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Is Passive Cooling + Dehumidification enough to keep us cool in Philly?

by Chad Ludeman on March 26, 2008 · 30 comments

in Building Science,HVAC

Lately the team has been struggling a bit with the HVAC budget and design. The original budget was around $10K which is doable until we starting putting things like an HRV and solar thermal into our wish list. One of the questions we have been asking ourselves from the beginning is whether or not we need a traditional Air Conditioning system in the home at all?

Philadelphia’s summer’s are fairly mild and do not have a high number of cooling load requirement. I have a number in my notes that says we have a mean total cooling degree days of about 1,172 degrees F (with a reference of 70 degrees F). The vast majority of these CDD’s come between June and August (861) which means there are only three months out of the year that we can really make a good case for needing A/C in an average home in Philadelphia (source data).

The real issue we have in our area is really not heat, but humidity. Since the air is so much stickier here than other areas of the country, it feels much hotter than it often is during the summer. It is not as easy to cool our homes with passive cooling due to the high humidity levels as we will be bringing humid air into our homes.

So this brings us back to our original question – can we make a home comfortable during the summer is we employ simple passive cooling techniques in conjunction with a whole house dehumidification system? I think the answer is yes, but we are not 100% sure yet.

Let’s break this down into two parts here. A traditional air conditioning system will both cool and dehumidify the air in a home. We are simply breaking these two functions apart and handling the cooling requirement my mostly passive means, while employing a whole house dehumidifier for the dehumdification requirements.


Let’s look at dehumidification first. The goal is to keep the humidity levels in the home below 50% ideally and 60% at the least. If we are successful in doing so, it has been proven that people are comfortable with their thermostats set between 78-80 degrees F during the summer. Since our baseline cooling degree days was calculated at 70 degrees, raising the target to ~79 degrees would drastically reduce and almost eliminate the total number of cooling degree days altogether. I don’t have the raw data to do this calculation officially, but there was a rough reduction of 600 cooling degree days for every 5 degrees added to the baseline from 55 to 70 degrees (source data).

Ultra-Aire 90HThe dehumidifier we are looking at is an Ultra-Aire 90H model that is capable of both dehumidifying the air as well as bringing in fresh air ventilation from outside (more on the ventilation benefits later). This model runs on a standard 110V outlet and only requires 6.3 Amps which means it should take much less power to run than a full A/C unit.

Passive (& some active) Cooling

Now for the cooling side. Here is a list of our cooling measures we are considering to employ in order to keep the inside temperature comfortable during the summer:

  1. Superior SIP Insulation – Keeps the house from getting hot and also remains a constant indoor temperature longer than a traditional home.
  2. Energy Star Windows – Low U values and proper glazing will help keep the home cooler.
  3. Cool Roof Coating – A white coating that will reflect more than 60% of the radiant heat from the sun.
  4. Landscape Shading – Trees in the back of the home will shade the south side and rear yard during the summer.
  5. Ivy Wall Covering – A wall of ivy over the entire south side of the home will shade and cool the home further from the summer sun.
  6. External Window Shading – Exterior shades above the south facing windows will block the summer sun from directly entering and heating the home.
  7. Low Wattage Lighting – Less heat from lighting indoors reduces cooling load.
  8. Ceiling Fans – Reduces the perceived temperature by three degrees (F) when running.
  9. Interior Thermal Shades – Increases the R-value of the window and can both help block heat from entering during the day or cool air from leaving during the night.
  10. Natural Ventilation – Natural ventilation from both the windows and the Ultra-Aire unit help to cool the home naturally day and night.
  11. High Thermal Mass – The exposed concrete slab floors on the ground floor (and possibly the bathroom) will absorb and store heat during the day and release it during the night if naturally ventilated. This is a cycle that can repeat each day.

Hey, that’s not a bad list. I have lived in a number of rowhomes and apartments in Philly now and many did not have any of these passive cooling features. We were still able to keep cool enough without A/C by simply opening the windows on both ends of the home. Many homes, we only had a window A/C unit in our bedroom to make it a bit more comfortable to sleep at night.

There are a couple of key features in this list to me. I think the first is the very basic ceiling fans. By lowering the perceived temperature by three degrees when blowing directly on occupants, we now can raise the thermostat to 81-83 degrees (F). With all of the other features helping us out, I don’t see how we couldn’t keep the home at that temperature simply by naturally cooling the house with outside air during the night when it is 17-19 degrees (F) cooler on average in Philadelphia. It could be 100 degrees during the day, but it will most likely drop to near 80 during some point in the night and this is a worst case scenario for Philly that would not last more than 1-2 days.

LEED & Budget Impact

So how will this effect our budget? With the model of dehumidifier we have chosen, we have the potential to eliminate both the HRV and the Ductless Split Air Conditioners from our budget. These two items are currently taking between $8K – $10K of our budget. I am assuming that installing the Ultra-Aire will be much cheaper than this figure and will free up some HVAC budget that we can shift towards a solar thermal system to handle all of our domestic hot water demands year round. I’ll take it if I can get it!

The last thing we need to verify is that we will not be violating any LEED prerequisite by not installing a traditional A/C unit to cover the calculated cooling load of the home. I would hope not and I don’t see anything at first glance, but we have MaGrann verifying this for sure now.

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{ 30 comments… read them below or add one }

1 Mark March 26, 2008 at 4:17 pm

Those are some great tips, for sure. I’m sad the dehumidifier is for full-house systems. Have you come across any that I could add to a 125-year-old house in a retrofit? We have steam radiators, so forced air really isn’t an option.

2 Rob March 26, 2008 at 5:51 pm


I live in Lancaster, PA and completly agree with your assesment of the required cooling in this area, dehumidification is the key! I definitly think the (mostly) passive cooling and dehumidification that you are providing will be more than sufficient.

3 millhouse March 26, 2008 at 10:59 pm

I always think in terms of if I end up owning this thing forever, would I want “x”, in this case a/c. You also have to remember that the average home buyer is not going to go through the thought process you did they will see the pictures, think it is small but neat looking (you know what I mean) and it does not have blah, blah blah in this case a/c or a garage or “x” so you can not keep cutting things out of the project.Great design can only overcome so many objections. This is just my opinion and I sweat alot so take it witha grain of salt.

4 chad March 27, 2008 at 11:52 am

Thanks for the comments guys.

Mark – Check the site that the dehumidifier is linked to. They have some free-standing models (Santa Fe) that are not for whole house use that might help you.

Rob – Thanks for backing up the concept with local input!

Millhouse – Thanks for the opinion. Try to think of it not as a home without A/C but without traditional A/C. We are still conditioning the air by actively removing humidity just like any traditional A/C unit would do. Then we are simply cooling the air with a whole bunch of passive and mildly active methods. Also, the person buying these homes will be far from the average homebuyer which gives us a bit of leeway…

5 Brandon March 27, 2008 at 6:20 pm

I’m assuming that with the humidifier being a whole house system you’ll be needing to do some traditional duct work as would be done with a traditional A/C system correct? If that’s the case than it shouldn’t be too difficult for a future owner to just buy an A/C unit and have it hooked up if they really wanted.

Assuming my above assumption is true, you might want to think about the design in order to make it fairly easy for a future owner to make this upgrade if they so choose. I personally think the passive cooling system should suffice just fine, but a lot of people do love their A/C.

6 chad March 27, 2008 at 6:52 pm

With only 6″ and 10″ round inlets and outlets I’m not sure if we will be able to accommodate a future A/C system but it is a very good point and worth looking into.

I would consider putting A/C for free in the future if the client was unhappy with the system. This makes me a bit nervous though as it requires determining someone’s unhappiness levels…

7 Y. Bzerious March 28, 2008 at 12:39 am

As a former realtor and long-time Philly resident, I’d agree with millhouse: you’d really be making things more difficult for yourselves by foregoing traditional A/C.

8 chad March 28, 2008 at 1:49 am

Sometimes difficult is a nice change of pace from impossible.

9 dbomberg March 28, 2008 at 3:50 am

Thanks for posting you house plans/progress. My comments (..please take what’s useful and toss the rest)…
1. those whole-house dehumids cost about $2200. A Sears Kenmore free standing one goes for about $230. Your place will be small enough one for the bedrooms at night might do it.

2. The kitchen counter-bar area makes sense but is the area behind it -the 10ft distance to the wall-the eating area? With a table in there, and the small passage between the fridge and counter, you may be constantly running over your housemate especially if in a hurry. Suggest you move kitchen counter to the wall and have a table on rollers for dining. Then put the fridge in the corner- where it is now everytime it opens it blocks the passage to the kitchen.

3. If you can turn the bathtub lengthwise against the wall you will gain a few feet of more useable floorspace. Otherwise the narrow area along the tub is just for getting in it. This will be especially good if a couple both want to brush teeth at the same time. It’s so romantic! you might even fit 2 sinks that way. woo woo!

4. the location of the stairs creates a narrowing of both the upper and lower floor. If possible, locate it along the shortest wall at the entrance to the house. that way a loveseat can go under it and a coffee table creating a reading nook. Or make it a bookcase, or put the tv under it on floor 1.

5. I am not sure how much passive heating you will get in the summer unless there is more sunspace/windows on the south side.

6. The tall narrow windows are nice, but suggest at least one “picture window” visible from couch area or eating area. If the local views are not nice, put it high enough to see treetops, clouds, or even birds on an electric line.

7. Make sure the diagonal views through each floor are as open as possible, preferably with windows extending the diagonal outside. Makes for nicer small rooms.

Thanks for letting me kibitz!!


10 Rob March 28, 2008 at 2:14 pm

The dehumidifier does not appear to be for a ducted system, and I think the whole idea of this is to reduce (less ducts), not to allow the homeowner to increase. This is being marketed as a green house and if you are going to do it, do it right and make it a green house. Not a house that sucks a ton of electricty so that its residents can have it refrigerator cold during the summer. I have lived most of my life (about 23 of 28 years) in this same climate and I see no need for AC, I am however not normal.
People who are intersted in this house will want to live there for the green benifits and will have to understand that low cost green can be acheived but with mostly passive systems. This means that the inhabitants will need to participate and close blind or open windows as the day changes, its part of living a lowtech, green lifestyle. If they want it all done for them this is certianly not in the 100K budget to have completly active and hightech systems…Rant over.

11 chad March 31, 2008 at 11:55 am


You are right on with your assessment! Feel free to rant here anytime.

12 chad March 31, 2008 at 12:08 pm


I’ll try to address your questions as best I can:

1. Room units may be a consideration for supplements to an HRV or ERV. They do not bring in fresh air as the whole house unit does though, cannot be hooked into a central control system (programmable thermostat), can be unsightly and have to be manually emptied if a convenient drain is not near the unit.

2. Noted. Ten feet in the city is actually a pretty large space to put a narrow, long table.

3. This was the original design. The new design is a bit more unique and adds a modern flair to the bathroom. Two sinks is out of the questions as it is excessive for our design…

4. This would require entering directly into a bedroom and passing through it to get to the bathroom and second bedroom. This orientation was considered and passed on early in the design process.

5. We are actually right in the range of window space needed on the south with about 11% of the floor area of the house. The problem is our house is not truly south facing and is more like south-west.

6. Picture windows are going on the south side where we can afford more window area. We are already slightly over the amount we were targeting on the north side.

7. Good suggestion. We will consider before finalizing the window placement.

Thanks for the comments and welcome to the blog.

- Chad

13 Ben March 31, 2008 at 11:10 pm

Have you considered an indoor air exhaust fan placed high near the roof line with inlet air provided by low windows near ground level ?

The only concern I have for a NON AC house in the city is noise abatement issue. AC is used to overcome street and neighbor noise (dogs, traffic, basketball bouncing, kids) all common to row house living. Your SIP panels and new construction should minimize this as compared to a normal row home.

14 chad March 31, 2008 at 11:53 pm


We are trying to design the windows on the second floor of the south side to be twice the area of the opening windows on the ground floor of the North side. When both sets of windows are opened this will create a natural current from the bottom north windows to the top south windows.

We are also looking into cooling techniques using the HRV/ERV that will still allow all of the windows to be closed during the summer if desired. This is especially important at night when people want more quiet to sleep. During the day it most city dwellers become accustomed to the “city noise” and actually grow to like it…

15 Ben April 3, 2008 at 10:32 am


Will the windows be left open when the house is unoccupied ?

Is security an issue in the neighborhood ?

Will windows if left open be shielded from rain ?

What is the operating style on the lower windows on the street side of the house ? Casement/awning inward or outward operating ?

Just thoughts from a lifetime Philly row house owner.

16 chad April 4, 2008 at 3:28 pm

The windows will not need to be left open while unoccupied. The idea of the whole house ventilation system is that the exchange of fresh, cooler air can be done through a series of vents without ever needing to open the windows.

If the temperature and humidity levels are acceptable outside, then the homeowner could choose to manually override the system and open their windows for natural ventilation.

17 AZ_Dez_Dweller July 12, 2008 at 7:26 pm

Many thought provoke-n items n this thread.
In my area of the AZ Dez; – a V Simplified
(=inexpen$ive) modality Needs-Be employed
AZ follows:
A ‘non insulated’ `box` in which to store Long
`Shelf Life` items such as noodles/beans/canned-goods/etc Will be placed
in the HOT!-end of 1700ish sq ft
-3br/2 & 3/4ba SnglFamDet ‘stick-hse’.
- Basic goal here is to (in beginning) lighten-load
on 2.75 ton cntrl roof-top heat-pump, there-by
reducing it’s % of ‘on-time’ *$ignificantly!!!

-Peeps unAware of HOW-*EXTREME! climate is here
likely have paradigms so far away from
‘where I’m at’
-understanding probability is perhaps remote.

Thinking here is;
-the ‘cool-box’ having non-insulated
surface area exposed to `hot-end`
of hse living area,
there-by allowing *FarLess ‘on-time’
-for kWh $ucking
Central AC unit while also
‘$ignificantly Improving COMFORT Level
in ‘day-time’ main inhabited area.

How can I go about emplacement of ‘stand-alone’
‘cool-box’ *Completely! `Not-Connected`
to ANY Util co(corp) ‘input’
- AZ regards energy input.?.
-I inform:
Am 100% Disabled, `$urviving`
-Only on SS.
- That said:
My budget Has NO!(nor will have)
*Debt-Srvc, so – bk 2 budget:
W/ $2k avail, – For 1st stage:
(not incl ‘cool-bx’ portion),
*where 2begin / what ‘Pkg of component pts’
will yield maximum *Cooling Only btu input?

Once ROI is realized; – repeat of
‘cookie-cutter’ approach will-be in accelerating
modality. – “Ideal”;
- *After *Cooling is basically a ‘non-issue’;
-looking 4wrd, either expansion to $ell-Bk energy,
move 2 also including heating-ability is decision
yet future. Long-range desire is:
-To become Completely autonomous from ‘out-side’
elec conection/need of, but;
-at this pt;
*- only how-2 begin this V modest,
‘Cookie-Cutter’ approach;
- given limited means IS MY Q. – TIA

18 tracy July 15, 2008 at 10:42 pm

First, I grew up on Long Island and there was no A/C except in my parents’ room. Just with a table or floor fan in rooms. It all worked out.

Second, I have never enjoyed A/C. But living in Austin now, you have to have it with 95-100 degree heat from May to September. But if you can make this house work without A/C, with all those other checks and balances for a good temp and air circulation, then hell yeah and bravo! And I also love the idea of radiant heat vs. the central (blown air) heat and cooling I have now.

I think a part of eco-friendly homes is realizing that with innovation, there are some things we take for granted that we can actually do without. And that way we live greener.

19 chad July 15, 2008 at 11:01 pm

Thanks tracy! This comment is going on the list for the ‘best comments awards’ post I’ll have to put together at the end of the project.

20 tracy July 19, 2008 at 11:21 pm

on world’s greenest homes on Planet Green channel, they featured an Arizona home out in the desert and he explained their radiant cooling system. High 15 foot ceilings with water pipes up at the ceiling that sends cool air down as the desert heats up to 110. he has a green roof. and of course he has fans too. and he has some outdoor spaces with misters. the owner/creator said his home was the only one he knows about to use water to cool a home.

21 tracy July 19, 2008 at 11:23 pm

oh and gracias for making the “scrap book” for best comments. :) i think all the comments in here are awesome. you got some smart folks joining the dialog. right on to all!

22 Kevin D August 24, 2008 at 6:39 am

The title of this thread is a valid question, and it needs a scientifically logical answer. Your HVAC/Mechanical guys should be qualified to give it to you, but it isn’t a simple answer.

In addition to the scientific analysis, it helps to actually “live it” for a while, since personal comfort levels differ between individuals. The computer model cannot generate a number to quantify your “coolness” when you are lying naked on top of the covers with the ceiling fan on. By the way, that scenario should work all the time, and if it’s OK with the homeowner, then A/C is unnecessary.

My answer to the question is no, and there are two issues working against you.

1. Personal comfort is the intent here. A/C systems are designed to deliver personal comfort. Dehumidifiers are designed for a slightly different goal, which is drier indoor air.

2. During those two hot months, the nighttime temperature doesn’t always drop far enough to get the house to say, 76F by morning.

23 Grant September 21, 2008 at 8:44 pm

I’m trying to design a house in Alabama and found this page searching for “passive cooling” in a “humid” environment. Here in Alabama, I am afraid that complete “passive cooling” is not possible, but I do hope to combine ideas to greatly lower my active cooling costs.

I have designed a belvedere with a cupola on top in the center of the roof to function as a solar chimney to passively pull air through the house. I have designed a “passive cooling” and “active dehumidification” chamber in the north side basement of the house to condition incoming air. Fresh air can enter from basement well windows heavily shaded by the foundation plants on the cooler north side of the house be dehumidified in the conditioning chamber and pulled through the house by the air rising and exiting out of the solar chimney (with all other windows in the house closed).

With cooler air entering the basement and hotter air exiting the roof belvedere, the higher moisture capacity of the exiting hot air should help further control humidity in the house (passive dehumidification). I also intend to install active ventillation systems in the bathrooms, kitchen, and laundry room to pull humidity and tainted air out of the house at the source, before it can circulate through the house.

I learned a lot when I looked into tying this active ventillation system into a whole house ERV… I didn’t think I would need an ERV with the passive airflow generated by the solar chimney and the basement dehumidification of the “replacement” air.. So I figured I would just actively vent the air in these rooms and have the replacement air come through the basement the same as the replacement air for the solar chimney.

However, with the higher number of cooling degree days in Alabama, I will likely need an active cooling system for at least one month of the year, plus the house will require active heating in the winter. On those days where the house is sealed tight, I expect the ventillation system and intake replacement air (required to maintain indoor air quality when the house is sealed for active conditioning — heating or cooling) will be a HUGE waste of energy without an ERV.

Studying PassivHaus design I have learned that very little energy is required to maintain a comfortable steady state in a super-insulated (effective R of over 50) sealed house with limited interior thermal loads and which has an active ERV and dehumidification system. In fact, such a “tight” and energy efficient PassivHaus design utilizing an ERV with a dehumidifier, essentially makes passive cooling techniques, passive ventillation design, and thermal mass benefits redundant at best and likely useless since it ends up cheaper just to leave the house closed and allow the active systems to continuously operate… I have come to the conclusion that the best overall bang for the buck when it comes to energy efficiency and comfort is to invest in a tight, super-insulated envelope with an efficient ERV and dehumidification system.

After following PassivHaus design principles, a relatively inexpensive, (even energy-inefficient) supplemental cooling system will provide the only additional cooling you might ever need, if any, at a MUCH LOWER operating cost than even a highly-energy efficient multi-stage geothermal heat pump can when it is trying to cool and dehumidify a house that is bringing in fresh replacement air without an ERV/dehumidifier!

Thus, after I have put enormous effort into a very effective “passive” design to save energy, the heating and active cooling months which require a “sealed house” are ultimately forcing me towards an active ERV and dehumidification system and a super-insulated always closed “tight envelope” as my most energy efficient option!

With so many heating days in Pennsylvania requiring a “tightly closed” but well ventillated home, the ERV makes perfect sense. Just keep the house “closed” year round and let the ERV maintain a year-round “steady state.” Limit the heat energy from appliances and fixtures inside the tight envelope (low heat lighting, convection cook top, laundry in the garage outside of the ERV conditioned envelope, etc.), and allow the ERV to help cost-effectively maintain a year round “steady-state.” If you do this, you likely won’t need an active cooling system in Pennsylvania.

I will likely still need some active cooling down here in northern Alabama. But the radiant floor system used for heating my home can also be used to circulate a cool fluid during the summer to pull some additional heat out of the home. Properly designed, the solar hot water heaters (used to heat the raidant floor as well as the hot water) can do an excellent job at cooling fluids during the night time, and thereby dumping heat from my home over night! I will use high thermal mass to prevent temperature swings from occuring during the day and the night and across seasons, and to thereby allow the ERV to be even more efficient at maintaining a “steady state.”

Anyway, I’ve been deeply researching similar concepts as you and this is some of my thoughts so far. I look forward to hearing how your project turns out and learning from your experience. I likely won’t begin construction for about 18 months.

24 chad September 22, 2008 at 2:54 pm

Grant – Excellent comment. I am more and more convinced that the Passiv Haus standard is the starting point for any environmentally friendly home, affordable or not.

25 ken October 29, 2008 at 8:45 am

Slightly off topic but prompted by the posts about cooling in Austin and Arizona.
I too think the Passivhaus standard is the best starting point even though not so well developed for cooling climates. Living down under we have a cooling problem as well durng summer with temps occasionally hitting 45C (113F) and often 35C (95F). Natural ventilation like opening windows or solar chimneys are not always an option when its as hot as that plus ours is the hayfever capital of the world. Add to that city noises and controlled ventilation is the way to go.
I’m a little surprised to see that nobody (as far as I can tell) has applied to cooling the equivalent logic the Passivhaus Institut used for heating (see
I’m just an amateur but a quick look at a psychrometric chart and a few quick calculations tells me that the maximum you can drop the temperature of the incoming air is typically about 15K (27F) before you hit the dew point which could cause discomfort due to too low humidity in the fresh air stream. This translates into a cooling load of 5W/sq.m compared to the recommended 10W/sq.m heating load. This is fine if your outdoor versus indoor temperature in summer is half that in winter because if you have 10W/sq.m in winter you will also have 5W/sq.m in summer. In my case they are about the same so my cooling load is also 10W/sq.m so what to do?

Best option seems to be to double the ventilation rate (to say 60 cu.m/hr versus 30 cu.m/hr) to transport twice the “coolth” for the same temperature drop. In summer this increased air flow may also aid comfort. On the down side it means higher power consumption by the ventilation fans and more fan noise. Also means the HRV/ERV needs to have twice the capacity in summer than it does in winter and would be more expensive to acquire and operate. However,the higher flow rate only needs to be used when the outside temperature warrants it.

26 Jes November 20, 2008 at 5:17 pm

Kudos to you! As with any paradigm shift, there is always a fear of letting go of what one is comfortable with (a/c). Soon there will be many examples like your place to demonstrate the positive results of a passive cooling approach. With air flow, and lower humidity you’ve addressed two of the key aspects of perceived comfort.

Just curious – did you calculate actual heating and cooling loads considering the row-house/party wall construction? Living through a gut rehab of my own rowhouse, the party walls create quite a consistent atmosphere – even in the extreme Chicago winter months the interior never dropped below 48 degrees in the untempered portions of the house – or above 82 in cooling seasons. Though, I have joked that completion of our home may help out the neighbors utility bills ;-) .

27 Martin January 30, 2009 at 12:18 pm

A belated comment, but I just read your post with interest and noticed how you were adjusting 70F heating degree days to get HDD with a different base temperature.

I work on a website – – that automatically generates heating and cooling degree days to any base temperature.

It should save you some hassle and give you more accurate figures in the future. Hope you find it useful!

28 Larry Rose February 23, 2009 at 5:07 pm

It seem to me that this could be solved with a creative use of a modified dehumidifier, one where where the hot and cold coils are in different air streams.
Given the the air supply will be at ground temperature with a the earth tube design for the Philly Passive house, the incoming air will be around 55 F, and probably near saturation. The passivhous std only heats / cools the incoming air. If the incoming air bypasses the ERV and is rerouted so it passes through only the cold side coils of the dehumidifier, the moisture will drop out of the already cool, saturated air. cooling it further. The house exhaust also bypassing the ERV, instead passing over the hot side of the dehumidifier, effectively dumping its heat load outside. Both dehumidifying and cooling can be accomplished in this way

29 Larry Rose February 24, 2009 at 9:30 am

Hold on here, it occurred to me that what I described was nothing more than a heat pump in cooling mode

30 Fafa July 10, 2011 at 7:01 am

I need a tutorial of passive cooling & passive heating
please help me, thanks!

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