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How will the use of a Thermastor dehumidifier affect the temerature of a superinsulated basement?

G33PvCERFo | Posted in GBA Pro Help on

Facts: The house in question is (3) story 4,800 sqft, designed with R-43 walls, R-60 ceiling, R-29 basement (where sub grade, the rest is R-43 at walk-out) and sub slab insulation is R-19. The slab is to be 4-inch. The location is Newbury, NH (Zone-6) I would like to address summer time humidity concern by installing a Thermastor (“Sante Fe”) unit in the basement Utility Rm. My son, who works for Thermastor has a similar set up in his Madison, WI house and he observed 70% humidity at the 2nd floor was reduced to 50% within a day (maybe less). The Sante Fe unit runs with a drain and the basement (normally cool anyway) absorbs the heat generated by the unit. My question is in regard to the potential for the slab assembly in the house mentioned above to absorb and mitigate the heat generated? Given the sub slab insulation, will this cause the basement to heat up? I could be wrong but was told that 1pint of water removed eqautes to 1000 btu of heat.

I don’t want to reduce the overall under slab insulation but would it be wise to do so in the Utility Rm. where the SanteFe is located. As an aside, the utility Rm is 13′-3″ below grade, deeply set below the Winter’s cold outside ambient air.

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  1. jklingel | | #1

    Why are you so sure you will have a humidity problem? Is there a vapor barrier under the slab? A vapor barrier in the house? If so, where? Is the below grade part damp-proofed, ground sloped, etc? A btu is the energy required to raise 1 lb of water one degree F, roughly 1/pt. 1000 btu sounds high; check with the manufacturer about how much the humidifier works to condense one pint.

  2. G33PvCERFo | | #2

    Yes there is a VB below the slab, perimeter drains cont. at the exterior ftg. and exterior vert. drainage mat aswell. The only VB in the house is at the 2nd Fl. ceiling plane. The walls are 2x6, filled with cellulose, structural sheathing, Tyvek and (2) layers of polyiso, 1x4 firring and siding. CCHRC's manual provided the general model. At the basement I've chosen to use 2-inch EPS with adequate perm rating so any capillary water migration will be able to dry to the interior, a 2x6 wall filled with cellulose finishes off the basement inside the EPS. I'm confident about keeping the water out that falls from the sky and runs on the ground as we are collecting rainwater with gutters, gutter topper (the whole nine yards) and piping it away to sub surface infiltration devices.

    In NH and much of NE we have relatively high humidity in the summer months (trust me it's a problem) and fresh air delivery system (RenewaireERV) although able to reduce moisture somewhat, achieves this by simply exchanging ambient exterior air for interior air. In our summers this will be useless and the house's design is tight enough that moisture will prevail. It will build up and be a menace.

    I got the numbers (1pt./1000btu) from my son and will double check them.

  3. dickrussell | | #3

    The 1,000 BTU/pint of water condensed is ballpark correct. There's an old saying: "A pint's a pound the world around." Dividing out 62.366 lb/cu.ft by 7.4805 gal/cu.ft and by 8 pints/gal gives 1.04 lb/pint. The heat of vaporization of water at 70 F is 1054 BTU/lb. Add in the work of the refrigeration loop equipment and you've got actually somewhat more than 1000 BTU/pint. So those numbers are right.

    The concern about where the dehumidification heat will go is valid. It won't leave the superinsulated structure too easily in the summer, and thus the temperature must rise, however slowly due to the thermal mass of the slab and foundation walls. On the other hand, a quick approximation of the rate of heat loss to ground may be useful for comparison to the rate of heat input from the heat of condensing x pints/hour of water from the air.

    I assembled a spreadsheet for the heat balance model for my own house (also in NH). The slab is roughly 2000 sq.ft., with R20 under it. The buried part of the foundation walls have an area of about 750 sq.ft., with R20 of insulation. The approximated heat loss through the slab is about 1800 BTU/hr, with a high degree of uncertainty due to assumptions made about soil temperature under that insulated slab. I hesitate to mention the winter time approximation through the foundation walls, as that is based on winter ground temperature profile. In summer, with a much warmer profile in the couple of feet just below grade, gain and loss areas cancel somewhat. Gains through the framed wall areas won't be much, averaged over 24 hours, due to low mean delta T, and the windows downstairs add a similar amount. Both would add to the dehumidification load, though.

    That tells me that a similar dehumidification setup for my own house (also superinsulated) would let me remove perhaps a couple of pints of moisture from the air per hour without substantial temperature impact, or at least no more than what a handful of light bulbs and a TV would do. The numbers are purely ballpark, no better than that, and your own house will have its own set of numbers.

    Humidity is a problem here in summer, at times, while there is no worry about heat gain during the day; neither your house nor mine will gain heat from the outside very fast, on the hottest day. Mine has a ground source heat pump, so it provides A/C in the summer, with dehumidification of course. I do have a problem at times, however, because the system is sized for winter heating and thus is grossly oversized for summer A/C. There have been many times so far when the house is cool enough but uncomfortably humid inside.

    I ordered my system with Climatemaster's "Climadry" method of reheating cooled/dehumidified air with heat removed in the process, so as to be temperature neutral while dehumidifying. Alas, I haven't had it running that way yet. I don't think CM has all their documentation up to date with respect to how to turn on the water flow before the compressor starts. Their usual suggestion, to run the line from the thermostat calling for heating or cooling through a slow-opening water valve, with the end switch passing the signal on to the heat pump, won't work with Climadry. There can be a call for dehumidification without a call for cooling, and that signal is over a different line. CM does provide a means for having the water valve tuned on by the heat pump itself with a 60 second delay before it turns on the compressor, rather than by interception of the thermostat signal, so clearly the designers had this situation in mind. My solution I think involves rewiring to use that alternate way of turning on the water valve, but I also must change the water valve to one that doesn't take as long to open.

  4. G33PvCERFo | | #4


    Thanks for your informative response. I gather you don't consider the potential heat gain from the "SantaFe" to be a significant or potential liability in regard to summer time comfort of this basement. The site conditions (steep slope and shoreline) of this project are such that a tall foundation wall (13'-3") is required and the Utility Rm. is nestled into the rear area where the sub grade depth is maximum. Because of the unique height characteristic, the Utility Rm's surface area (less the floor) is 477 sqft, the floor is only 147 sqft. There are insulated wall and sub slab assemblies however that will resist normal conductive losses and this is why I am concerned. My understanding of thermal dynamics is relatively modest and I have used the work of other super-insulation pioneers to model my wall and floor assemblies. This being the case, I try to listen when others with specific intelligence in thermal dynamics and building science offer insight.

    Given that the Utility Rm is so deep I wonder if reducing the sub slab insulation or even eliminating it altogether (just in the Utility Rm.) would allow the heat generated by the SantaFe's dehumidification to be more easily absorbed. My concern is mostly for the summer months when the dehumidification will be a wonderful and energy efficient way to provide comfort in this lake house where the walk-out basement is being utilized as quality living space. It might be running 24-7 in the summer. Of course I wouldn't want to have created an energy loss liability during the winter months. As you know, In NH we are by and large a very cold climate.

    It seems the heat created by the SantaFe is relatively minor, so perhaps I'm worrying needlessly and should leave well enough alone. I am excited by the application of dehumidification in this house and the considerable advantages offered by products like the SantaFe to provide relief from the summer humidity and consider the air scrubbing from the merv 11 and optional Hepa filtration to be a nice bonus.

    One aspect of humidity that I never understood with clarity (being an art major) is that humidity seeks less humidity and infact it's driven towards less humidity. This being the case (please correct me if I'm wrong) providing one of these dehumidification devices in the basement of a tightly built structure will suck the humidity from the entire interior envelope, including the floors above. If it does this while also reducing particulates and allergens from the interior air and can provide comfort by reducing the need for more energy consumtive AC, this strategy seems to be a win-win. Is there a lesson here?


  5. dickrussell | | #5

    It isn't really that "humidity seeks less humidity." The water vapor molecules are bouncing around actively, bumping into each other and into the oxygen and nitrogen molecules in the air. There is a random movement to all these molecules bouncing around, and there is a net movement of any kind of molecule from a region of higher concentration to a region of lower concentration simply because there aren't as many of that kind "over there" to be coming back as there are "here" going "over there."

    If you run a dehumidifier in the basement and leave the door at the top of the stairs open, the air downstairs will be dried more by the dehumidifier down there, but there will be a net diffusion of water molecules upstairs to the region of lower humidity downstairs. Whether the effect is sufficient for comfort upstairs is a matter of geometry. If you have mechanical circulation of air within the house things will even out more rapidly. How well the diffusion down the stairwell works vs the inflow of outside humidity via your ERV is another matter.

    You seem to be reluctant to use mechanical A/C energy, yet you seem willing to add mechanical dehumification. For a house such as you describe the size of a unit for A/C would be rather small, perhaps just a ton. Without pumping heat out, you'll have to depend on whole-house ventilation at night to remove heat absorbed from outside and generated within by human activity during the day. That takes a concerted effort and the right timing, and there are some nights when the outside air doesn't cool off as soon as you'd like. What is the heating system?

  6. user-988403 | | #6

    I think you got some good advice already but I just want to make sure you are not getting rid of your sub-slab insulation. I live in MN quite a ways away from NH but same climate zone. My ballpark estimation is that you will "loose" 10 kBtu in the winter if you go from R19 to just 4" concrete slab in your utility room. That is definitely more energy and money than the potential cooling demand caused by your dehumidifier. I am not familiar enough with your summer climate to validate your dehumidification strategy. However, if the dehumidification was causing overheating (which I also don`t think) it would definitely be smarter to go with a Minisplit AC. A dehumidifier basically has the same first step (cooling the air to bring down the absolute humidity) and then reheats the air before releasing. So definitely more energy intensive. I do know that under some circumstances "only dehumidification" is necessary and may be so in your climate. If you are worried about this look into products that can do both., the Daikin Quaternity is one example. I have never used it but it sure looks promising (No idea about the costs).

    As a site comment I have to disagree with Dick on one topic. I do not believe in random movements of atoms. In fact water vapor and air are independent from each other and you were right, water vapor travels from high humidity (high pressure) to low humidity (low pressure) in general. However Dick is right about his concerns of how effective this may work.

    Hope this helps

  7. homedesign | | #7

    Dick Russell,
    I do not agree with Philipp...
    I thought your description of Molecule movement was very Good.

    I have found these KhanAcademy videos to be helpful....
    (in the Chemistry Section....see ideal gas equation videos and partial pressure video)

  8. user-1022459 | | #8


    If your slab is more than a couple of feet below ground level, the sub-slab temperature should be nearly constant year round. If you are conditioning your basement, the sub-slab (and the slab) temperature will be below the conditioned air temperature. This will result in a net heat loss from your basement to the slab below which will be a positive thing in cooling season and a negative thing during heating season. Insulating sub-slab will provide a lower return on investment than insulating your basement walls and your above ground enclosure. I definitely recommend insulating under your slab if you are placing hydronic in the slab as the energy loss from a heated slab is considerable.

    Insulating below the slab will slightly raise the slab temperature which will reduce concerns from condensation during the cooling season; in other words, an insulated slab can handle slightly higher dew point air above it than an uninsulated slab without forming condensation on top of the slab. The insulated slab will also provide more comfort if the floor surface is finished. This is my way of saying that the insulated slab decision is likely one of the least important decisions you will make regarding your building enclosure.

    The Santa-Fe dehumidifier will produce heat in your house when it is dehumidifying. It will convert the latent heat (water vapor removed from the air) into sensible heat - this is where the 1000Btu/pint number originates. The Santa-Fe will also produce additional heat from the electrical motors within the dehumidifier - this heat will be roughly equivalent to the power consumption of the dehumidifier. all (refrigerant) dehumidifiers will produce the 1000Btu/pint heat as they remove moisture, but the efficiency of the dehumidifier will determine how much additional heat is produced by the power consumed while dehumidifying. The Santa-Fe dehumidifiers are high efficiency dehumidifiers, so the additional "power consumption" heat should be less than most other dehumidifiers. The rate at which a dehumidifier produces heat will be great enough (when it is operating) that heat transfer through your slab will not fully offset the dehumidifier heat and your basement air will warm up. Over the long term, your basement air will be warmer with a dehumidifier than without. How much warmer will depend on a number of variables - your desired indoor humidity (dew point), the outdoor dew point, the rate at which outdoor air enters your house, and the amount of moisture created by the house occupants. All of these variables will affect the required running time of a dehumidifier - the dehumidifier is only adding heat when it runs.

    Your highly insulated house will reduce the cooling and heating costs by reducing the heat loss and gain during the heating and cooling seasons respectively. Unfortunately, the moisture "load" on your super insulated house is much less affected than the temperature "loads". Creating an airtight enclosure will help to reduce the moisture "load" from outdoor air infiltration, but you still need to ventilate with outdoor air when the house is occupied. The occupants inside the house (and their activities - washing, cooking, bathing...) will generate water vapor within the house that needs to be removed as well.

    During much of the heating season, ventilating with fresh air from the outdoors (which has a low dew point) will provide dehumidification and a mechanical dehumidifier will not be required. During the cooling season and the "in between" shoulder seasons, the outdoor dew point can be high enough that mechanical dehumidification is required. Air conditioning can remove substantial amounts of moisture during the cooling season - when it is running. During the shoulder seasons you can encounter the need to dehumidify and heat, or dehumidify and cool depending on the outdoor air conditions.

    You have an ERV which provides fresh air into your house. I bet you also have other devices/appliances which will ventilate as well (kitchen hood, bath fan, clothes dryer). These devices will bring fresh air into your house including the water vapor in the outdoor air. The ERV can perform some moisture exchange, but it will not be able to single-handedly dehumidify your house during periods of high outdoor dew point. The ERV will help take the "edge" off of the incoming high dew point air, but mechanical dehumidification will be required to reduce the dew point further.

    Your house may require more dehumidification in the cooling season than a typical house since your air conditioning run time will be much less than a typical house. As I stated above, there are a number of variables that will determine the amount of dehumidification required. You should understand your ventilation requirements - over-ventillating during high outdoor dew point periods or when the house is not occupied will unnecessarily increase your dehumidification requirements. A CO2 meter can be used to estimate how many air changes you are getting in your house.

    Minimizing the water vapor created and left in the house by occupant activities can also help. Operating bath fans while showering and the kitchen hood while cooking can reduce the residual moisture left in the house after these activities. Line drying clothing instead of using a clothes dryer can reduce outdoor air infiltration.

    Air within your house will stratify based on the air density; warm air is less dense than cool air. This will define the thermal gradient in your house unless you mechanically move the air to disrupt this gradient. Water vapor will move freely through the air in an attempt to reach an equilibrium (see Brownian motion and diffusion for the mechanics). If your house a ducted HVAC system, it can be used to distribute the fresh outdoor air (by connecting the ERV) and to disrupt the natural thermal gradient in the house. A whole house dehumidifier such as the Ultra-Aire units (made by the same people as the Santa-Fe) can be connected to the HVAC ducting to provide fresh air ventilation, dehumidification, and air filtration.

    Good Luck with your house!

  9. G33PvCERFo | | #9


    Thanks for that informative response. Indeed all the answers received have helped with my understanding of the conditions that will likely be encountered in this NH residence. The climate in NH does challenge mechanical design considerations. The cold dry winter and hot humid summer (with more humidity than most imagine) are the primary aspects but the shoulder seasons are different. In fact those seasons are quite nice. They typically have some heating load and the humidity is variable (sometimes comfortably low with occassional higher humidity). I haven't focussed on the degree of humidity in the shoulder seasons (haven't noticed it really) and my impression may be in err, but suspect when we aren't having a tropical storm or serious rain event the shoulder season ambient humidity can be useful for mitigation of interior humidity. Your detailed discussion about the possible mechanical system challenges propells me to better describe the overall system for your understanding. Please understand, I greatly appreciate the suggestions that come forward and next time I travel these roads, I will budget the use of a mechanical engineer (beyond what the supply house provides).

    My current design is to exhaust Bathroom humidity and Kitchen humidity directly. I have previously tried to salvage heat energy from steamy shower areas but this experience proved too troublesome and I've given up on that.

    Fresh air will be supplied to all corners of all three floors with a dedicated duct system simply for that purpose. This is simply ventilation not HVAC (only "V") and the duct system and cfm will be sized for only those requirements. The local mechanical supplier "The Granite Group" has recommended a "RenewAire" ERV for mechanical heat recovery and I understand that the limitation for humidity management of this system will be the humidity of the outside ambient air. (10) ceiling fans are dispersed through out living spaces and all bedrooms to move air around. Budgetary constraints are likely to eliminate half of these ceiling fans but wires will be supplied for future use where units are eliminated.

    My client prefers radiant heat and use of gas fireplaces ((3) are planned) to provide heat. The shoulder seasons will largely be when the fireplaces may provide all the heat required. The lake house does have considerable view oriented glass so the heat loss from this does make need for additional heat a requirement between Dec.-Feb.

    Mitsibishi (slimjims) ductless minisplits were part of my original design to provide additional heat but most importantly AC in the upstairs bedrooms and the living area. Both cost and my client's aesthetic dislike of their look (both the inside and exterior units) has recently questioned if the units will survive the final budgetary shake down. I have advised against their elimination. My view is that loss of the AC component from large houses is one of the decisions most greatly regret.

    My sense is that even without a more involved (ducted dehumidifcation system) which is a budget buster at this point, a unit like the ThermaStor "SanteFe" will provide critical dehumidification during the summer season and provision for an additional unit at the 2nd floor may be a consideration if humidity becomes an issue in the future. I will pass on the importance of owner participation in management of humidity through lifestyle adjustments (use of cloathes line etc.) when possible.

    Thanks again to all (Tim, JohnK, Dick, Phillip and JohnB for your thoughtful review, sharing of Thermodynamics knowledge and advice.


  10. user-1022459 | | #10


    It sounds as if you have the situation in hand. Ductless mini (or multi) split systems provide a good solution for homes without HVAC ductwork. Often a small indoor coil unit is installed on each living level allowing each floor to be a "zone" with independent thermostat control.

    Be careful adding many indoor electrical appliances - the power consumed by the appliances will become heat that will need to be removed from the house during cooling season. A ceiling fan left running in an unoccupied room is nothing more than an interesting looking heater.

    I suggest that you plan and make provision for the dehumidifier and mini split system(s) even if they are not installed initially. The homeowner may find the budget $$ after living through a few seasons in this house and will appreciate that the provisions were made at construction.

    I expect that indoor moisture will be a problem for much of the year. 75F indoor temperature and 50% humidity requires a dew point of 55F. Remember that the occupants of the house are creating a moisture load within the house when it is occupied. The outdoor dew point will have to be substantially below 55F for the ERV to remove the internal moisture load created by the occupants. The ERV will be working against you in this situation - it will be moving moisture from the exhaust air stream to the incoming fresh air stream. I suggest that the occupants keep a thermo-hygrometer in the house to monitor the moisture level.

    The Santa-fe dehumidifier can be easily installed (even by the homeowner) post-construction as long as the required provisions (water drain and electrical branch circuit) are provided. If you wish to avoid mechanical cooling, the Santa-fe dehumidifier will be a good choice as they will input the least amount of heat per unit of water removal due to their high efficiency. The mini-split units will require professional installation, but location(s) can be chosen pre-construction which will allow the electrical branch circuits to be installed at construction to reduce the cost of post-construction installation if desired.

  11. jXnSpGVR9v | | #11


    The bottom line is that your basement will be slightly warmer and much dryer with the Santa Fe than without. The end result will be a very comfortable space.

    The slight warming of the space is to your benefit, since it further reduces RH. For every degree you raise the temperature, your relative humidity drops about 2% (assuming no additional moisture is added). Warming the space with dry air gives you a “double whammy” in terms of humidity control and, in a way, limits the heating potential. As a side note, due to the efficiency of the Santa Fe it will produce less heat than other, less efficient units (per unit of water removed).

    My advice: insulate well and don’t worry about the extra heat. Based on years of operating a Santa Fe, you’ll be amazed when you walk into the basement and it doesn’t feel like a basement. Good luck!

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