Can anyone tell me what an optimum indoor air humidity level is for winter-time. I’m in Northern Vermont, and heat with wood, I know those are two strikes against me, but I’m trying to make the situation better and don’t know what to aim for. My gut tells me that 50% should be about my maximum, or we’ll have trouble with molds, any insight would be helpfull.
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My gut tells me that 50% should be about my maximum, or we'll have trouble with molds, any insight would be helpfull.
Good guess. We also use a woodstove, and keep a pot of water simmering. Rarely do we get above 40%, often in the 20%'s. You are monitoring it? A lot will depend on your air changes, what type of house, leaks, air system if any.
PAHS Designer/Builder- Bury it!
Now that I re-read my orig. message, I realize how many details I left out. We have a small 850sq.ft cape, upstairs has low ceiling with shed dormer for one full side. House is constructed with 2x4 exterior walls, insulated with icynene, 1/2 of roof is 2x6's with icynene, dormer is 2x10 with rigid foam board and fiberglass comprising a R-value of just about 40. 2/3rds of the house has a crawl space under it, dirt floor but covered with poly, other third of basement is 5'6" with poured concrete floor. All crawl space walls have 2" foam board outside, and icynene sprayed on the inside to +/- 3' below grade. All new windows and doors, insulating glass, spray foamed around edges prior to trim, caulk, paint. Exterior siding is cedar shingles, 5" exposure, over typar, exteriors of windows is wood trim, caulked, painted. Roof is steel, cold roof, vented from facia to ridge. Additionally we have a propane fired forced-air back-up furnace, run's occasionally, but rarely more than 1 hour per day. I hope this is enough info.
We have a small digital hygrometer, tracking in the high 20's to low 30's. We too keep a pot on the stove, which evaporates about 1 gallon/day, and a small humidifier in our bedroom. Pretty much I'm trying to determine how much higher optimium RH would be so I can determine what else I may need to add for humidification.
Now that I re-read my orig. message, I realize how many details I left out.
You sure didn't this time. Including several that I'm not particularly familiar with. But the primary thing seems to be what volume of air you're humidifying, including air changes, which you're apparently not planning for. Can you experiment with a second pot on the stove and measure how you're doing with that extra gallon? Or, conversely, go a typical day without the pot and see how dry it gets. That'll tell you what 1 gallon/24 hrs accomplishes in your house. Then you can pick a rh % and plan a humidification system that puts out sufficient water to reach your goal.
Couple of years ago I replaced windows in a house with low e insulated units. Homeowner promptly decided she could then run a huge humidifier. Had water running down the walls. Never bothered to monitor so she had no idea where she started or where she ended up. Dew point will happen if you keep adding water.
A little experimentation and you'll do considerably better. PAHS Designer/Builder- Bury it!
Just went to a mold seminar yesterday. Mold will start on wood at 60% relativbe humidity.
the problem is that there can be "hidden areas" where the relative humidity spikes because of air inflitation into cold spots.
For example, I saw this condensation and starting mold growth on the rim joist behind some FG insulation in a crawl today. It has been pretty cold, and the crawl didn't have a full vapor barrier. I don't know what the RH in the crawl was, but this illustrates the concern:
View Image
I recommend 35-40% myself.
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Bob,
You stated "I recommend 35-40% myself".
Any particular reason or source for this range? Is this for winter or year 'round?
Tim
Tim,
I don't have a particular, specific source, but it is based on a few things: a combination "average" of recommendations I've heard over the years; my experience that the humidistat "charts" / calibrations tend to be pretty inaccurate; my experience that in cold weather when I see humidistats set at 50% (for those with a "humidity" scale) the houses almost always show signs of excessive condensation at the windows, and a theoretical concern that in older, leakier houses where there is a higher need to add humidity, it seems to me to be a higher chance of hidden condensation inside walls etc.
I should have added that I tell my clients that I recommend a highly scientific approach to setting their humidistat: "turn it down if there's condensation on the windows, turn it up if they are going through too much skin cream."
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Several donations have arrived! Thanks and God bless!
Its a good range, from my experience, and coincides with the reference I use most frequently (ASHRAE). Just curious if there was a source of recommendation that I was unaware about.
Scottie - I posted here recently that I vent my (electric) dryer indoors in the winter and got slammed. .... but I've been doing it for a month, and I have to say that the house feels MUCH better .... still get static electricity shocks all over the place so I know we're well under 50%RH ... I think it's a great way to keep the humidity up.
I'm in Boston area ... used to live in Northern VT though (Jericho) .. wish I had thought of the dryer thing back then .... Bill.
Scottie - I posted here recently that I vent my (electric) dryer indoors in the winter and got slammed.
Must have missed that thread. We too vent our limited use of a clothes dryer indoors during the heating season. Helps out our woodstove water pot. The only problem would be excess humidity, which we monitor.PAHS Designer/Builder- Bury it!
What would be great is if somebody invented a duct baffel for the dryer that was controlled by a humidistat controller. That way when the house was bellow a preset rh the baffel would divert the dryer inside. You get some heat too!
The humidistat that work with the humidifier on my furnace has a table on it plotting outside temperature agains the "correct" humidistat setting. This make me think that it depends on the outside temperature. For example, when it is zero outside, the humidistat recommend setting it to 10%.
By the way, the humidistat is located on the return-air duct by the furnace, so it reads an "average" humidity for the entire house. Obviously, humidity levels will be higher in closed bedrooms at night, or in a kitchen with a pot of pasta boiling.
Yes, the outside temp makes a big difference. Check this message.
http://forums.taunton.com/tp-breaktime/messages?msg=39169.14
The range of recommended indoor relative humidity levels for health and comfort, according to the American Society of Heating, Refrigeration and Airconditioning Engineers (ASHRAE), is 30 - 50 %. The range takes into account eye and nose/throat irritation, growth of various types of fungi and appearant comfort. In the winter, you would try to maintain the minimum and in the summer, the maximum.
Tim
I checked out a 1997 book titled "The Healthy Home Handbook" and for a 1500 sqft "tight" house it spec'd a 6.5gal/day humidifier. I think I could swim in mine if I put in that much moisture. I have all cherry wide plank floors, 1x6 beaded pine ceiling, and lots of glass. With 6mil vb on ceiling and walls I can't figure where this much moisture goes, we have a heat pump and no burning apliances.
I use a couple of small humidifiers when the gaps start showing up in the floor and ceiling. I cut back or stop when I see condensation on windows. I am hesitant to put humidifier in ductwork as I see it feeding any dust in the ductboard and flex. I always seem to get a slight bit of condensation on the aluminum threshold portion exposed under french and patio doors.
Edited 1/26/2004 2:24:48 PM ET by RASCONC
Ok, now I'm really stumped!
About a month ago I started a thread asking about the need for heat recovery air exchenge systems in tight homes and the concensus was that you had to have them because your cooking and breathing, etc would eventually make the house (even in a cold climate) feel like like a sauna in the winter. The newest issue of FHB even repeats this opinion in the energy-efficient house article.
Summer seemed to be a non-issue to me since I like having windows open.
But here we have several people stating that they essentially do everything they can to humidify their house except flood the place with a garden hose and they still can't get the levels above 40% or so.
Something doesn't add up. can anyone clear this up for me?
"...and the concensus was that you had to have them because your cooking and breathing, etc would eventually make the house (even in a cold climate) feel like like a sauna in the winter.."
I can't speak to the "consensus", but my experience is (specifically with the design and troubleshooting of HVAC systems in commercial and institutional buildings) that I have never heard of such conditions existing in a residence (with code ventilated appliances) in areas with extended heating seasons; highly occupied, heated therapy pools and natatoria excepted. If no bathroom or kitchen exhaust were present, products of combustion were not properly eliminated of other uncontrolled moisture sources were present, energy reclaimation or humidity control would be the least of the occupants concerns.
The use of an heat/energy recovery is recommended for positive ventilation control, for a superior environment and to save some energy on the air exchanged, there are practical limitations and drawbacks on their use in a residence. One is that few if any builders/contractors provide positve, continuos ventilation. Exhausts are sporadic. Homes are become more expansive for the occupancy (i.e. McMansions) which leads to adequate ventilation by leakage even in a "tight" home.
Tests were conducted years ago on the "tightness" of modern "high efficiency" homes and found that the leakage rates in air changes per hours or ACH ranged from 0.2 to 2.0 homes in North America. At the minimum of this range in a 1500 sf home with 8 ft ceilings, works out to be 40 cfm. Air at 70 degF and 30% rh has 4 times the moisture of saturated air at 0 degF. If I crunched the numbers right, this works out to be the equivalent of just over 11 oz/hour. Not a lot, but enough to keep your house from becoming a sauna.
Have you ever experienced "...the house (even in a cold climate) feel like like a sauna in the winter.."? Me neither.
Tim
Hey, thanks for the response!
Seriously, almost all the answers I got in the other thread had to do with relieving humidity levels. This puzzled me given my experience with perpetual dryness in our Minnesota winters. So what is the need for this extra "system" in a house?
Are they for the folks who never open their windows in the summer?
Are they for people who, for whatever reason, produce toxic fumes? (insert joke here)
Or is it another example of technological overkill.
There is no real need in a typical residence. If the occupants are particularly sensitive, allergic or have any very specific needs for air quality, that could be an exception. If one uses commercial cooking equipment with a commercial exhaust rate, that would be another exception.
I designed and installed the system in my home. It works great. I have no energy recovery. I do plan to add a "Skuttle" for positive pressurization, maybe.
In commercial, institutional, medical and high density residence types of buidings, codes dictate specific mechanical ventilation (both outside air and exhaust) rates. I regularly design systems that utilize an energy recovery unit that serves as both the source of fresh air (OA) and the toilet and/or general area exhaust fan. In situations where specific OA and exhaust rates are defined, relatively constant at very specifically defined periods, the HRV/ERV/ERU is ideal. None of these circumstances apply to a typical, modern residence.
I have a large home. In the winter, we regularly (daily) have wood fires, have all gas (lp) appliances, all using inside air for combustion-vented outside, and pretty much have a negatively pressurized house in the heavy heating season. Air leaks in at a rate that I have fairly closely predicted, based on the construction, number and quality of openings. With two people, 4 dogs, 3 cats, and 180 gallons of aquariums/fish @77 degF, I have to run the power humidifier (AprilAire) to keep the relative humnidity (RH) up to the minimum recommended 30% level.
Otherwise, our windows are open from April through September unless the temps get above 90 or below 50.
I would say that an HRV/ERV is overkill for a typical residence. I have designed specialty residence HVAC systems that included them, but they are the exception.
Edited 1/27/2004 4:24:45 PM ET by Tim
Thanks again!
I can't tell you how rare it is (or seems to me) to get a "naw, you don't really need that" type of response to any inquiry here, especially when it comes to things seen as cutting edge and/or health related. Yours seemed like a very common sense and informed opinion.
And I don't just say that because it matches my opinion - based on nothing but intuition.
I read that previous thread and there was some very good discussions, especially those posted by (the creator of Wendy's) Dave Thomas. But he lives in Alaska, has an (by his reports, which I do not dispute) extremly tight house and is a very knowledgeable fellow about these things. There was also a bunch of useless babble by people who only know "of" these devices, not actually anything "about" them.
If you can build a home that has a leakage on the order of DT's, you would need to seriously consider this (the addition of an ERV to your home) further. If you are to build a home that is conventional/high efficiency (like stick built, 2x6 with blown cells for insulation, etc..), after the first winter, you will most likely need to add moisture to the home not remove it.
Size and occupancy, as well as tightness is important. 8 people in 1500 sf is much different than 2 in 4500 sf, obviously. Habits must be taken into account as well. Do you cook three meals a day or one?
There is no "right" or "wrong" answer.
Edited 1/28/2004 10:19:23 AM ET by Tim
"Seriously, almost all the answers I got in the other thread had to do with relieving humidity levels. This puzzled me given my experience with perpetual dryness in our Minnesota winters. So what is the need for this extra "system" in a house?"
I think that two different types of condition are being talked about:
Case 1, too much humidity:
extremely tight homes will need mechanical air exchange devices to keep the inside air from becoming too high humidity (and to exhaust indoor polutants). This kind of home is going to have been built like a tupperware container with almost no air leak anywhere (these usually cost a real premium and are presented as "extremly energy efficient", or what have you).
Also, typical (leaky) homes may have too much humidity if the appliances or bathrooms are not properly vented (can be deadly CO concern), or if there are lots of people living in a small house, using lots of cooking, bathing and breathing that raises the humidity level (like during Thanksgiving when you've got ten guests and two stoves and an oven going all day long - the windows just run with water after a day like that in almost any house).
Case 2, too low humidity:
typical homes may need humidification if there is very little cooking, few residents (who breathe, cook, bathe) or high air leakage that cause the interior air humidity to drop below the human comfort level (somewhere below 20%)
When you asked your question before and got answers about how to reduce humidity they were thinking about the former. On this thread people are talking about the later. At least that's my take on the situation.
Note that $10 ~ $20 humidity gages are notoriously inaccurate. I bought five (two came with humidifiers, already had one and bought two more on sale) and ended throwing away two of them because they read SO far off from the other three (all located in the same spot, monitored several times over a few weeks to decide). Using one RH guage can make you really confused if its information is totally wrong (one of my bad gages always read ~20% too high!). Now I have one for each floor (3) and know that they are all at least accurate relative to each other!
Hope this helps,
Norm