Condensation on sill log
Hi everyone,
I am trying to figure out the root cause(s) and the possible fix on the bottom log of my house.
A bit of context first:
House is located in Southern Ontario near the Georgian Bay. Being surrounded by the Great Lakes, Spring is humid, Summer is very humid, Fall is humid, Winter is humid. Only in the dead of winter on the coldest nights I will get some dry-ish air outside (RH of 30~40%). As an example the other night it was -10C but the RH was still 80% outside (cold air might be “drier” in term of water contents but in the end it is the proximity to saturation that matters, and outside air here is almost always close to saturation year round). As a result, the air in my house has a high RH too (I don’t have central heating, just a wood stove and a direct-vent propane wall furnace as a back-up). I set the mechanical ventilation as low as possible (trough-the-wall HRVs), to keep the RH from ramping up. So in average I have about 65% RH in the house when it is 80~90% RH outside.
As I am off the grid, I try to avoid running a dehumidifier (summer is fine as I have excess power during the day, but late fall, winter, early spring it’s a no-no except during generator time).
That excess humidity is causing me trouble as my bottom logs, which are naturally cooler than the rest, experience some condensation (right at the subflloor / sill log junction).
Apart from that, house is performing all right, I use 3.5 cords of wood for the entire heating season (November to March) and manage to keep the house at 20+C.
From bottom up, I have 3/4″ ply, sealant, aluminum flashing, sill gasket, flat sill log.
What I tried so far:
I thought first it was some air infiltration cooling the underside of the sill log, so I caulked both on the outside and inside where the log meets the subfloor, but no change, condensation still occurs when it gets cold outside.
Now I am thinking the metal flashing is a major energy nose-bleed, cooling the bottom log excessively.
So I would like to pull the flashing out or cut it flush with the subfloor and add rigid insulation against the rim joist from the outside (the rim joist is recessed with respect to the outside face of the bottom log, so I can lay 1.25″ foam plus 5/8″ skirt board and the whole assy will still be a bit recessed — right now there is 2″ skirt board against the rim joist). I hope that will increase the temperature of the indoor face of the sill log enough to avoid condensation.
Does that sound like a good plan, Am I overlooking something?
Cheers.
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Replies
Etamine,
My guess:
1. Your house has severe indoor temperature stratification, with hot air near the ceiling and very cold air near the floor during the winter. This is typical in a log house, because log houses are leaky, and air leaks lead to the type of stratification I just described.
2. You are misunderstanding some basic facts about relative humidity, leading to a bad decision on ventilation and higher indoor relative humidity than optimal. It's unusual or impossible to have 65% RH indoors during the winter, so you may have a defective hygrometer. But if you know that your indoor RH is too high during the winter, you need to increase the run time on your ventilation system -- even if the outdoor RH is 80%. That outdoor air is very dry -- trust me -- and it will help lower your indoor RH.
The two steps you can take to improve the situation are the following:
1. Perform air sealing work on your house -- not just near the bottom log, but everywhere. This is challenging, because sealing air leaks in a log house is like ordering the ocean to recede.
2. Lower your indoor RH by operating your ventilation system more frequently during the winter.
Etamine,
To expand on Martin's second point: If the temperature outside was 0C, and the RH is 80%, when you introduce that air into the house, at 20C it will have a RH of 22%
Martin, Malcom, thanks.
Yeah I get it now, when you warm up a mass of air, it expands, which creates more room for the water vapor present inside it, making the mass of air less saturated indeed.
Time to crank up the ventilation.
Regarding Martin's 1st point: the inside of my lapped corners have been on my to-do list for air sealing (there is an expanding foam gasket in there but I do feel the air coming in when the wood stove is burning high).
Would a large ceiling fan help with regard to stratification?
Regarding the outside: the sill log sticks out by 2" over the subfloor, that leaves me with only 6" of solid wood between outside and inside at the bottom (my "logs" are 8" square). I think the 2" skirt board does not keep any cold air from washing the underside of that overhang. So if I stuff some backer rod between the bottom log and the skirtboard, and caulk it, is it good enough or should I replace the skirt board with some kind of insulated sheathing to keep that bottom log a bit warmer (with the same sealing strategy)?
Cheers
Ceiling fans most certainly do help with stratification. They are especially helpful in rooms with very high ceilings. The ceiling fan will “stir” the air and blow the warm air back down towards the floor. If you have significant stratification issues a ceiling fan may help make your room more comfortable by evening out the temperature of the air in the room.
Bill
Etamine,
Bill's suggestion -- to run a ceiling fan -- isn't a great solution if you are off-grid, because you don't have much electricity to spare during the short, cloudy days of winter. The best solution by far is to perform air sealing work.
Martin makes a good point. I didn’t intend to imply a ceiling fan was a way to avoid air sealing. The ceiling fan is a good solution to problems with stratification that remain *after* you’ve sealed the place up as much as possible. The ceiling fan won’t use as much power as a dehumidifier, but if you run it constantly the relately small electric consumption of the fan will add up.
Bill
Bill, Martin,
The ceiling fan is indeed a top-up after the air sealing work. I am pretty sure I will still have a bit of stratification issue after addressing air infiltration as my home is a 1 1/2 story with an open area above the living room and dining room (1st floor to peak is about 22 ft). The 2nd floor material, 2x6 T&G, is also the ceiling of the 1st floor, so even in the other rooms the air may leak upstairs. The larger fans (6 to 7 ft) can operate at a lower speed to deal with the rest of the stratification. The 7ft fan takes between 4 and 40W depending on the speed, say 20W on medium speed -- that is still less than my inverter dead load or my HRV's. Running it full time will cost me 480Wh per day, half of that if I run it part time (I do have the Wh budget for running it full time, the problem is more the cost of the fan, efficiency doesn't come cheap). The other option (more local) is to install the blower designed for my wood stove, that would move the air horizontally in the main room, but that blower fan is more power hungry than the ceiling fan.
Destratification is more efficient when done at the source (far less cfm needed). Ie, you want to blow around the hot air as it comes from the furnace or stove so that it better mixes with room air before it rises. Also, you don't want to increase airflow on people (this cools them off) and this can be hard to avoid with a ceiling fan (depending on location and height).
It is true that large, slow moving fans are much more efficient (cfm/watt) than small blowers.