Uninsulated toolshed: Air tight or ventilated?
canada_deck
| Posted in General Questions on
Hi All,
Last year I built a very efficient toolshed that was insulated, heated and air tight. I dry it with desiccant as needed.
This year, I’m building an uninsulated shed for things that don’t need to be kept warm. I live on the coast near the ocean in a climate similar to Seattle with very moist winters. Should I make it air tight and/or vapor tight and use desiccant (a bit of a pain to be honest) or should I just install a few vents (like a ventilated crawl space)? My main concern is if it will get too musty/moldy inside during the very long periods of cool high humidity weather if I have the vents.
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Because this new shed will not be heated or insulated its interior temperature will be very similar to the outdoor temperature. Air sealing will not help reduce moisture as the interior temperature will be in equilibrium with the outdoor temperature and humidity. Air movment (ie ventialtion) requires a difference in temperature for air flow. Convective ventilation such as a gable end vent or ridge vent won’t hurt but you will be drawing in the exterior air at the same humidity and temperature it was outside and bringing it in. If you air sealed the hell out of the shed humid air would still work its way in, though maybe slower.
I would focus on allowing the building the ability to stay dry, shed water, and be rodent proof. Think generous overhangs, vegetation away from the walls, wood a good distance from the ground, and rodent proof. Direct sunlight if possible.
Sealed, unconditioned buildings tend to have a problem that moist air can get trapped inside and when the temperature drops you get condensation. The condensation accumulates over time to the point that it's always humid inside and you get rot and mold. Shipping containers are highly susceptible to this, Google "shipping container condensation" for more details.
Ventilating the building gives humidity more of a chance to escape. If you can site the building where it will get solar gain the heat will provide vapor drive that will dry out the building and its contents.
One way to think of it is that with lots of air exchange, the indoor and outdoor conditions will closely match (for better or worse).
If the shed is sealed, your interior conditions will lag exterior conditions. Ultimately those conditions depend on:
‣The absolute moisture content inside-- based on introduced moisture (wet tools), removed moisture (desiccant), and diffusion in/out.
‣The temperature, which will fluctuate more slowly than with lots of ventilation.
I wouldn't venture a guess which actually yields the best result in your climate. The first option may be more of a 'known' though. If that 'known' is known to be undesirable, you could try to build it tight, monitor conditions, then add ventilation if interior conditions are worse than exterior conditions.
A couple other variables that will affect this:
‣Ground coupling: will lower temps in summer and raise temps in winter.
‣Solar gain: will raise temps always, which reduces RH*
‣Shading: the opposite of solar gain...
‣Wet objects that can increase diffusion into the structure, such as reservoir cladding, nearby plants, or a slab without a good vapor barrier.
And it bears repeating that the moisture content inside will depend largely on introduced moisture. If you are introducing moisture such that there is more vapor inside than out, it needs to be removed and ventilation will do that. If you are not introducing any moisture, then it's theoretically possible you can maintain favorable conditions inside without air exchange.
*It's worth noting that the sun can create significant vapor pressure (solar vapor drive) and drive moisture into the building, which will then rely on diffusion to drive out... the catch is that there's no sun on the inside to drive it back out (I suppose with windows there could be some). In other words, reservoir cladding or similar such situations could do you in.
canada_deck,
I keep all my tools in an un-insulated shed in the same PNW climate. The first winter I closed off the vents and encountered all the problems the other posters have brought up. Since then, with the vents open, I've had no problems. Sandpaper gets a bit soft, some steel develops a bit of rust over the years. That's about it. The walls and roof are fine.
I live in this climate and have two shipping containers. Not once have I seen condensation on the interior. I also have an uninsulated wooden shed for storage. Same thing, never once seen condensation or had issue with mold. I do live in an area with a lot of breeze and southern exposure, I think that probably helps.
joenorm,
We have one outside our fire hall used to store supplies for emergencies. Until we conditioned it there was a fair amount of condensation. Venting might have reduced that, but effectively venting a container is difficult.
Interesting. Mine just has the factory vents, nothing more.
It may be as simple as it is in an open area and gets a lot of night time radiant cooling.
God help us if there is an emergency. There is nothing very useful in the container anyway.
Thanks all. Very helpful. I will make it ventilated. It's going to be a simple lean-to roof so I will probably just use vented soffits at the low and high side.
Moisture from the ground is something I hadn't given much thought to. This shed will store stuff like bikes, kayaks, and lawnmowers so to keep it simple I was thinking of using a row or two of cinder blocks as a foundation for the walls and gravel as the floor. I figured if I put stuff away wet, it would just drain into the ground. Now I'm wondering if I want to put a vapor barrier underneath the gravel to prevent moisture from continuously coming up?
I had a chance to talk to a local today who has built a number of structures in this area, including ventilated unheated sheds. As a reminder, I'm near the coast in Canada (similar to Seattle but colder.)
This fellow has made an interesting observation over the years:
- During the winter, snow will build up on the roof of an uninsulated shed.
- That causes the roof sheathing to become freezing (32F or 0C).
- It's not uncommon for the air temperature to rise to slightly above 0 in the days after a snowfall and to be very damp. This can happen in a very well ventilated shed (e.g. no walls) and can be especially bad in a more tightly buttoned up shed (with a large floor area with no vapor barrier that is providing both heat and moisture.)
- When this warmer very damp air hits the underside of the roof, it condenses and the sheathing develops mold and then rots.
His solution has been to install rigid foam insulation on the underside of the roof. I don't like that plan because then the plywood is sandwiched between two vapor barriers but he says it works well.
canada_deck,
The roofs of unheated sheds on the PNW function very similarly to the ventilated attics that most houses there have. The sheathing does see swings in temperature and dampness due to snow and night sky radiant cooling. Rigid foam, carefully sealed might help keep the sheathing warmer, but at the risk of limiting drying.
I would ask whether there really is a problem worth addressing? The sheathing on these roofs (both house attics and sheds) can see some cosmetic damage, but in the absence of other problems does fine.
I you want to dig into this deeper, this paper by RDH addresses these issues:
https://www.rdh.com/wp-content/uploads/2017/08/RCI-2015-Problems-with-and-Solutions-or-Ventilated-Attics-GFINCH.pdf
Thanks for this! That is an extremely interesting article that confirms the experience here. I have to admit I'm a little worried now about all the extra insulation I had blown into the attic of my house. The article points out that increased ceiling insulation may be reducing the heat available to dry the underside of sheathing during the winter and making mold problems worse.
I'm trying to think about how to use this information. I don't like sandwiching the plywood between two vapor barriers so I have two other options:
1) Build a roof assembly that places the insulation between the roofing material and the plywood. An example might be some type of a metal roof that I could install on top of a layer of foam (which would be on top of strapping or plywood)?
2) Build a roof assembly uses a permeable roofing material (e.g. cedar shakes) and then install rigid foam underneath the plywood.
canada_deck,
The nice thing about vented attics is you can easily see if the sheathing is developing problems. My bet is that as long as your has adequate venting, and air-sealing , it won't develop any.
If you want to add roof top insulation - and again I don't think it is necessary - the easiest way is to add 1/2" or 3/4" foam on top of the sheathing, and attach metal panels right through that into the plywood.
to keep the sheathing warmer, the foam should really be above the sheathing.
Agreed. Now I will have to think about how to make that happen. Seems like shingles are out of the question.