Woodshop super-ventilation: Any reason to air seal / insulate?
I’m building a 900 sqft woodshop outside of Portland, OR (Climate Zone 4c).
Based on Bill Pentz’s research, the best way to avoid fine dust-related health problems is to ventilate all sanding/cutting tools to the outside with ~ 1000 CFM of air, which is enough flow to fully change the shop air every 7 minutes.
Recirculating air filters don’t cut it for fine dust — it has to vent outside.
Taking this airflow as a given, is there any reason to:
1) Worry about air-sealing the shop wall envelope? (There will be an open garage door or intake fan providing makeup air.)
2) Insulate the shop at all?
It seems to me that the constant airflow would effectively keep the internal shop conditions the same as the outside (sans rain).
As for winter comfort, it seems like my best option is radiant heat dishes, since heating up 1000 CFM of makeup air would be a huge waste.
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The insulation and air sealing would be primarily for when nobody is actively sanding and cutting--either when they are doing other tasks, or when nobody is in the shop. You might be thinking that you won't bother heating when nobody is there, so there's not need for insulation then, but if you have been running radiant heat all day, the thermal mass in the shop will have been heated by it even if the air wasn't much, and when you shut off the air flow and the heaters, the air temperature will actually go up a bit when some of the heat in the lumber and the equipment goes into the air. So you'll retain some of that heat, and prevent the overnight temperature drop from happening inside as much as it does outside.
I'm not sure whether this is a daily use professional or semi-pro shop, or a weekend hobby shop. But in either case, it's useful to avoid temperature swings when nobody is there because temperature swings mean swings in relative humidity. Swings in humidity aren't good for wood stability, and upward swings in humidity can rust tools.
A large HRV on your exhaust air would pre-heat the makeup air nicely, but would be prone to clogging. It would need the large-particle dust removed before the heat exchanger, but it would probably still slowly get coated with fine dust and lose heat exchange efficiency and air flow. So you'd want one that could be vacuumed out and/or washed out easily.
Air sealing is pretty easy. I'd do, just so if you ever decide on another use for the space, you won't have to retrofit.
Kevin,
We have a large shop in a similar climate (Vancouver Island) and decided to separate out the functions. The main woodworking area is covered but unheated. The only problem we have there is having to seal the surfaces of table saws and joiners against surface rust. A quarter of the shop is conditioned, providing storage for kiln-dried wood, assembling and finishing. The activities that that generate fine particulate are done in the covered area with dust masks. A third small area is reserved for the storage of finishes we don't want to freeze. It is well ventilated but maintained above freezing.
Charlie: It'd be a semi-pro shop used daily by one person. I have no idea if there are 1000 CFM HRVs and how much they cost --- it seems like most homes need on the order of ~200 CFM for ventilation.
Malcolm: Splitting into conditioned/unconditioned spaces is an interesting idea. Either a conditioned "building" within a larger unconditioned space, or a small conditioned building with a massive enclosed porch.
As for rust, is that from condensation due to large air temperature swings in the unconditioned space?
Kevin, It's pretty minor and no different than storing your tools in an unheated shed. Most tools are completely unaffected, and it must be common enough that Delta recommends a surface treatment for all their table saws.
My ideal shop in this climate would be a gable-roofed structure with a large breezeway for working flanked by two conditioned areas. i might think of enclosing the breezeway with a chain link fence or overhead doors so it could be secured when not in use.
1000+ CFM HRVs do exist, for commercial applications. For example,
http://www.lifebreath.com/products/commercial-hrv/1200-fd-and-1200-dd
or, a more efficient one with an ECM motor, slightly larger
http://www.lifebreath.com/products/commercial-hrv/1500i-ecm
I don't know how much they cost.
Just for background, you can find commercial HRV systems up to at least 4000 CFM, but really big commercial systems are usually ERV, and go up to at least 300,000 CFM! But they use desiccant wheels and might recirculate some of the dust.
Malcolm: Got it, thanks for the info. I assume rust comes from condensation (tool cooler than moist air) so if it shows up I'll need to either reduce the temp swing (via insulation) or reduce the air moisture (via dehumidification). Both approaches would require some degree of airtightness, no?
Then again, it depends on the values of the relevant variables and if you're in the same climate zone and having no problems with tool rust in unconditioned sheds then that's a compelling argument = )
The worst case for condensation and rust on tools in unconditioned spaces is a cool night followed by a warm humid day. The thermal mass of a good thick steel tool stays cold while the humid air flows in during the day, and deposits condensation on them. From Malcolm's report of few problems in your region, it may be that those scenarios are not as common as they are in some regions. But in any case, insulation will help avoid the tool getting as cold at night; while air sealing will help avoid having as much humid air coming in during the day.
I believe that unprotected steel will rust at high humidity even without condensation, but I don't know the threshold where that starts, and you probably don't have much completely unprotected steel.
Smart controls on ventilation can help, by bring in outside air when the absolute outdoor humidity is low and avoiding it when the outdoor absolute humidity is high. There are a few commercial systems that offer that capability.
Charlie, I don't really understand what mechanisms are at work. For twenty years I have kept my tools in an unheated shed without any any visible corrosion (the table saw and bench tools are at the larger shop I share). The cladding on the north side of my shed periodically shows signs of mold, as do the posts on my porch.
Perhaps you are right. It is the sudden swings in temp and humidity that cause the rust - which are different conditions than those creating favourable climate for mold growth.