impacts building area.

  8. Robert Opaluch | | #10

    You mention the shed has one door. That door might be a large contributor to cold air infiltration as well as to conductive heat losses. A wood door, or insulated steel door? Or you might put some rigid insulation on the interior side of a wood door. When I calculate heat losses on a building, doors and windows are a significant portion of heat losses, comparable to walls and ceilings/roofs.

  9. Keith Gustafson | | #11

    Air sealing the door on such a small structure probably gains importance

    I bet the exterior dimensions are what are used for the 100 sq ft, so there is a downside to excess insulation

    An insulated concrete floor will help carry temps through the night. My first building project as a 900 sq ft barn with the concrete insulated from the walls[1986 the concrete guys looked at me funny] and a whopping 1 inch of exterior foam. With a bunch of south facing glass it would pretty much stay at 40 degrees in Massachusetts winters with no regular source of heat. Nothing ever froze. Eventually put a furnace in it and it took little to heat.

    Is there a height limit? a 12 pitch roof gives some nice storage. Or a 9 foot wall allows high shelves.

    Remember that house levels of insulation are based on a lot of degree days on a large area.

    When you lower your degrees days and square footage you decrease the amount of insulation that makes sense.

    1. Expert Member
      Malcolm Taylor | | #14

      Keith,

      I deal with these small accessory buildings constantly, and there are a lot of misapprehensions about what permits they require and where they fall.

      The BC Building Code says it does not apply to " accessory buildings under 10m sq in building area that do not create a hazard". The second part of that sentence opens them up to regulation based on the inspector's judgement on any number of grounds. The most common way they are interpreted as creating a hazard to their occupants is if they are used for sleeping, in which case a permit is required, and all the requirements for bedrooms (egress, smoke detectors, etc.) apply regardless of size.

      While these small accessory buildings may not fall under the code, they are still subject to all the provisions of any local bylaws. This is where things like proximity to the lot line, lot coverage, and height are laid out and are required to be followed.

      1. Canada_Deck | | #24

        I can make it fairly tall under my local zoning so I do like the idea of a lean-to roof that allows me to make full use of the tall wall.

      2. Keith Gustafson | | #27

        Obviously everything is up to the local inspector, but generally if you do not have heat, you do not need insulation at all.

        One thing to think about with wall thickness, assuming the are hard about the 100 sq ft, is that the walls on a 4 inch wall take a little under 13 square feet of usable floor space. A 6 inch wall is 19 square feet, so you lose 6 square feet or 6 percent of your total available space by going with a 6 inch wall. If you went the other way and maybe site built sips for the walls, a 2 1/2 inch wall is a little over 8 square feet, so that is an almost 11 square foot swing in a 100 square foot building

        Because of the stated use of the building, the differences in heat use between the insulation levels would be difficult to measure.

        Not a decision making process common in houses, as usually no one but the tax man cares about the actual dimensions of the house.

  10. Expert Member
    Akos | | #13

    Instead of raising the shed off the ground, I've had good luck with a simple floating foundation on the ground.

    Build a slightly bellow ground perimeter out of stone blocks (6x8x12 typically used for retaining walls works well for this). I like to set the stone over a small gravel pocket but can also be set directly over compacted dirt if you have sandy soil.

    On top of the pavers set a 4x4 or 6x6 PT perimeter frame. These can be anchored to the stone bellow with a couple of expansion anchors.

    Fill the inside with sand, level and set standard patio pavers as your interior finished floor.

    The walls can now be anchored to the pressure treated base frame.

    For an insulated shed, the only thing I would change is to add in some gasketing and a layer of rigid under the patio pavers.

    This keeps everything near grade but lifts the wood structure a bit above dirt level to avoid direct ground contact. You also don't have to deal with floor joists, insulation and critters making a home under the shed.

    1. Canada_Deck | | #25

      I do like that idea. The current site is already a sloped concrete pad but this gives me a lot to think about. Perhaps instead of elevating off that pad, I should be thinking about how to leverage it.

  11. Canada_Deck | | #26

    Thanks all. This has given me a ton to think about.

    As I think through it more, I am realizing that even if I chose a single nominal size of lumber (e.g. 2*6,) I am going to need a bunch of different lengths so it's unlikely I will be able to order a full skid. I prefer to order a full skid because I end up with higher quality wood. I find when I place partial orders, I am more likely to get all the bowed stuff off the top of the pile that people picked through. I think I will just have to find a way to manage that issue with this small project. If I remove that constraint, it opens up a lot of opportunities to optimize.

    There were a number of comments about building the shed on the ground (instead of wooden floor joists/beams elevated on blocks) to take advantage of warmth from the earth. Given my goals (keep the shed just above freezing,) it may be better to be exposed to the earth than the air. On one hand, the earth (especially with a poorly insulated floor) is an annoying heat sink if you are trying to keep a building at 70 f. On the other hand, the earth may be quite useful if you are trying to stay at 33 f.

    That's a little challenging on my site but I am going to give it some serious consideration. I also really the idea of a concrete floor vs having to mess around with vinyl.

    I also appreciate the comments about the door. It will definitely be a weak spot and there are greatly diminishing returns of insulation elsewhere if most of the heat is being lost through the door. I am now considering basic insulated fiberglass doors (one swinging out and one swinging in) which is probably cheaper and more effective than one extremely good door.

    So here is one interesting option coming out of all of this:
    Wall assembly from inside to outside:
    - Plywood sheathing on the inside (partially for functionality - being able to mount hooks anywhere on the wall)
    - A layer of Comfortboard 80
    - 2*4 walls filled with Rockwool batts
    - Exterior plywood or OSB sheathing
    - WRB
    - Rain screen furring strips
    - Siding

    *Note: No vapor barrier

    Roof assembly from inside to outside:
    - Plywood sheathing as the ceiling (again - to be able to hang hooks from anywhere)
    - A layer of comfortboard 80
    - 2*6 or 2*8 rafters (depending on snow load calculations) filled with Rockwool batts
    - Exterior layer of sheathing
    - Tar paper and shingles

    Floor assembly:
    - I will build on the existing sloped concrete slab. There is no reason that the floor of a tool shed needs to be flat.
    - I would probably pour a small 6" foundational wall to keep all the wood off the ground and to achieve a level base for the structure.

  12. Keith Gustafson | | #28

    To me the key is keeping a concrete floor isolated from the outside air. I would probably pour a floor over foam and surrounded by foam, as the cost would be minimal and the upside is a ton or so of material that does not want to change temperature rather than a ton of material that slowly drops in temperature over the course of the winter.

    And moisture, you can seal out the moisture from rising

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