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Question DetailsAsked on 7/24/2016

WHAT TO DO ABOUT HIGH HUMIDITY (51-60%) LEVELS IN YOUR NEW HOUSE BUILT ON A CRAWL SPACE WITH A/C ON ?

HOME IS BRAND NEW (3 MONTHS OLD), A/C UNIT HAS BEEN CHECKED AND IS FULLY CHARGED, CRAWL SPACE IS SEALED, ATTIC INSULATED TO R-32 BUT HUMIDITY STAYS HIGH INSIDE HOME. I'M TRYING TO DETERMINE WHAT'S THE MOST COST EFFECTIVE REMEDY. SHOULD THIS BE AN ISSUE FOR THE BUILDER TO RESOLVE ? ITEMS IN MY PANTRY (COOKIES, CRACKERS, CHIPS) GET STALE WITHIN DAYS OF OPENING AND CANNED GOODS OR BOXES STICK TO THE SHELVES AFTER 4-5 DAYS ON THE SHELF.

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That is a good 10 maybe 15% too high a humidity - maybe even 20% high on the high end for summer, especially if anyone in the faily is allergic to dust mites or mildew such.


I am going to assume you are getting the correct temperature coming out - i.e. that the A/C is cooling the air as it should, and of course that the A/C is actually running - not that the system is just running on FAN setting.


Assuming you are not leaving windows or doors open to let in warm humid air, that crawlspace and/or attic is not open to the house (access panel or door not sealed or left open or door to attic has gap under it rather than weatherseal), that you are getting actually significant amounts of condensate off the A/C evaporator into the pan and out the drainline (and pan is actually draining, not just re-evaporating into the airflow) which would mean it is actually removing moisture from the air, and are not generating major amount of indoor moisture then yeah - sounds like a complaint under new home warranty for the builder to solve. And I would be dealing with the person you paid to build the house, not taking it up with the HVAC sub (though obviously any questions he has answer honestly) - becasue your home warranty is with the builder or developer who sold you the home, or your General Contractor if he designed it. If you had an architect design it for you, then the reasponaibility may lie there if he designed and specified the HVAC system and the builder jsut installed what was specified.


Major indoor moisture sources - human evaporation from skin and breathing out moist air (though that is normal for a house), lots of house plants or large aquariums or terrariums that are not covered, not using kitchen / bathroom exhaust fans while and for 15 minutes after frying/boiling food and during/after showers and baths. Also - uncovered hot tubs and such with standing water, and clothes dryers venting into the house instead of outside as they should. In some cases, and especially with hot-dry and steam-generating units, dishwashers can put a lot of hot vapor into the air too, especially if run on a daily or more frequent basis.


One rarer source of moisture, and should be pretty well stabilized before 3 months, is moisture from wet framing/subflooring in houses that sat in the rain significantly and did not dry out before being closed in and drywalled/floored.


Another rare source of this - summer only - would be bathroom or kitchen fan vents that were not run through the roof and terminated in the hot humid attic or which do not have the required flappers to prevent backflow, in which case you should be able to detect (with damp hand or piece of tissue paper) airflow coming into the house through those sources when the fan is off - depending on wind direction or time of day possibly.


You said A/C unit is fully charged - but did the HVAC contractor check the high and low side pressures and that the evaporator coil temperature is correct during operation (commonly in the mid to high 40's) so it will condense moisture from the airflow and remove it when indoor humidity is high ? During humid summer days, an A/C will commonly remove quarts of water from the air - gallons in extreme cases.


Another major probloem with modern installations - the ASHRAE guidelines on ventilatiopn and airchanges per hour has changed recently (going back up to counteract too many houses built too tight), but the airflow design for evaporators has not in most cases - so in some cases the higher airflow from the fan is keeping the evaporator too warm so it is not removing the condensate like it should as higher humidity air passes over it. Some systems use a humidity gage in the air downflow of the evaporator and a thermostat on the evaporator to reduce the fan speed (with variable speed fans) during high humidity so the air flow over the evaporator is less, so it runs colder and removes more moisture from the air.


Also, and you can check this with a decent humidity gauge or just by wiping concrete surfaces with your hand for moisture, basement moisture coming through foundation or basement slab, or to a more limited extent from uncovered sump pump pit. If you have high moisture in the living area upstairs coming from the basement you will most likely have condensation or dampness visible on concrete surfaces in the basement because of the cooler ground temperature pulling down the surface temps in the basement.


You said crawlspace is sealed - did you mean from the outside (BAD idea), or crawlspace is ventilated to outdoors well but there is fully covered and edge-bonded vapor barrier on the underside of the floor joists ?


Problem could also be too tight a house without enough makeup air exchange. A LOT, LOT of architects and HVAC contractors are putting in furnaces and A/C air handlers that totally recirculate the air, without exhausting a portion of it and replacing it with fresh outdoor air as is normal for commercial installations. This can cause a buildup of moisture in the house - and commonly is associated with noticeable persistence of odors in the house as well. Did not used to be a problem with fairly air-permeable houses and open furnace/water heater/boiler makeup combustion air vents through the outside wall, but with tighter "5-Star Energy Rated" houses and closed-combustion appliances that draw their combustion air and exhaust it directly from/to the outside, so they are not "consuming" and exhausting indoor air as they operate, it is becoming a BIG problem. Saw one house last winter that was brand new (less than 2 months since completion and was fast-track construction, in less than two months) with MAJOR mildew/mold problem due to lack of any source of fresh air, aggravated by the fact it got pretty wet before roof/siding went on (totally framed and sheathed then let sit in wet weather for several weeks before roof/housewrap went on). The blower door test done by the remediation contractor showed less than 1/4 air change per hour. In the good old days" when we used Pterodactyls as ventilation, fans the standard used to be 4-5 air changes per hour was considered suitable to control humidity and odors, then it became 2-3, then 1-2, now I have seen some recommendations as low as 1/3 airchange per hour - which definitely is an invitation to trouble. In fact, in many highly energy-rated homes air exchange is so inadequate that after all that trouble making the house "tight", they then put in a whole-house ventilation system to exchange air with the outside, which is a waste of energy (both is lost conditioned air energy and in fan electricity) and money (though does filter the incoming air). Makes about as much sense as the houses in our area with massive south-facing windows for winter solar heat gain - then they have to put one or two air conditioners in for use in the other three months, in an area where air conditioners are not a routine appliance for houses.


Unfortunately, many builders will take just that route to solve an issue like yours - just open up some outside ventilation without consideration for which area(s) really need it. For instance, basements are a common source for this, and pulling the source air for the furnace/AC from there (as return air from the living spaces) can result in this being handled in many cases, I have seen outdoor vent being installed to provide more fresh air even in very high humidity areas, where (during the A/C season) all that does is result in MORE moisture being drawn in from the outside.


BTW - the cans and boxes sticking to the shelves may or may not be related to the humidity - this is a common event in new/remodeled homes, especially ones that did not air out for weeks or months during the sale/construction process, and most especially with acrylic and latex paints. Generally goes away in a year or so IF you rotate items so all parts of the shelves get a chance to be exposed and dry out.


If you would care to, use the Answer This Question yellow button right below your question (just as if you were answering your own question), to let us know what the solution is that works for you in this case, after all is said and done.

Answered 2 years ago by LCD




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