Posted By chrisbiker on 05 Apr 2013 08:47 AM


All homes I have seen up here, boiler/rads, central ASHP/GSHP, underslab/hydronic, whatever, run shockingly(lol) too dry in mid winter without humidifiaction being added by some means. Outdoor dew point air in the teens/single digits gets in and just drys the place out. This is on typical homes and that is what I am referring. Not homes of energy experts who do all the proper stuff to get the shell tight.

Again, that's primarily a ventilation rate/infilrtation issue.  Outdoor dew points are typically in the teens near me, but even at 20F dew points you're on the low thin wedge left side of the psychrometric chart, where even dramatically lower dew points have neglible effects on the grains/lb. of moisture content in the ventilation air, and the amount of "makeup moisture" necessary to achieve a comfortable & healthy 35-40F dew point. Look at the grains/lb scale on the right edge of the linked-to chart.

At a 40F dew point the moisture content of air is about 35 grains per lb of air.

At a 20F dew point the moisture content of air is about 15 grains per lb of air, thus you need 20 grains of make-up moisture to hit the healthy-comfortable range.

At a 0F dew point the moisture content of air is about 8 grains per lb of air, so you need about 27 grains of make up.

Even at polar-arctic -100F dew points the difference in makeup moisture required is barely double what it is for +25F dew point air- it's just not that big a difference.  (Just how far "...up here.." are you , really?)

Even if you're at the north pole, it's not very different from being in Worcester MA in terms of how much moisture needs to be added to the ventilation air, but let's run the comparison on the more temperate climate of Edmonton Alberta for yuks:

The mean January outdoor dew point in my Worcester MA neighborhood is about +15F or ~12 grains per lb, to Edmonton AB's +5F or ~9 grains per lb. according to weatherspark.com datasets. Those are shortfalls on the ~35 grain "healthy air" level of 23 vs 26 grains, respectively, not all that different in terms of how much ventilation/infiltration would make it too dry indoors. At a given rate of humidity input from people & plants/bathing etc I'd only have to back off my ventilation rates to 23/26 (88%) of my current levels to achieve the same indoor humidity in Edmonton that I'm getting in Worcester. That's not a very big change in tightness requirements, and definitely not super-tightness levels.  Is it colder and drier in Edmonton? You bet! But it' doesn't make much much difference in how much moisture is lost out of the house maintained at human comfort & health levels.

And I'm not living in one of the "...homes of energy experts who do all the proper stuff to get the shell tight..." by any means- I've just fixed the biggest and most obvious holes left over from the circa 1923 timber-framed construction, and pumped some cellulose into the plank-sheathed never-tight wall cavites (or at least most of 'em.)  I'll be blower-door testing the place at some point later this year as I work out some of it's less-obvious issues, but the interior RH as-is never drops below 30%, even during cold snaps.  Even a half-assed stab at air sealing a house with plywood or OSB sheathing & windows newer than my double-hung antiques would be tighter than my house. (In most houses it's just not all that hard to hit under 3ACH/50 with even a modicum of effort, which is why it's the new standard for IRC 2012.)

Bottom line, it's the ventilation rate that dries out the place, not the heating system (though air handlers can drive infiltration rates way up on leaky houses), and the right solution for the benefit of both humans & building is to tighten up the place, not pumping moisture into the air with humidifiers.  (And at 0.18 BTU per cubic foot per degree F that adds up to real energy savings as well.)

I agree and understand the science. All depends on infiltration, not the heat source.

I am in SE Pa, so not that FAR north and definately not on the north pole (your a funny guy). No need to be snarky. I have seen many homes around here. The ones without some humidification drop to the low 20% RH range in cold snaps. Not terrible, but it makes for some sparky situations and dry throats. Also makes you feel colder than if the RH was 30+. These are typical code homes. My in-laws just got a new home a few year ago (local) without any humidification. Dry as a bone in there, but its a bit large for two (2300 sqft) and has exhaust fans in every bath. The point is every situation is different. A smaller home with 5 kids, dog, fish, and 18 plants is another story.

I know you are being modest, but you are one of those energy experts. Those holes and celulose and the many drafty windows that were fixed make a nice difference. I have my place dialed in pretty good, and prefer (not need) a little humidification during the winter. Next would be re-side wtih ext foam, but thats not happening soon.

To change to a heat pump from oil, is the issue of this thread. You point to me is that humidification is not needed. OK.

I guess I needed more emoticons- it was intended as more humor than snark.

I'm north of you in a cooler/drier but somewhat comparable climate, (Harrisburg's January dew point average is 4-5F warmer than Worcester's) and yes, most homes here leak like sieves (just like mine did before I moved in) but most can be easily and cost-effectively retrofit air-sealed to where mine now live or better. Yes, I've learned a bit about where to look, but moving air isn't very hard to detect with a wet-hand when the place is so leaky that you hit sub-20% RH in cold snaps.

My house isn't all that different from your in-laws: ~2400' of fully conditioned space + 1500' of semi-conditioned basement, 3 human (if an almost-13 year old fully counts as human- some days are better than others...;-) ) one hamster and one 6' ficus tree that gets watered once a week- that's it. Everybody 'ceptin' the ficus & hamster are out of the house for 10 hours out of every 24- it's not a super-high occupancy rate for the available air volume.

Air sealing isn't rocket science and it's not everybody's idea of a great DIY time, but it's worth it, no matter what you have for a heating system. Even if you're paying the pros with blower doors and IR imaging to nail it all down all at once, it's worth it, on higher comfort, lower energy use, and making it a more resilient building. IIRC there are even subsidies available for retrofit air sealing in PA.

Mind you, I've done remarkably little in the way of tightening up the antique double-hungs, though there are a couple that I'm ready to pitch & start over on. The prior owner put on some cheap triple-track storms which surely helped, but wind would still whistle in through the pulley holes. I added some pulley seals this year, but the place never got below 30% before that fix. The biggest holes were into the attic spaces behind kneewalls upstairs, and the foundation sill/band joist, both of which got at least partially fixed during insulation projects, though I had to crawl in and add a cardboard air-barrier where the porch roof space was fairly open the adjacent attic space after the open cell foam guys missed the 4" x 100" gap under a beam completely. It was dead-easy to find, if a PITA to fix, but it was the equivalent of ~2 man-hours, and it made a difference in the wintertime humidity levels. Any air sealing pro would have found it and nailed it in a heartbeat.

Most of the time, when you fix most of the big holes you will fix the comfort problems- this was no exception. Any of the bigger holes is worth all of the window & door & plumbing/electrical leakage combined, so start with finding & fixing the big holes first, especially at the bottom & top of the house (too kill off stack effect.)

Really? What's your total house volume, occupancy rate, and where does it test out in ACH/50 (or cfm/50)?

If you are running any mechanical ventilation, at what rate (and why)?

"Grains" in this context == 64.79891 milligrams of water mass per grain. (Historical background.) How that became the standard measure of moisture fractions of air in IP-units psychrometric charts & the HVAC trade is obscure to me. But it's easy enough to just go with it for comparative purposes when reading the IP-unit charts rather than trying to convert it to other units that we all use with greater familiarity.