Insulation for Off-Grid Cabin
AndyPeifer | Posted in Energy Efficiency and Durability on
I have questions regarding the insulation details on a wall assembly I am designing for our new, off-grid cabin build in New York, Zone 6a. I am up to speed on the 15+5 (code) vs the 51% exterior insulation (best practice) conversation.
I am planning on the following assembly, meeting 15+5 code, from inside-out:
5/8″ wood paneling
Rockwool or fiberglass in framing cavity
1 1/2″ EPS board (R-5.75) with taped seams
ferring strips for rain screen
My questions are:
- Is being thin on exterior insulation a wise assembly with the knowledge that the 4-season structure is only occasionally heated (woodstove) with no A/C and no mechanical ventilation?
- What happens when we blast it with warmth for 4 days when whole cabin is 15 degrees? Will Membrain and cavity insulation prevent moisture accumulation on interior of sheathing? Does using Membrain mitigate the thin exterior insulation concern because the wall can dry to the inside?
- Is the fact that this is an occasional use cabin present greater challenges or minimize trapped moisture concerns?
I am having a hard time understanding my dew point and moisture risks since there isn’t much written on these types of buildings.
Thanks in advance for your time. It is much appreciated.
Thanks in advance for your time. It is much appreciated.
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Please give us some more details on your planned use. What have you planned for heat and hot water. Will the heat be on year round or just when you are there?
What size cabin and foundation ideas?
Heat will only be on when we are there.
Cabin size is 1 1/2 story, 1200 sq ft.
Foundation will most likely be a CMU wall on footers with small crawl space. I started planning on piers, but the benefits of a conventional foundation have convinced me to spend a little more money.
The heat source will be wood stove only.
Water system will be rainwater catchment, to cistern with submersible pump, through particulate and UV filters, to pressure tank. I would prefer a well, but am concerned about the pump demands on a solar system in a region that is unfriendly to say the least!
Hot water will be on-demand propane unit.
We live in Philadelphia and this is a cabin that we will, for the foreseeable future, use for weekend and week-long getaways year-round. However, new building code in NYS has us doing it right in terms of energy efficiency and perhaps down the road it would be a more semi-permanent residence.
Occasional use structures provide an interesting variation and it is tough to make specific recommendations. My gut feeling is that you're at lower risk with occasional use than with constant use. Advance warning: the post below may get long.
It all comes down to moisture content, time and temperature. If wood products are held at a high enough moisture content for long enough at the right temperature, they get moldy and eventually rot. For most purposes, those conditions are >30%WMC for more than 72 hours at a time, at temperatures above 70F. Longer times at lower temperatures will also allow growth. Below about 50F there is little or no risk of mold/fungus growth.
Wood Moisture Content (WMC) is related primarily to the relative humidity of the air around it unless there is bulk water leakage. But if you have bulk water leakage, you're screwed regardless. Rough numbers here: WMC>30% equates to RH>90%, or near-condensing conditions. The thing about unconditioned structures is that they generally stabilize at the local average humidity. In NYS, this can be above 90% at any time of the year which is why those seasonal cabins get musty/moldy during long periods of disuse. The solution to this problem is to provide some conditioning to the indoor air to always keep the RH below 70%-80%. That's just enough to keep the mold at bay.
Your question about condensation and wall construction is complicated by the fact of the occasional use dramatically changing conditions. Most of our discussions (and most real-world experience and data) are based on near steady-state conditions. As an example, if you arrive at the cabin when it is 15 degrees outside, it's probably pretty darn cold inside too. There's a good chance that it will also be damp enough that the wood is near its saturation point of 28%-30% WMC. But the wood is cold, so no harm done. When you heat the place, the wood warms up. Of course the interior panelling warms up more than the studs, and the sheathing doesn't warm up at all. But when you warm the air, the RH goes down and that starts drying out the wood. So that's good. The warmer the wood gets, the dryer it gets. Still good. All else being equal, you won't get any more vapor drive during the heated times than the unheated times, because the total moisture content of the air stays constant.
Any increased risk would come with the introduction of more moisture to the air when the house is occupied. Cooking, cleaning, bathing, breathing all add moisture to the air. Some of that moisture load will penetrate the walls. The Membrain will act to limit that. An HRV would help wash the moisture out of the indoor air and replace it with dryer outdoor air. (Even at 100% RH, 15F air holds almost no moisture).
But I'm not so sure about the idea of the walls drying to the interior, except when the house is occupied and it's warmer outside than in. During unoccupied times, the interior of the house normalizes to the average outdoor temperature and humidity, with a substantial time lag due to the insulated and relatively airtight walls. You would probably see the house reflecting something like a 14 day trailing average. Unfortunately again, in NYS, the 14 day average might be near that 90% mark on occasion and once again, the house gets musty.
All of that was just a long-winded way to say "I don't know." After thinking about it, I would say that your condensation risk is lower because there is less total moisture being introduced to the interior air because of the occasional use. The risk would be lower if you use an HRV when present in winter. It would be interesting to put a couple of Temp/RH dataloggers in the place to see what the typical unoccupied and occupied conditions are. Post the data here in a year or so...
Peter, thanks for your feedback. I learned a lot from your post, especially the consideration of moisture added through use (i.e. hot showers). Your assessment that our risk may be less because of minimal interior moisture added is helpful.
Being off-grid, a HRV would be tough to pull off, although I have been brainstorming air movement/exchange measures to minimize that musty cabin experience.
I didn't hear you express concern about condensation at the sheathing , so that is reassuring in terms of sandwiched mold growth....didn't consider the fact that when that would occur it also would be low enough temp that mold growth wouldn't occur.
I have an off grid cabin that we completed last year in zone 7a/7b, I can give you my perspectives:
Insulate for comfort, not energy efficiency. Adding another log on the fire is easy, cheap, and harmless. The ROI of super insulating a building that is only heated intermittently with free firewood is not good. I did 2x4 framing with roxul insulation, works good for me. Some of the thinking that goes into assemblies with exterior insulation is the continually heated building will keep the interior face of the sheathing warm and below the dew point (condensation point), this will not be the case with a weekend cabin. I think you can throw out much of the modern building science for this type of project because the implied assumptions are wrong.
Moisture loading will have major spikes from heating piles of water for cooking, washing, coffee/tea etc. and without mechanical ventilation (especially a kitchen exhaust fan) it can get muggy. I insist on having the window(s) cracked at all times, even -40c, for ventilation and CO mitigation. This leads to arguments with the wife, and more wood on the fire, but what can you do.
The spikes in moisture loading is offset by the cabin being unoccupied for long stretches and being able to dry out. Also, the wood fire is good at drying things out. Build your assemblies with drying potential, inwards and outwards. This means 1 layer of VB somewhere in the wall assembly.
I used a 6mil poly sheet behind the drywall as our air/vapour barrier and was diligent about sealing it well with caulking and tape. This is great for keeping the moisture out of the walls but makes for an airtight assembly which is challenging with no mechanical ventilation.
I would install a cold air intake for the wood stove., this also helps with CO concerns.
Battery operated smoke and CO detectors are cheap and quite reliable if ensure you have spare batteries around.
ECOfan is an awesome way to move heated air around the cabin without electricity.
We were going to install a solar array and batteries for electricity, but have found that we can light our cabin just fine with LED light strips powered on AA batts. We probably spend $50 a year on batts for this purpose. We have a couple of powerpacks for charging phones and a music speaker, and we are now questioning the solar thing altogether. Eventually we will have a fridge and stove on propane.
Good luck, would love to see pics as the project progresses.
Thanks plumb_bob. Sounds like you have a good thing going up there in zone 7.
I would love to scale back the insulation, but with the new 2020 residential code in NYS any heated structure needs to meet energy code. In the end, perhaps it will work to our benefit if it ever becomes a place we spend significantly more time.
The scope of our solar system is TBD. I am prone to take your approach and live into it a little bit and gauge our needs.
One of the important concepts in the Zero Energy Ready Homes (ZERH) program from DOE is the "ready" part that establishes a standard set of solar infrastructure requirements to be met. These include such things as conduit, space for meters and inverters, stuff like that. This infrastructure is inexpensive to install during a build but expensive and finicky to detail after construction is complete. You should consider going this way so that adding solar later is cheaper and easier.
Point taken and I think this will be wise to compensate for.
Getting the air sealing right is far more important than the amount of insulation according to Dr. Joe Lstiburek. In that case, I wonder about your using wood paneling on the interior. If you do have high levels of humidity in the cabin on occasion it might leak into the walls and condense on the back side of the exterior sheathing so maybe a solid layer of paperless drywall under the paneling would be a good idea?.
Also, you might consider using a desiccant dehumidifier while you're there to reduce the indoor humidity.
I would not use poly on the inside if you are using rigid foam on the outside since you'd in effect be creating a wall that will not dry to either side..
As for the foundation, I would look up every article written by Dr, Joe on crawlspaces. What do you plan on doing with the roof?
I'm not an expert but I'll pitch in my two cents:
- Keep it simple. You don't have AC. You want to keep moisture from inside the cabin from getting into the wall. You can't beat polyethylene for that task. The smart membranes are a compromise IMHO.
- Make the assembly more breathable to the outside. Replace the EPS with Rockwool Comfortboard.
- Be thoughtful about ventilation. Exhaust fans use very little power and having an exhaust fan located near your point sources (kitchen area, bathroom) will be helpful. Will you have a shower in this cabin?
- With a simple design and good air-sealing, you will need to think about providing fresh air for a wood stove if you have one.
- You will have to be thoughtful about the cabin getting musty when not in use. This seems like the trickiest problem. If you have a solar system, this gets a lot easier as you may be able to hook up a small dehumidifier. Maybe something like this: https://www.eva-dry.com/product/edv-2500-mid-size-dehumidifier/
- If you aren't thinking of solar yet, consider wiring up a few things (lights, exhaust fan, etc.) on 12V wiring and you can easily bring a sealed 12v battery with you which can get you through a weekend. You can always take it offline and jumper it to your car when you need to recharge it. You could do 120v and add an inverter but that might not be worth the trouble.
If you have some (consistent) sun during winter time, you could use an "air collector" with a small PV-powered fan to blow heated fresh "dry" air into the cabin to replace stale moist air.
very DIY and doable
If you decide for that from the beginning, that collector could maybe function also as cover over the entrance etc. - also as pre-heater for the air even when the cabin is occupied.
Thanks everyone, this has been really helpful.
Modifying this assembly from some of your feedback:
Rockwool Comfortboard (over housewrap) for a more vapor permeable exterior (rather than rigid foam) and then use Membrain on the inside as a vapor permeable air barrier. Seems like my priority needs to be preventing air leakage and this assembly might be more forgiving when vapor does finds its way in with drying happening primarily to the outside.
Rockwool would dry better to the outside than rigid foam, correct? And added bonus is that it is a recycled product.
The modifications you suggest will make the wall more durable, both in semi-occupied and fully occupied modes.
I have a somewhat similar cabin in process.... I'm monitoring interior and exterior temps, humidity, etc...but hard to determine at what point I get concerned about anything?
A bit of an eye chart attached....but hourly data of interior temp, RH and dewpoint, as well as exterior dewpoint, and some have 120hr averages overlayed (5 day)
You can see daily small rises in RH as temps get above freezing and snow melt occurs outdoors, too..... and now the interior RH is getting closer to stabilizing around 60%...