Replies

1. GBA Editor
   Martin Holladay | Jan 20, 2015 06:04am | #1

   Alan,
   Q. "I am wondering if lower temperatures increases the air change rate."

   A. Yes. One of the main drivers of exfiltration and infiltration during the winter is the stack effect. The rate of stack effect leakage is a function of delta-T (the difference between the outdoor and indoor temperatures) -- so the colder it is outdoors, the greater the rate of stack effect leakage.

2. 
   AlanB4 | Jan 20, 2015 06:35am | #2

   Thanks for answering, i thought i now have very little ceiling leakage (only one ceiling fixture) and i made the attic access airtight, i will have to look more carefully for leakage.
   How much is the effect increased, does it go by temperature difference or absolute temperature?

3. GBA Editor
   Martin Holladay | Jan 20, 2015 07:19am | #3

   Alan,
   Q. "How much is the effect increased, does it go by temperature difference or absolute temperature?"

   A. As I wrote, the rate of stack effect leakage is a function of delta-T (the difference between outdoor and indoor temperature). The actual equations for calculating stack effect leakage are shown in the image below. (These equations come from "Field Validation of Algebraic Equations for Stack and Wind Driven Air Infiltration Calculations" by Iain S. Walker and David J. Wilson.)

   In the equation, T(in) - T(out) is the delta-T. Since this is a factor in the equation, a higher delta-T raises the air leakage rate.

   If your indoor temperature is constant, it can be said that the colder the outdoor temperature, the higher the rate of stack effect leakage.

   .

   
4. 
   dickrussell | Jan 20, 2015 11:38am | #4

   Alan, in equation (8) of the reference by Martin, those temperatures are absolute, not F or C, although it doesn't matter in the numerator. Another way of looking at the effect of outside temperature on leakage is to consider that leakage is driven in part by air density difference. From physics, the volume of a mol of gas (air) is ZRT/P. Since at one atmosphere the compressibility (Z) is essentially one, volume varies linearly with absolute temperature, and thus density varies inversely with absolute temperature.

   To use a good winter example, consider interior air at 70 F, and outside air at zero. The outside/inside air density ratio will be (70+459.67)/(0 + 459.67) = 1.152. With that outside air being 15% heavier than inside air, it's no wonder houses leak so much in bitterly cold weather. Now, add the extra pressure of a 20 mph wind against the house and ......

5. Expert Member
   Dana Dorsett | Jan 20, 2015 02:29pm | #5

   "The humidity this winter is a lot better, but i am noticing down to -5C the humidity will stay about 50% which is excellent, at -10C its about 40% and at -20C its just under 30%."

   Can I assume those are outdoor temps, and the indoor temp is between 18-22C, and that humidity is relative to the INTERIOR temperature?

   If yes, in a wood-sheathed studwall type house those numbers are a bit too high to be healthy for the house, unless your interior wall air-tightness &/or vapor barrier are PERFECT. (In things constructed by humans there is good, better, and best, but no such thing as perfect.)

   When it's cooler than -5C outside it's better for the house if the indoor RH is 35% @ +20C, since the dew point of the interior air is +4C, much higher than the sheathing temperature, thus any air leakage or vapor diffusion from the conditioned space into the wall cavities accumulates moisture in the sheathing over a southern Ontario winter. At 0C or lower you really don't want anything NEAR 50% RH @ 20C indoors, which has a dew point of +9C.

   The dew point of 40%RH @ 20C has a dew point of about +6C...

   ...30% RH @ 20C runs a dew point of +2C.

   The lower the dew point, the less moisture accumulation in the sheathing, which in turn lowers the fungus / mold / rot risk. The "healthy for humans" range of humidity recommended by health professionals is 30-50%. To be healthy for both house & occupants, shoot for the low end of that range in winter.

   Numbers that high in a house with your blower door leakage numbers is an indication of significant moisture sources too. Making pasta all day with no lid on the pot and no kitchen exhaust fan running, are we? A large collection of tropical fish aquariums, mayhaps? :-)

   It could also be measurement error- what instrumentation are you using to measure that RH? (I'm gonna go out on a limb and say it's not a sling psychrometer. :-) )

6. 
   AlanB4 | Jan 22, 2015 03:08am | #6

   I don't know how to use that formula, but i was asking if stack effect increases at the same rate at 20C inside and 0C vs -10C (10 degree difference) or -10C to -20C (10 degree difference), and is does the same temp drop increase the effect linearly? If that makes no sense then i am asking how temperature drops increase the effect, 10C below room temp say causes 10% increase, 20C below room temp causes 20% increase, 30C below room temp causes 30% etc, or is it something else, say 10C below room temp causes 2%, 20C below room temp 8%, 30C below room temp 30% and so on.

   @ Dana, you assume correctly, interior temp is 20C, humidity is what i described and measured by a cheap home depot humidity meter. If 30% humidity is too low (and caused by massive forced ACH) then why are humidifiers for furnaces sold? I have no humidifier on the new furnace, and no aquariums, vast pasta making or anything else, but i do wash dishes, and i have noticed showers will increase the humidity by 10% and within 12 hours it has dropped back to where it started. I do have a crawlspace but since thats where the majority of the air leakage is it should be tracking the outdoors rather well (i'm guessing)
   The house is very old, heres a post i made about it not too long ago
   https://www.greenbuildingadvisor.com/community/forum/general-questions/38991/how-long-ago-were-2x4-actually-4-inches
   Its funny you mention aquariums, when i moved in there was the 30% RH causing furnace and mold on a wall in one room, i speculated the previous resident had an aquarium or other high humidity habit because at 30%RH i had no mold created whatsoever, it was far too dry here.

7. Expert Member
   Dana Dorsett | Jan 22, 2015 11:30am | #7

   30% humidity isn't too low, 20% is. ASHRAE says 25%, for comfort, medical professionals say 30% for health, but it's not a firm fine line. If you're at 30%RH @ 20C during the coldest weather, you're fine. That is not a symptom of "...massive forced ACH..." at all.

   The "...down to -5C the humidity will stay about 50%..." is way too high for the health of the house though. It doesn't take "...massive forced ACH..." to bring it down to 35% when it's -5C outside. In VERY tight houses the RH can rise that high even without bathing /cooking /aquarium moisture being added, but it doesn't take high ventilation rates to bring it down. Some heat recovery ventilators are outfitted with dehumidistat controls, which is useful to keep from over-drying the house in winter, the way it can if controlled simply by duty-cycling it. Those aren't large ventilation rates.

   The answer to "...why are humidifiers for furnaces sold? " falls in the same category as why active attic vent fans, radiant barriers, and cigarettes are sold. It's because it's legal, not because it's a great idea. For very air-leaky houses interior humidity plummets in winter humidifiers are something of a "solution-problem", solving the comfort level issue while putting the house at risk for mold problems when operated by folks who think of 50% RH as "average" ,and thus the right number to shoot for, when it fact it's at the high end of what's healthy for humans, and over-the-top high for increased mold risk inside the wall in cold climates.

   In a house built to IRC 2012 tightness levels (sub 3ACH/50 ) humidifiers have NO use at all, unless the occupants insist on higher ventilation rates.

8. 
   AlanB4 | Jan 22, 2015 11:37pm | #8

   I think my house is close to a century old so no poly vapour barrier (i got cellulose put in the walls so i can confirm there is none) and at 14.85ACH50 its quite leaky. I don't think i mentioned its about 700sq ft, so not a big place. I guess i don't know whats causing the humidity, though i prefer the 50%, my hands don't dry out, my asthma doesn't bother me much, i'm coughing less.
   In the spring/summer/fall the humidity is about 60-90%, much higher and except for the last 10 years there was no central air. Even last summer i barely used it (high energy consumption and low output, it took 3-5 hours to cool the house night or day).
   I notice i can stand 40% humidity but 30% is too low, it takes about 70% to get condensation on the windows @ -15C, though they are double pane vinyl and replaced in 2003 (i think)

9. Expert Member
   Dana Dorsett | Jan 23, 2015 10:49am | #9

   Letting it get above 70% RH in summer is a serious mold & fungus risk, and a likely contributor to asthma symptoms!

   Sealing & insulating the crawlspace walls, and installing a ground vapor retarder will reduce those risks.

   With a 700' house it doesn't take a lot of human-generated moisture to raise the indoor humidity, but it takes that much less wintertime ventilation to control it. It may be worth installing a dehumidistat control for the bathroom fan to manage it better in winter. If 40% RH is your personal low-end tolerance limit, that's where to run it in winter, but 35% would be better for the house. In summer you'd have to use air conditioning or a dehumidifer to hold the line at 50% for an upper bound.

   The reason for the 50% upper bound is that above 50% RH dust mite populations grow rapidly, below that point they slowly die off. Above 60% RH skin & respiratory fungus infection susceptibility begins to rise. Above 70% RH mold spore counts soar.

   If only 1/3 of the house has the crawlspace, what's under the rest of the house (open piers, full basement, slab on grade or ...???)