Reasonable Energy Efficiency Benchmarks for a Workshop
We’re homeowners and newbies to green building techniques (although this site has been a very helpful jumping off point). We feel a little better equipped and supported when it comes to making improvements to our house, but we’re having trouble with the give and take when it comes to building an outbuilding.
We’re working on plans for a roughly 30’x30′ slab on grade two story outbuilding for woodworking, metal working and stained glass. The shop will need to remain the in 60-80 degree range (to keep tools from rusting) and we’re in climate zone 5a (Michigan). What I’m wondering is- how much is too much?
For the separate house upgrades we’re following our energy auditor’s suggestion of adding R60 to the attic, additional R20 to above grade walls, R15 to the foundation walls, triple pane windows, heat pump installation- all to get us to ACH50 1-1.5 and net zero in the future. For the shop this seems like overkill. What sorts of R values/insulation specs would you suggest for a building that needs to conditioned at 60-80 degrees? What’s a reasonable ACH50? The pole barn construction gives us the opportunity to insulate 6″ deep wall cavities. The shop doesn’t need to be a the same level of comfort as the house and holding back from pricey windows would free up more funds for (increasingly expensive) wood for projects.
Thanks in advance for any suggestions!
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Replies
Pole barns are notoriously difficult to air seal well. Spray foam is often the only way to really do it, and even then it's still tricky to do well.
You will want some insulation under the slab, but you didn't mention that. Don't leave that out.
You're planning to heat your shop to the same level as a house, so you'll have similar energy consumption to a house of similar square footage and exterior wall area. That basically means you'll end up wanting similar insulation levels to what you'd have in a regular house. I would not put money into triple pane windows here though, I would just use decent double pane windows and put the savings towards... Wood for projects! :-)
Remember that you need to keep humidity under control to minimize rusting too -- it's not all about temperature. I would also consider a radiant slab, which has some ability to buffer rapid temperature changes that can cause condensation, and the slab's thermal mass helps to do this even if you happen to loose power.
Bill
One thing to consider is that if you are aiming to make this building as air-tight as possible (and I will second Bill's comment about trying to air seal a pole barn- it's difficult) is bringing in fresh air. We ended up in this discussion this past year as a client was worried about clogging filters on an HRV and a mini split wall mounted unit in a woodshop. It was a fair point and we didn't have any great answers other than to make the filters as accessible as possible and to install a simple pre-filter that will capture the largest of the particles- Ideally one that didn't add too much static pressure.
The triple pane windows seem a bit much for a shop. Casements would be good in lieu of double hungs or sliders. To take a shop as far as a house seems like a bit much, but from an energy standpoint I can't argue with what Bill said. If the shop was only used at certain points of the day or certain days of the week one could make an argument for lesser assemblies as this would be less overall energy (assuming the ramp up to heat it from the 'away' setting didn't outweigh the constant temp). Certainly going all-electric is good and if it is offset with renewables one could argue that the money could be better spent on the PV instead of the upgraded walls. It will probably be a calculus based on local rebates and installed PV costs (not to mention insolation at this site)
In my experience in climate zone 6, rusting tools is not a issue in an insulated, unconditioned shop. I have had a few times in uninsulated garage that a rapid warming, from say 30 deg F to 60F in a day, will cause condensation on everything metal. I believe a insulated shop buffers the outdoor temperature and humidity swings enough to prevent condensation. I don't think it's worth it to condition a space full time to prevent rust when some wax or oil on the table saw, welding table, etc, will do the job good enough. Is it not as easy in more southern climates?
This building is as big as a house and is being heated like a house. So you've got pretty much the same energy calculus as a house. I would recommend standard stick-built construction rather than a pole barn, with 2021 IECC minimum-code insulation and air sealing at least. Pole barns are not only tricky to seal and insulate properly, the cost ends up being similar to standard construction once you do everything right. There are several threads on here about insulating a pole barn. Read them and make your own decision.
Note that for a shop environment you are going to either need aggressive air cleaners or substantial ventilation air, or both. Ventilation air can get expensive to heat and cool, and as Josh mentioned above, there is legitimate concern about clogging the core of an HRV/ERV.
If this is going to be a wood shop with a dust collector, and the dust collector will be vented outdoors as it really should be (the reciculating/filtering kind really aren't good enough), then the makeup air needed for the shop is going to swamp anything an HRV could do.
Bill
To my mind the real questions is what are your goals with this building?
Is net zero your top priority?
An R60 attic is great but the payback over R38 could be very long.
A Pole building is great way to make dry square feet to keep your tractor out of the rain but it costs so much money to make then into energy efficient heated building all the cost saving from the pole construction will have disappeared. If you want a tight well insulated space built it like a house and not a barn.
Understand the quality of windows you choose will determine your indoor humidity level and comfort level. Cheaper windows will have cooler surfaces that will condense more water from your air giving you lower humidity and less comfort in the winter.
Walta
Thanks for all your input so far and questions. I'll try to recap, add more context, and respond to questions.
I think what we're looking for is something like "Pretty Good Shop"—a compromise between performance, pragmatism, and economics. Our hope is to have both of us use the space to do woodworking, metalworking, and stained glass, among other things. All of those demand space, so it's large even though we know that reducing area would be one of the standard simplifications.
We're planning on adding a large (as big as the utility will allow), ground-mount PV system.
A number of folks have expressed concerns about sealing a pole barn. We have discussed the project with high performance builders ($$$) and were trying to research the other end of the spectrum (pole barn). If we talk to the high performance folks then we're trying to figure out how much to lower their standards ("it's just a shop") and if we talk to the barn builder then we're trying to figure out how to raise their processes ("why would you spend that much insulating a barn?").
Our assumption was that we should insulate the slab, as suggested. I'm trying to figure out local sources for reclaimed insulation.
We were hoping that a larger temperature range for conditioned space (60-80º? 55-85º?) would somehow transform the insulation requirements but nobody seemed to think so. The suggestion to insulate but not condition is interesting (and we hadn't considered it, perhaps because we're used to an uninsulated, unconditioned garage right now).
Others have expressed concerns about dust filtration, especially when combined with an ERV. We've been researching this also and assume we'll need multiple approaches (pre-filters, separate air filtration, larger scale chip "dust" collection). I don't think that external venting for the collector is necessary, but I may be mistaken.
Thanks all for your help with this!
>"We were hoping that a larger temperature range for conditioned space (60-80º? 55-85º?) would somehow transform the insulation requirements but nobody seemed to think so. "
The allowable range just makes it easier to maintain the "setpoint". Energy loss is all about temperature differentials, so you loose more heat when it's 0*F outside and 85*F inside than you do when it's only 60*F inside. The range doesn't matter, only the instantaneous values. The only thing a big range will do is change how often your heating (or cooling) system cycles. If you want to limit energy losses, try for a very narrow allowable range that is as low as you can go (assuming heating here, but cooling season is just opposite of this). This means if you have a 50*F absolute minimum, and your system can reliably maintain a 2*F range, then you'd maintain maybe 51-53*F to give yourself a 1*F buffer. If your system isn't that reliable, then you'd have to move the setpoint higher, so that you can tolerate more variation without dropping below your minimum. The lower the setpoint on the inside during heating season, and the less you deviate from that in the "hotter" direction, the lower and more controlled your energy losses will be.
There are health risks with the indoor filter-type dust collectors since very fine particles go right through the relatively open cloth-type filters they typically use. If you vent your dust collection system outdoors, you don't have any problems. I would recommend a cyclonic type dust collector vented to the outdoors. There are a lot of articles about the health issues from fine dust from dust collectors, and even some woodworking articles about that fine dust messing up high-gloss finishes on workpieces.
Bill
A higher quality cyclonic dust collector with HEPA is worth it, but I agree that for a home shop you don't need to go outside with it, and doing so would complicate matters.
One could question if HEPA is enough I suppose (and they need periodic maintenance) but they seem pretty reliable and efficient. I have an Oneida that works for me, though admittedly I've never measured for airborne particles during use. It depends on a lot of other factors too, like how good your sanding collection system is, which both creates some of the finer dusts and often doesn't use the dust collector anyways (at least not the sander itself, but a sanding table hooked to the collector is nice.
I wager that with a good cyclonic HEPA collector, the bigger contamination issue is from tool to hose losses and not through the filter (i.e. some tools are hard to collect all the dust from).
Happy to be proved wrong (actually I wouldn't because that would mean I would have to fret over my current collection set-up).
Bill,
I read his post about temperature range as being summer/winter range, not instantaneous. If they maintain 50F in winter and 80F in summer, of course that will reduce energy costs from maintaining a constant 75 year-round. And, with reduced energy costs, extra insulation makes less economic sense.
I'm going to be building a new shop soon, so I'm going through the same exercise. I still think a "pretty good shop" would have close to code-required insulation levels if it's going to be conditioned, even with a larger seasonal temperature range. It's probably overkill to go to code+. Some time spent with energy modeling software could help dial in the sweet spot for cost/performance.
I don't worry too much about fine dust particles in my wood shop, but that's just me. I generally don't do fine lacquer finishes, or if I do, I clean the place really well and dedicate my time to finishing until its done. I'm planning on a recirculating dust collector, but allowing for exterior venting if it seems to need it. I expect to skip the ERV because I think it's just going to take too much maintenance. I've found that a box fan with a MERV-13 filter does a great job of general air cleaning and I'm going to build some nicer ones for the next shop.
The metal shop is different, and possibly the stained glass shop if still using lead caming (is there an alternative?). In those spaces you can be dealing with metal fumes, solvents and lots of other immediately hazardous substances. You need exhaust hoods and makeup air, and the cost of conditioning your makeup air is going to drive your energy costs, at least during the times that the ventilation is active. I'd still insulate at near-Code levels, though because there are lots of hours when you're not going to be aggressively ventilating.
That would make sense (regarding the temperature range). The closer the inside temperature is to the outside temperature, the lower the total energy losses will be. If the wintertime heating level is only 50F, then lower insulation levels could be tolerated with similar total energy losses to a home that was heated to a higher temperature but had more insulation. It would end up being a cost/benefit calculation at that point. I would try to get to code anyway, since it's not usually a huge step up in cost on a new build, and it essentially saves you money forever after you've completed the build.
Bill