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Overheating from South Windows

During hot sunny weather, our second floor sometimes overheats

Image 1 of 2
Our house has extensive south-facing glazing. The ductless minsiplit unit is visible on the right side of the photo.
Image Credit: Andrea Lemon
Our house has extensive south-facing glazing. The ductless minsiplit unit is visible on the right side of the photo.
Image Credit: Andrea Lemon
A view of the south-facing windows from the exterior.
Image Credit: Andrea Lemon

For the past two years, Ted Lemon and Andrea Lemon have been living in a single-family Passivhaus which they built in Brattleboro, Vermont. Ted Lemon wrote the essay below in July 2012.

Andrea and I recently started sleeping in the house, so some things that I’ve been noticing about the heating profile of the house are starting to become clear.

When we finally got the HRV running a few weeks ago, I was disappointed to notice that we don’t have the kind of evenness of temperature between the upper and lower stories of the house that we were hoping for. My first panicked theory about this was that the HRV wasn’t working. Panicked, because the ducts are all nice and snug behind walls now, so if we got it wrong, it’s going to be a real pain to fix.

The good news is that I don’t think the problem is the HRV, although I’m contemplating one tweak to the vent layout which we could do without any wall surgery.

Our original plan included exterior louvers

When we talked with Peter Schneider (one of the two energy consultants on our project) about the possibility of overheating, he reported his experience with some of the houses that Efficiency Vermont has designed up in Charlotte, Vermont. These all have lots of south-facing windows, and Peter hasn’t seen problems with overheating in the summer. Based on Peter’s reports, we were kind of hoping to get away without doing one of the features we’ve got on the plan: active shading on the south-facing windows.

If you look at the drawing of the house shown below, which is a view of the south face, you’ll see that the lower windows have what looks like louvers on them.

These are part of the solar shading plan. The idea is to install the louvers in spring and keep them around until fall, and then take them off and stash them for the winter. These louvers will prevent high-angle light from making it through the window in the summer and heating the interior space. By installing them on the outside, we minimize heat gain inside — hopefully most of the heat that is generated when sunlight is absorbed by these shades will re-radiate into the outside air, rather than heating the inside of the house.

We specified high-solar-gain glazing

We’re experiencing some overheating. Why is Peter seeing different results? I don’t know, but I have a theory. I think the windows in the houses in Charlotte may not have the same solar heat gain coefficient as ours, and may reflect more high-incidence light, while letting low-incidence light in, so that they gain more heat in the winter than they do in the summer, even if the same amount of light is hitting them.

The way our house is set up, we have a heat pump indoor unit on the wall downstairs, right in the center of the house. We have nothing upstairs. So the reason I was worried that the HRV wasn’t working is that one possible interpretation of the data is that we’re cooling the house adequately, but the HRV isn’t doing its job in redistributing the heat evenly throughout the house. If that’s the case, it’s kind of a big problem.

But having just spent the morning sitting down next to the windows with the heat pump off, I have a different theory. For most of the morning, it was nice and cool downstairs, but as more sun came in and the day went on, it started to get hot, just as it is upstairs in the afternoon. In other words, the heat coming in from the windows is heating the upstairs and downstairs evenly; the reason that it feels hotter upstairs is because the cooling effect of the heat pump completely counteracts the heating effect of the windows. I’ve confirmed this by turning the heat pump back on.

So this gives me some real confidence that when we get around to building and installing the louvers, we will stop experiencing overheating in the south part of the house. I’m still a little tempted to add one more ventilation supply vent (fresh air grille) on the south side of the house upstairs, but that’s something I’m hoping to have a chance to debate with the guys at Zehnder. If it needs to be done, it’s a really easy fix, because I can do it up in the utility loft, which doesn’t have a finished floor.

A recent update

In August 2014, Ted Lemon wrote the following update on the overheating issue.

Over the past two years, we’ve learned that there isn’t a problem with overheating in spring. As summer approaches, before it gets really hot, night flushing is completely adequate to keep the house comfortable.

As the summer progresses, the sun gets high enough that there is minimal solar gain. Not none, but minimal. This turns out to be beneficial, because summers in Vermont are humid, but not much hotter than our preferred indoor temperature. With a little solar gain through the windows, we wind up running the heat pump in air conditioning mode enough to bring down the humidity; without the solar gain it would get too cold.

There is a bit of an issue with overheating in late summer, when the sun is lower but it’s still warm out. Louvers might help a bit with that, but I think we’re talking about somewhere between two weeks and a month, and as the weather gets crisper, night flushing should start to be effective again.

We were having trouble with overheating upstairs last summer, and I started to use a box fan laid flat to push cold air up and basically stir and mix the air in the house. This turned out to be quite effective. So partway through the summer we had Bill Congleton install a Haiku fan in the atrium. This draws about 35 watts on full blast, and we don’t generally run it on full blast because it isn’t necessary and creates too much of a breeze. Once we had the fan in, we haven’t had any problems with temperature gradients — it’s about the same temperature upstairs as down. The fan’s very pretty, too.

Low indoor humidity during the winter

The bad news is that we’re having problems with low indoor humidity during the winter. We’d originally imagined that because we were sealing the house so tightly, we’d have indoor humidity issues, but that was based on an incorrect understanding of the reason why houses get humid: lack of air exchanges. With the HRV running at 30% or 60% of capacity most of the winter, we were seeing humidity getting down into the 20-30% range, which was uncomfortable.

This is the one big failing of the fancy HVAC arrangement so far. It’s actually quite hard to counteract because there are so many point sources of air and so many exhaust ducts that there’s no good place to install a humidifier where it will affect the whole house other than inline with the HRV.

We talked with Zehnder about this, but they have had bad experiences with whole-house humidifiers causing condensation in the ducts when customers have installed them, so they strongly recommend against the installation of a humidifier.

I think it would be possible to have a very tightly controlled humidification system coupled with an HRV that wouldn’t cause condensation because it would be sensitive to the inner temperature of the house and the humidity of the input air, but that appears to be a product waiting on someone to decide to make it.

We can get the humidity up a bit by drying clothes indoors, but we don’t do enough laundry for this to be a complete solution to the problem. So this is our little bit of remaining future work; I have a few ideas, but nothing concrete enough to talk about yet.

Ted Lemon lives in Brattleboro, Vermont. He is a computer programmer and network expert who works for Nominum Inc. and is on the administrative/editorial board of the Internet Engineering Task Force. Ted and Andrea Lemon write the Vermont Passive House blog (formerly known as the “Almost Passive House” blog).


  1. GBA Editor
    Martin Holladay | | #1

    Indoor air is dry during the winter
    Since your house is designed to meet the Passivhaus standard, I'm surprised that you are having a problem with dry indoor air during the winter.

    My first guess is that you are overventilating. Have you tried operating the Zehnder HRV for fewer hours per day?

  2. user-1135248 | | #2

    reflect it away
    How about Reflectix shades you can drop down inside the big
    windows during the afternoons? Works for me. Available in
    big rolls from HD.


  3. tallywa | | #3

    Dry winter air
    We have tried everything with the Zehnder to get the air to be more humid. We switched from an HRV to an ERV core, we run it at its lowest setting. We do not turn it off—that's actually not so easy to do. Furthermore, the air quality suffers a bit when we run it at its lowest setting: there are still some volatiles outgassing after two years. The goal was to have clean, fresh air in the house, so solutions that compromise that aren't attractive.

    Hobbit, the problem with shades on the inside is that they do not affect the amount of heat that comes in; only the amount of light. I don't mean to say that they can't ever be useful for mitigating overheating, but in our situation we have high-performance windows and a very high performance wall, so dumping heat into the windows and the wall is mostly going to result in that heat re-radiating into the living space, possibly over a longer period of time, but ultimately to the same effect, because we don't lose much heat overnight.

  4. tallywa | | #4

    Actually, we've been slowly finishing the interior of the house, so now that I think of it some of the volatiles are from surface treatments that occurred even in the past six months... :)

  5. user-626934 | | #5

    ERV v. HRV

    I agree with Martin, I'm surprised you have low humidity in the winters...especially after trying out an ERV core. If you used the Zehnder Comfo 350ERV, it has a rated winter moisture transfer rate of about 60%. You should have seen an IMMEDIATE difference if other conditions remained more or less the same (air change rate, outdoor moisture and indoor moisture generation).

  6. ahamel | | #6

    Overheating and humidity
    Hey all

    We also live in a house designed to meet the passive house standard. We've installed the Zehnder Paul Novus 300 ERV and we have no problems to maintain a humidity rate between 35 and 40% in winter, The house is very tight and the walls are built to serve as hygro buffers, they actually are very efficient to keep the ratio quite stable in all seasons. We use a daikin quaternity (9000 BTU) to help in summer as it have a deshumidification only feature.

    We design the house with oversized roof overhang and motorized louvers for the second floor and i built some nice Stainless and wood sun breakers for the first floor.

    Every passive houses have some overheating in spring and fall as the weather is warmer and the sun still lower but this is a flaw we can easily control or live with, it just depend where is your comfort zone. I'm quite confused about your humidity problems, normally if you are under .06 ACH as you are supposed to be and there is two or three peoples living in the house, who produce humidity daily with showers and cooking, the Zehnder ERV should not have any problems to maintain appropriate humidity conditions.

    The only thing that can keep this kind of dry conditions is air leakage, there is a flaw somewhere in the building envelope. Just run another blower door test to be sure, in negative and positive pressure, If normal amount of humidity is produced by the occupants, it's the only way humidity can escape.

    Which kind of envelope design did you used, how was your blower door test result and when, during the construction process and under what kind of exterior and interior conditions did you do it.

    It's impossible that the problem causes are mechanical, do a simple recommissioning on the envelope and install appropriate shading devices (outside) and everything should get back to normal.


    Hey Martin, how can we post photos on the blog ?

  7. GBA Editor
    Martin Holladay | | #7

    Response to Alain Hamel
    To attach a photo, click the words "File attachments" that appear under the comment box. Then click "Browse" to find the file on your computer that you wish to attach. After you select the photo, click "Attach," and then finally click "Post." Let me know if these instructions are unclear.

  8. wjrobinson | | #8

    You can take advantage of
    You can take advantage of your situation and add enough plants to remedy.

  9. ahamel | | #9

    File attachments
    Thanks Martin

    Didn't saw it, now, it jump right at my face...

  10. wjrobinson | | #10

    Too much glazing
    Too much glazing upstairs.

    Need ability to turn off Zehnder.

    Should use products with less VOCs.

    Should not live in a home sealed up tight while applying VOC off gassing products

    New construction needs time airing out. Open up a home windows etc for months... Relatives of mine just moved into new construction, and the smell of new was very apparent.

  11. rjparker | | #11

    Central Humidifier Not Essential
    A standalone humidifier can work as the humidity in a house will equalize on it's own. Central is nice with proper installation since it can be permanently installed and plumbed to a water supply. But, like drying clothes inside, a standalone humidifier will raise the humidity level.

  12. user-3549882 | | #12

    High Solar Gain, Overheating (South), and Too dry in winter
    Some things to consider:

    - In the photo, there's a nice work area next to the windows at the lower level. Is it too hot from the room temp? or humidity? or from direct and/or reflected sunlight? Infrared (IR) heating will make it feel overheated even when the temp and humidity are OK.
    - If you shield yourself from the IR temporarily, does it still feel too hot? (Give it a couple of minutes.)
    - Is the direct sunlight a good work condition for you? (I prefer diffused light for desk work.)
    - Aside: I don't see any curtains. That's a good look. Sometimes interior diffusers can enhance the work environment though they likely do little for energy efficiency.

    It sounds like the Haiku fan is reducing the temperature gap between the levels but at a cost. I'd expect about a +2 to +4F higher temp at the upper level as normal and would not use electricity to overcome this.

    - It sounds like issues of appearance, view, convenience, and initial cost make louvers undesirable. Guilty and cannot be proven innocent.

    - Louvers can keep out unwanted IR with no energy penalty. In fact, a/c electricity will decline.
    - Louvers will deal with direct sunlight and with reflected light from the exterior surroundings.
    - Louvers are designed to reflect both IR and visible light (VL) leaving the room cooler and darker. Solar grates reflect VL inward while absorbing much of the IR, leaving the room cool and bright with diffused VL and some residual IR. Grates are often translucent, providing even more VL whereas louvers are usually non-translucent.
    - Solar screens are another option.
    - Louvers can be permanent or seasonal. They can be stored in place (shutters) or removed for storage. For energy efficiency they should be deployed during a/c season.
    - Your upper windows appear already partially protected by the PV awning shown in the diagram. Louvers on upper windows may not be cost justified and are more effort to remove. The saving available from louvers there depends on sunlight exposure and area
    - The louvers/grates need not be an all-or-nothing decision. Some windows can be left uncovered to maintain a valued view. A louver can have gaps or be installed 'cafe' style.
    - Grates turn a high shgc window in winter into a low shgc window of summer. Install and forget for the season.
    - With less direct lighting, the temp unevenness is automatically improved and any a/c needed can run whenever desired.
    - House flushing can still be used but it will be needed less.
    - Even with low shgc windows, homes in Charlotte, VT might still have some issues with summer direct heating from sunlight. Charlotte invites more understanding.
    - Exterior devices such as louvers, grates, and shutters are beautiful in their function. We just need to recognize the ugly duckling as a swan.

    - This condition is normal for an old, legacy, leaky house.
    - If we have to run the hp to depress summer humidity and then heat the air to keep it from being too cold, it sounds like the hp is running too long.
    - If the winter dryness is not due to condensate somewhere, it sounds like too much cold, dry air is being introduced. Since the house is super sealed, the ventilator should relax. Save some electricity. Save on that new humidifier you've been coveting.
    - Consider what happens on cloudy, humid summer days. The hp would normally trigger with temp. With clouds, the temp changes only slowly and the room should still feel nice from last night. If it feels humid, another sign the HRV is working too hard.

  13. jackofalltrades777 | | #13

    Window SHGC
    What is the window SHGC number for the south facing elevation? Are those Intus uPVC triple-pane windows?

  14. gusfhb | | #14

    Low humidity
    really the only

    Low humidity

    really the only answer is you are ventilating too much. enough painting already


    I have to cover my skylights with solar screening from The Despot every summer. Yeah, not a great plan but not the end of the world either. One could fashion awnings from such that could be operated from the ground and deployed daily if you felt like it. Wouldn't catch the wind either

    I am a fan of trees that drop their leaves early, what poplar maybe?

  15. jinmtvt | | #15

    Ted ...
    I do not wish to sound rude , but i do not understand why there is no proper overhand and sunshades for the first floor.

    You installed high SHG windows, fine.
    PH insulated house, fine.

    But then you let the sun in almost without any shading for year long ?

    From what i can see on the pics you've linked .

    1- your roof overhand is too short for the windows height and position , but at least existant.

    2- you should've installed either a cheap fixed faux-toit ( sorry do not know how it is called in english ..Martin a little help here please ?:p )
    or install some more PV panels on some custom fixtures, or at worst get some restaurant style stretched fabric on some cheap tubes , that could've been folded up during winter time.

    This is a very basic design error, and it should be adressed BEFORE working on "confort" enhancements from the interior.

    you are at approx 42deg of latt? something in the vincinity of 40 to 50deg ( 90deg being vertical ) of shading to all south windows should've been worked in the design.

    But now you do know you are getting too much sun in the summer
    ( do you enjoy the direct glare?? i sure don't )
    So choose a method, and install a system.

  16. Nick_Grant | | #16

    HRV won't distribute heat
    I think the comments contain the answers. I did wonder though if windows are opened in cold clear weather in winter in order to dump heat which would lead to dry air.

    As it is such a common fallacy it is worth mentioning that HRV can't be used to usefully redistribute heat,

    Say 30m3 per hour ventilation to an area X 0.33W per m3 per hour per K X say 3K temp difference = 30W

    Say this is venting a living room of 30m2 = 1W/m2 compared to peak heat load of 10W/m2.

    Someone else can translate this into heritage units and you can use your own assumptions.

    Whilst high solar gain glass (say 60%+) does give a couple of kWh/(m2.a) energy balance benefit in typical house we try and hit the PH target with 50% g if we can, especially for non domestic buildings such as schools. Window energy ratings in the UK do however favour high g which is pushing the market that way.

    Also we try and avoid using the windows as a heating system, too expensive and difficult to control - see other blog posts on this site.

  17. tallywa | | #17

    Wow, lots of opinions.
    A couple of observations. First of all, the Zehnder spec says "up to 65% moisture recovery" which sounds a little vague. But suppose you have five air exchanges a day, and are not putting much moisture into the air. Remember that the air coming in is cold, and is being heated by the ERV. So the moisture level in the air is going to be really low. I'm going to claim that it's completely dry for the sake of easy math, although I'm sure there is some moisture in it. So if you start out at 40% humidity at the beginning of the day, you'll be at 11% at the end of the day. The residual moisture in the intake air, plus whatever moisture is added to the air in the house, keeps it in the 20% range, but that's too dry.

    Several people have suggested using fewer air exchanges. This is a hair shirt strategy. The point of building an energy efficient house, at least for us, was not so that we could freeze or suffocate at a lower cost: it was so that we could be genuinely comfortable, with comfortable indoor air, both in terms of temperature and in terms of humidity, without presenting an undue burden in terms of carbon production.

    If we doubled our solar capacity, we would be comfortably past zero energy for this house; at our current capacity, we are pretty close, and since we can source power in Vermont from carbon-neutral methane, we don't feel a strong urgency to adding capacity—we're waiting for newer technology.

    We can achieve both comfortable temperatures and comfortable humidity, but achieving the humidity we want is expensive in time and to some degree energy, because point-source humidifiers are really inefficient, have to be refilled, and use wicks that have to be replaced regularly. To keep the house comfortably humid during the winter involves running a large humidifier full blast at about a 50% duty cycle, or less than full blast at a 100% duty cycle. This consumes something like 60 watts on full blast, and is quite noisy. I'm sure this can be done better; the reason it hasn't been is that most people think 60 watts continuous isn't bad.

    An additional inefficienty with point source humidifiers is that we are depending on the humidity of the return air: most of the moisture produced by the humidifier gets pulled into the ERV return vents, so there's an immediate 35% loss of humidity when it's returned. A central humidifier would be putting the moisture in at the supply vents, so there would be no such loss; although of course the moisture would be pulled out in the natural course of events, we'd get a much more even distribution. Our supply vents are in the very rooms we spend the most time in, so having those be the driest rooms in the house means that we need to make the humidity at the point source that much greater.

    On the topic of plants: we have a fairly substantial hydroponic garden in winter. Unfortunately that isn't a very helpful source of moisture, because the room it's in has a return vent. I am occasionally tempted to put a tree in the atrium, but I think it wouldn't survive the summers, because during summer it would be in shade all the time due to the roof overhang.

    Someone suggested that our problem is that the house isn't tightly sealed. It is actually very tightly sealed, and we have excellent thermal performance. Our blower door number was 0.29 at ACH50. The belief that it is sealing that is the issue is one that I succumbed to when we were building the house; that's why I was so surprised by the actual humidity numbers when the house was done. In fact, it doesn't matter how the air leaves and enters the house, whether it's because the house is poorly sealed, or just well-ventilated using mechanical ventilation. In either case, you are pulling in dry winter air, heating it to make it even more dry, and then mixing it with more humid interior air. The ERV counts as heating, just as if the air were being heated by a forced-air furnace, so despite being a much more energy-efficient way of keeping the inside air warm, it has the exact same effect on humidity. The enthalpy transfer helps a great deal, of course, but it doesn't solve the problem—it just mitigates it.

    Someone also suggested that leaving the thermal gradient in place was a good idea. I disagree. In this scenario, in order to keep the upstairs comfortable, we would have to make the downstairs 4 degrees cooler. In addition to making the downstairs uncomfortable, this would consume more energy, since it would require more cooling. The Haiku fan has a DC motor and is extremely efficient: at maximum power, it's consuming 35 watts, and we never run it at maximum unless I just want to cool off after a summer workout. We don't run it much in the winter, because the temperature gradient in the winter isn't significant.

    Someone also suggested that we overglazed. Our south windows have an SHGC of .6 and a whole-window U value of 0.14, center of glass U value is 0.10. The windows are Schuco windows, because we wanted in-swing windows and high-performance in-swing windows were not available from a North American manufacturer when we were shopping (the high-efficiency window market has improved a bit since then).

    With these windows, we get a great deal of benefit from solar heating in the winter. They are not oversized for the available thermal mass inside the building. The criticism with respect to sun-drenching is perhaps valid, but the bright space in winter is very pleasant, and aside from the humidity issue, both Andrea's and my experience of the house in winter is that it is exceptionally pleasant. If you look at the picture, there are several shaded areas that are connected to the atrium space, so if you want to be exposed to the bright light without sitting in the direct sunlight, this is not a problem.

    I get the impression that some folks reading the article have concluded that we are dissatisfied with the outcome of our building project. This is not the case. The house is really wonderful—the most comfortable living space I have ever inhabited. But it is not perfect, and I think there are a relatively few tweaks that, were the technology available, would make it even better. To me, it's an ongoing experiment, not a final result. If Zehnder does come up with a solution that adds humidity, or one of their competitors decides to do so, and I believe that it will work without creating condensation in the tubes, I'd be really interested in trying it. None of my carping about the ERV should be taken as an indication that I think it's a bad solution. It's a great solution. It's just not as complete as I think it could be.

  18. RZR | | #18

    Interior Mass
    @ Alain "Hygro buffer walls" care to elaborate? What material?

    @ Ted what material are your walls and ceiling made of common to conditioned interior air? What design tool did you use to determine moisture levels?

  19. Brian Knight | | #19

    Optimally sized overhangs?
    The low wintertime humidity is peculiar and in my opinion, envious. Do the baths have dedicated spot exhaust? Might help skipping that when too dry or close the ERV exhaust vents there if not.

    The summertime overheating seems to be the bigger problem of course. As to Jin's question.. I know northern builders often skip lower level overhangs, was this an aesthetic decision? The top level could have been better sized for unwanted gain it seems?

    Function wise, I prefer properly sized overhangs to anything removable or that needs to be adjusted. Future homeowners wont always get it. Great job on your home though! Thank you for the useful details.

  20. tallywa | | #20

    Buffer walls and bathroom exhausts
    We have ERV return vents in the bathrooms, so there's no way to reverse those in the winter without rebalancing the system. Which might be tempting, but we didn't balance the ERV ourselves—Aubrey from Zehnder came out and did it. So that would be a bit of a learning curve. And I'd be tempted to actually reverse them rather than closing them, since otherwise we'd have to leave the bathroom doors open all the time to get a good transfer of moisture out of the bathrooms into the rest of the living space.

    Overheating is not a big problem for us in the summer, as I said in the blog. We have a pretty massive overhang on the south wall of the house, but it's a tall wall—we'd probably need a 12' overhang to shade the lower windows in summer, and that would have been a nightmare from a structural perspective, in addition to being quite ugly.

    Lower level overhangs would have compromised heat gain in the winter and spring, and also would have looked ugly. Given that we've been able to make good use of the summer heat gain to control humidity, it would have been a shame if we'd been too proactive in sizing our overhang. What we have is just right on all but a few days of the year. The only way to attain perfection would be with an active shading system, and we were strongly discouraged by our builder and our energy consultants from doing that. Rightly so—it would have been a complete boondoggle.

    Our walls are 9 1/2" dense-pack cellulose insulation with drywall inside, zip sheathing outside, taped, 4" of polyisocyanurate on top of that, then 1/2" vertical strapping 24" on center with FSC-certified cedar cladding imported from northern Vermont. The studs are i-joists stood on end to minimize thermal bridging while providing plenty of strength. The ceiling is constructed similarly, but has a moisture barrier on the back-side of the drywall to slow the movement of moisture into the insulation. The insulation is 24" of dense-pack cellulose, zip sheathing on top, and then a vented roof on top of that. Possible overkill, but when we designed the house I was worried about moisture getting into the insulated space.

    We haven't done any moisture modeling other than the in-wall dew point calculations Mark Rosenbaum did before we decided on the wall construction. I just have a cheap, not terribly accurate digital hygrometer, plus my flaking skin, to guess the approximate moisture level.

  21. jinmtvt | | #21

    You are not overheating during summer because your AC runs full time.
    Do not try and tell me that your 0.6SHG full unshaded south wall isn't causing any discomfort during summer time. Believe me , i've been there .

    Why do you all through the trouble of designing a PH house, installing PV system, getting Deutsch windows , but skimp on a very simple basic detail such as sun shading.

    Let me explain it simply.

    Your roof overhand is large enough, but your windows are too low for it to be really effective,
    but at least its there. Could've been better, but you must get some shading from 11-14h during summer.

    You never want to extend a second floor overhang so it covers first floor windows, obviously.

    First floor needs to be treated independently, using either a fixed false/roof or some other method of shading.

    Then you say, you would loose benefits during winter ..mmm no.

    Take some time to verify sun angles for your heating months,
    and you will be surprise as to how LOW it goes up to.

    Lets say you set for blocking sun at higher than 45degree, then you'd need to choose a position within the window to do this, and any time the sun would be lower than 45degree , it would obviously be NOT BLOCKED ( cie all through heating season )
    That is why you DESIGN overhangs and sunshades .

    Since you already have your windows installed and height cannot be changed
    ( using wider but smaller windows helps with overhang size )
    You either need to have and adjustable/removable shade, or choose a compromise with a fixed one.

    If you don't wish to install sun shades on the first floor, be my guest.
    But it shows as to how you missed a critical step in your house design .
    ( cie basic thermal control BEFORE indoor climate control )

    Other than that, nice project! bad color :p

    ah yeah, i agree to wait for cheaper solar solutions to upgrade for now.

    my last .25kwh

  22. ahamel | | #22

    Shading and my walls details
    This is my shading strategy and it's working very well and not so ugly ;-)
    I attach a PDF of my walls details so you can visualize how we are able to buffer humidity, in fact RH never go under 35% and average around 40% most of the time. The paul Novus 300 F have a humidity recovering rate of 0.73.

  23. RZR | | #23

    @ Alain you wrote: "and the
    @ Alain you wrote: "and the walls are built to serve as hygro buffers, they actually are very efficient to keep the ratio quite stable in all seasons".

    The only way that is possible is if you used 30% clay such as kaolinite and illite or simulr substitute as a binder in place of OPC. Just because your ERV solved your humidity control does not mean it will in any home, any climate zone in the world. Nice overhangs and stainless btw, I do not think that is Ted issue since they do not add breath moisture in the winter.

    I like this hot climate Arizona, USA builder bull nose wing reflector deck made out of stainless plate to paint the interior with soft light on the radial roof. The home is poured-earth. Very nice!

    He has other passive heating and cooling methods including drop down shades on his site:

  24. RZR | | #24

    Does Vermont have alot of
    Does Vermont have alot of German's or what? ;) I read of alot of passive homes with moisture problems there. There is one in the Q&A section overheating with moisture control and storage issues that used drywall too.

    I think the words we need to get more familiar with in material design is "hydrophilic" and "moisture content". Clay plaster is a good example, contributed to pore size, a fundamental component is the incredible relationship it has with water. It’s catatonic charged nature makes it highly hydrophobic due to the fact it bonds electromagnetically with the polar water molecules quite readily. Coupling this with clay's high surface area, it’s molecular form is plate-like. It attracts and holds large amounts of water without deformation.

    Man made gypsum along with the inert fillers found in most drywall does not have these properties, high levels of dense portland cement, sand, rock, found in concrete no better. It has a very low moisture content and ability to store, hold, and release it. It breaks down in the presence of moisture.

    You read alot about "dew point" and "perm rating" design concerns which is very short sited, when in fact, as noted, there are other properties to consider when selecting a material that is in direct contact with interior conditioned air.

    With drywall, moisture infiltrated naturally or mechanically has no storage system and is ex-filtrated at the same rate. An ERV is not smart in the sense that has a moisture content input from interior mass senors, so it will not solve the issue. If it had the capacity to keep up with the moisture drops in winter it would need to run non stop, if high heat and humidity are the issue the AC would need run non stop, we have seen that situation in passive homes too often out here.

    Tight construction has no way to naturally infiltrate moisture in winter, nor ex-filtrate it in summer, nor balance it with nature and outdoor clay-trees, we need a mechanical means depending on the home design and climate. It is VERY difficult to model this not only for moisture content but, it's effect on heating and cooling loads. Most get it wrong or do not consider it hence the issues. All you can do is consider the moisture content and hydrophillic properties of materials that create a dynamic moisture storage system and dynamic balancing system in the home using interior mass, and in some cases add interior-exterior water falls, humidifiers, ervs, combo's....

    Here are a couple of video's to demonstrate the upfront planning for moisture control. Passive homes can be perfect if you design them right, consider moisture and IAQ. Drywall and most conventional manufactured material data sheets show they fall short of quality interior mass as described above.

    All you can do is resolve by mechanical means and costly electrical loads. If you have room you could add an internal water fall, or solar heated plant-water fall room addition shown in video #2 but, you still have the cost of a water pump vs humidifier.

  25. Brian Knight | | #25

    Terry, Iam sure GBA would let you write your very own article on the magic of clay plaster.

    I still agree with Jin and that maybe a better overhang strategy might have led to a different title of this blog. Passive solar designs will always experience some overheating, usually in the very late summer and fall but overhangs above each floor level will do much to mitigate it. Shameless examples:

  26. tallywa | | #26

    Shading design...
    Alain, your shading design is indeed very attractive, and we'd been planning to do something similar, but concluded that it was not necessary.

    Jin, I don't like to live in an overly humid house. Vermont summers get very humid. So yes, the air conditioning is running, and yes, it is acting to bring the temperature down, and yes, if the window were more shaded, it could run less and produce the same indoor temperature. You are quite correct. But it would be quite a bit more humid inside the house, and would be uncomfortable.

    One of my goals of building our house was that I wanted to showcase the fact that an energy-efficient low-carbon house (as opposed to an inefficient house with a giant rack of solar panels) could be _more_ comfortable than a traditional house, not less. I think we have achieved that. I think that pursuing the strategy of convincing people to just accept discomfort isn't the right one for the U.S.: at present, we can be comfortable in our poorly insulated homes, or comfortable in our Passivhaus-style homes, and I would like to argue for the latter, not the former.

  27. Marc Rosenbaum | | #27

    Using PHPP in the North sometimes results in overglazing
    Here are a few observations: HRVs are not intended to even temperatures out around the house, period. Even if they were, the air flow rates are way too low to be effective.

    Using PHPP to meet the 4.75 heating criterion often means homes in the northern tier have more glazing than optimum, because the software doesn't prioritize cooling load as much, being a German product. The Lemon house has 218 s.f. of net south glazing for a 2,392 gross s.f. house with no mass (it's on piers). I think of my house as slightly overglazed, and it is 1,000 s.f. smaller and I have about 111 s.f. of net south glazing.

    Dehumidification isn't possible without a closed-up house, and in that case usually the house will overheat if it doesn't have some cooling. Ted and Andrea could install a small stand-alone dehumidifier (the best ones are from Thermastor) and use that in concert with the heat pump to manage RH separately from temperature, but it will be used marginally in their climate.

    Concerning the dry indoor air during the winter: I'm wondering what the ventilation rate is. The Passivhaus Institut tends to over-ventilate in my opinion, using 0.35 ACH as a guideline. In this house with just 2 people, that's too much. I balanced my house to 50 CFM with 2 occupants and the winter RH bounces depending on indoor temp between high 20s to low 40s - just right. In the plot below you can see the inverse relationship between temperature and relative humidity, and also see that I have too much south glazing :-)


  28. tallywa | | #28

    CFM ratings on Zehnder
    Our ERV has three settings. On the low setting, it's drawing 67cfm. On medium, 110. On high, 156.

    One of interesting quality of the ERV when it's on the medium setting is that I can smell the outside air. This is a really nice feature, although I will freely admit that it has nothing to do with energy efficiency, and may make allergy season worse for me. :)

  29. RZR | | #29

    If you took the time to watch
    If you took the time to watch Rock and Roll Handy Randy Bachman's from BTO narrative of his home in the video I posted, his home has a natural smell of the outdoors all the time because he recreated it inside his home, his home solved his allergy problems, he has indoor moisture regulating system that does not rely entirely of ventilation or ERV rates, nor CFM, and the material he used does not get any greener or non toxic, low cost to boot.

    German design tools are created by German's for Germany, we have different natural and manufactured products here in the USA. Why would anyone expect to use a design tool that was never created with a USA bill of materials in mind and expect to have accurate results. As if determining heat flux and moisture loads in not already challenging enough. Until there is way to parameterize materials(which includes specific heat, moisture content, plasticity, expansion, attierburg limits, thickness, density, perm rating, dew temp, etc) diurnal RH, it will never be an accurate tool. To do that the design tool has to be tested and back calibrated to actual data like the above. It really boils down to local resources and an in depth understanding of their mechanical, thermal, properties and how moisture interacts with those properties. I could probably guess as well or better than PHPP just by looking at material data sheets, as Randy's builder did. It takes years to gain the knowledge in mechanics of materials, chemistry, and physics, these design tools have difficulty getting right. The tool is only as good as the user, trash in, trash out.

  30. tallywa | | #30

    It produced good enough results to get us where we are. What I would like to see is an open source tool that does the same thing, and that can be extended as you have described. The fact that it's a proprietary excel spreadsheet makes it pretty inflexible: not only is excel basically write-only, but if you modified it you couldn't share the modifications.

  31. jinmtvt | | #31

    Ted and RH.
    I understand what you mean , we have very humid summers here also ( very similar climate to yours here in central Quebec ) and it is hard to get rid of the moisture
    because the AC does not run enough.

    But then, it is not normal to try and get 40%RH when the outside RH bouces between 60% and 80% all summer.
    Use standalone dehumidifier instead of the sun,
    they remove moisture and produce some heat, which is then compensated by more AC which removes more moisture.

    Sun glares may be comfortable during winter time,
    but i don't get how you can endure such heating rays while eating on your nice wooden table during summer time.

    Alain: nice stuff !

    I am jalous of your nice window shutter, but what is the ROI on those ?
    ( other than meeting PH levels :p )

    Nice mix of materials though , i like it much!
    ( manque seulement un peu de plantes grimpantes pour compenser les parties industrielles! )

  32. jackofalltrades777 | | #32

    Alain Hamel
    Where did you get those exterior louvers from? It looks like commercial based stuff.

  33. jackofalltrades777 | | #33

    Marc Rosenbaum & Terry Lee
    Based on the homes square footage to south glazing ratio, the ratio is around 9%, which is about the average for a PH. I usually see it fall within the range of 7%-12% of square footage to south glazing ratio. What becomes vital is the SHGC of the south glazing. If a 9% glazing ratio has a SHGC of 0.60 versus 0.40, the difference in heat gain can be significant. Of course, overheating in the spring - fall is based on a lot of other factors like the ones mentioned in this discourse.

    Exposed interior thermal mass helps to moderate the temperature swings and prevents overheating in the swing seasons. Concrete (or tile) floors and masonry interior walls help to moderate. A PH usually has exposed thermal mass to help deal with this.

    I agree that running software calcs that are specifically designed for a German/European climate will not bring accurate results in the continental U.S. That is why I prefer the direction that PHIUS is going as the director is developing climate/region specific PH standards. So if you are building a PH in Georgia it will not be the same parameters as it would be in Denver, Colorado. That's the logical way to go with PH in the USA. I think that is what led to the huge blow-out and split with PH Germany and PH USA. The German PHI wanted everything set in stone and refused to modify it's standards for the regions in the U.S. It was one size fits all but in reality, that doesn't work.

  34. RZR | | #34

    I found the suggestion I write an article for this site funny, Martin and the readers know I am not the best speller or writer as hard as I try I'm a keyboard dork :)

    Here are some of my observations having just come from a one of largest test facilities in the world where I tested MANY polymer's and sealants, took them to failure with thermal and pressure fatigue cycle test. Less is more when it comes to the use of sealants, tapes, adhesives, and seals, I have seen that time and time again. The blower door test is just a snap shot in time and tells nothing about the sustainability and life cycles of the seals. We can mature the life by engineered test and count the cycles to failure, in aircraft those cycles are closely regulated, redundant in some flight critical systems, which is usually low life depending on the design (mating materials) and the environment that the seal is placed in. Here again, it would be nice if the seal or sealant manufacture would disclose the life cycle test on their product data sheets. A hot box test can take temps down to -100 F and up to 200 F or to ignition and/or fire test, add pressure, moisture, CO2, CO, create an environment per a test standard such as ISO or ASTM but the testing is expensive.

    I would think Germany has the maintenance, replacement, reliability and empirical data, perhaps depending again if the seals are manufactured with the same materials as the USA. I would like to think that is part of the PCPP although I have never used it and probably never will. Maintaining or replacing seals can be an expensive undertaking depending on the design, especially if buried behind dry wall. I do not think most will get a century out of them like the mass designs offer. I know hempcrete and limecrete is readily available over there in Germany and Europe and not in the USA, this mass is a good choice for both walls and floors, earth another providing not moisture barriers or retarders, but moisture management systems. Cast and poured in place both eliminate sealing of walls studs, floor joist, rafters, etc., although fenestration's need sealing. Mass design do that by default although they too can have failures from pressure, thermal, and moisture cycles if not designed right.

    I would think the cost of sealing stacked assemblies with peel and stick or sealants is more expensive and will not sustain as well. Yep, I agree concrete is good mass storage for heat-coldth, ORNL quantified that in their DBMS and mass calculator, they also proved that ICF and interior foam is not as effective. Commercial grade concrete in general is not for a good moisture manager and RH regulator nor is gypsum. Portland cement, natural or synthetic gypsum, sand, rock, do not have the crystalline or molecular structure for it.

    This product I am looking at would probably do better than SPF since it does not have the formaldehydes, blowing agents, fire retardants, fillers, etc. that reduce lap shear and tensile strengths, life cycles, potentially pollute IAQ, and it is deliver under high pressure. Cost < $1000 per 2500 ft-2, need an airless paint sprayer but it gets in deep into the nicks and crannies. For redundancy and sustainability I would use a taped Huber air-moisture barrier (as Ted did) but I’d use their R-ZIP sheathing in place of polyiso since the R-ZIP-foam systems adds more redundancy for air-moisture sealing. In my mind the redundancy gets closer to the life cycle of mass designs but, the cost skyrockets especially when you factor in the labor cost of sealing layers. Some cast or poured in place designs do not require a render and are hard to beat.

    In any case, with any sealant or seal, I would design in an insulation to shield them from hot-cold pressure cycles, maybe just some loose fill glass, cellulose, or mineral wool, in cavities if possible. Mineral wool would be chemically inert and a good choice.

    I think it was Alain that mentioned the possibility that Ted may have lost his seal. The foam Alain used is susceptible too. Both designs, the drywall has little to no mass or moisture management, the concrete floor in Alain’s does add thermal mass.

    Ted, you wrote: The ceiling is constructed similarly, but has a moisture barrier on the back-side of the drywall to slow the movement of moisture into the insulation. The insulation is 24" of dense-pack cellulose, zip sheathing on top, and then a vented roof on top of that. Possible overkill, but when we designed the house I was worried about moisture getting into the insulated space.”

    It appears you have sealed cavity with the potential to trap any moisture that does get in it?

    Using stainless as a reflector on a flat facade as Alain did makes sense in the summer to reduce solar gains through windows, although that gain is also a lost to mass needed in climates with low night time temps as well as the winter when the gains are wanted. The radial reflector deck I posted does a better job at reflecting the gains by the suns direct beam which accounts for 90% efficiency as opposed to diffused beams through the upper clerestory windows to ceiling mass in summer for night time heat, and opening up or diverging the radial reflector deck for winter gains to the lower windows. The sun travels over 45 degrees north-south when it moves from east-west, so a flat reflector on a facade efficiency will vary depending on whether it is seeing the suns direct or diffused beam. Stainless is not cheap here in the USA, a more cost effective solution is the radial deck, or a movable plate or mirror that aligns to the suns direct beam but that can be costly. Same holds true for PV arrays.

  35. ahamel | | #35

    Jin, Peter, Terry and Ted
    Jin, no ROI calculations have been done because this is simply what we wanted, this is my conception of an almost perfect house and for doing a return on investment calculation, you need a starting point to compare. This house is our dream house and we built it the way we imagined it, we do not see the super insulation, fiberglass windows or other not so standard features as add-on, it's simply the way it should be.Thanks, we also really like the semi-industrial look, plants will grow up with years as this house is almost news (1 year old) but we do not want to add any climbing plants to hide some of the industrial parts.

    Peter, the motorized roller shutters are from this company in Quebec city and for the fixed louvers, i've done them myself, if you want details on the design, just ask and i will post some close-up pics.

    Ted, i like you vision of an affordable and low carbon house, your house maybe isn't perfect like mine but, you are working hard to improve and understand the complex interaction between humans and buildings. primarily, houses are shelters and they are supposed to protect us and maintain almost perfect conditions for human being without being energy hogs or environmental hazards. there is still a lot of work to do but things are moving because of people like you.
    Cheers and enjoy your pretty good house.

    Terry, i love your conception of a more natural and organic house but, for now, most of the builders are just beginning to understand the basics of building science and before running they have to learn about walking. There is a learning curve, new sustainable high performance building are, for now, just using enhanced construction techniques and materials similar to older ones but, in few years you will see more and more better building as education bridges the gaps.The majority of people who read or share informations on this blog are leaders in their own specialty or region, we are all helping to improve the most important aspect of evolution, knowledge transfer. This is a good start, but, as we all remember, we shall never lose sight on one very important thing, often the missing link between statu quo and evolution.
    ``Imagination is more important than knowledge`` Albert Einstein

    And about thermal mass, here's some pics of the inside of the first floor.

  36. RZR | | #36

    Thermal Mass & HRV
    Nice job Alain...If your happy with it then it IS perfect :)

    Nice placement of the brick thermal mass, looks like it is being loaded by glazing. I find it difficult to design thermal mass to emit at a predetermined lag time, ideally 6-12 hours after it has reached it's storage capacity. There are alot of variables as I described above that any software would have to have as a design tool. You would also need a smart sensor tied to the HVAC system to include thermal couples, CFM, with a phase lead time in the program to not over heat. The heat-coolth emitted when the temperature difference between the wall and surrounding air would be slow depending on the difference and thermal characteristics of the wall. Low CFM HRV/ERV flow would distribute low emitting flux to other mass in the house and aide in evening out hot-cold-moisture spots making Marks statement in accurate, besides some the systems are pushing some good CFM these days.

    Marc Rosenbaum wrote: "HRVs are not intended to even temperatures out around the house, period. Even if they were, the air flow rates are way too low to be effective."

  37. ahamel | | #37

    Thanks, house is very comfortable and temperature are near even, maybe 1 or 2 degrees difference in the worst case.
    RH is constant in all seasons and easy to control with the Daikin Quaternity 9000 BTU's. There a bit of overheating but we now handle it with a Nyles Geyser heat pump, there's a sensor in the dinner room, when temperature reach 22.5, the heat pump switch on and stock the excess heat in a 12 Kwh phase change thermal battery, this heat will be used for heating or to produce dhw.

    I've used the basic calculation for thermal mass in weight and thickness, unexposed or exposed found in the book Tap the Sun : Passive Solar Techniques and Home Design. It give you basic guidelines for mass to glass surface ratio and it's working !!!

    The home is not netzero for now, we are fine tuning mechanical systems to lower the overall energy consumption and we will adjust the number of pv panel when done, we have 5.4 Kw pv array but the system is planned to handle 8 Kw.

    By the way, our home is located in Saguenay, Quebec, Canada. we have 10 447F heating degree/day @ 65F

    It's a work in progress

    Have a nice day

  38. RZR | | #38

    Nyles Geyser heat pump
    Alain wrote: there's a sensor in the dinner room, when temperature reach 22.5, the heat pump switch on and stock the excess heat in a 12 Kwh phase change thermal battery, this heat will be used for heating or to produce dhw.

    Alain, I never knew there was such a product thanks for posting. Watched the insulation video that said the pump is not strong enough to have the lines too far away from the storage unit. Also appears that the unit is only capable of pulling surrounding humidly-heat in the room it is installed. Do you have some additional "thermal battery' somewhere and how far away is your storage tank?

  39. ahamel | | #39

    The storage is located right beside the heat pump and it's in a vented mechanical room. Maximum heating power needed in the coldest day of winter is 2267 W.
    Heat is distributed by radiant floor but surface temperature is set to 21.5 maximum, this setting optimize solar gain and minimize overheating, radiant floor is set to continuous circulation in all seasons to evenly distribute solar heat gain to the unexposed and exposed spaces.
    Serious think thank sessions were done and lot of building science involved in this special project, we formed a nice team, engineer, architects, students, raters and specialized trades were involved since the beginning of the project, as i told you, it's a work in progress.
    if you want, just do a google search on Alain Hamel kenogami house, you will learn more about.


  40. RZR | | #40

    Heat Pump
    I'm thinking push-pull fans mounted on the utility room to increase efficiency at the whole house level although that small heat pump does not look like it has the capacity to move a lot of hot humid air and phase change to DHW, it looks like it was designed to reduce the radiant heat phase change heat of the water tank to air only in the utility room not needing tank insulaton. It be interesting to add the fans, tank insulation, note the whole house heat and humidity level drops. I'd imagine it would depend on levels, how high they were in the specific home. The fans would need to be turned on with the thermocouple, humidstat, sensor too. A kit like that could possible be the solution to existing homes with centrally located water tanks that are overheating or high in humidity again, depending the capacity of the pump. My guess is with efficient fans the energy bill is still negative. Be also interesting to figure a way to use this device as a humidifier in winter to the HRV instead of ERV, or supplemental moisture to an ERV. Ted's issues be solved :)

  41. Gergman | | #41

    Shading & Moisture Ideas
    Sorry to be late to this party. My parents (91 & 93) just moved into their new PHIUS+ (cert. pending) in mid-July, so they've not yet experienced its humidity performance in the winter, but 2 weeks ago we removed their homemade shading device for the winter. Their home is attached to ours by a short breezeway (U.S. parlance).

    We're using a CERV so the temperature, VOC & CO2 levels combined with our set points control its operation - although it also provides data on humidity. It is normally "off" but always sampling the air about every 13 mins. If it determines that a set point has been reached it sets its dampers for fresh (filtered external) air or just recirculation (filtered interior air) & determines whether to use its heat pump for heating or cooling & then turns on its 200cfm fans to make the adjustment quickly. All interior & exterior data are recorded & presented graphically in (more or less) real time. Perhaps we'll have some humidity complaints to share this winter though ;o)

    We have 1,436s.f. & 12,625c.f. of treated area (24' cube). (.28ACH@50P, 3.4kBTU/s.f./yr.) The beta version of our shading device only addressed 182s.f. of our 308s.f. of S.-facing windows but since it wasn't installed for the 1st month of occupancy (mid-July thru mid-August) we were able to assess its effectiveness (somewhat) & declared it to be very effective. (Temporarily we used a sunshade for 106s.f. of the 134s.f. of W.-facing glass.) We've Intus windows: .088U, .49SHGC W. E. & N.; .11U, .62SHGC S.

    I used [send some money his way if you find his work useful] to design a very open, cheap, adjustable, DIY seasonal device of 1 x 10 hemlock boards suspended on 18" ctrs. on jack chain from a cable-pulley system that employs a small, removable winch for raising (spring) & lowering (fall). All 7 pieces are then stored away for 7-8 mos. (See pic) We intend to extend this solution to the kitchen & basement windows, & to use vertical louvers for the big W. windows - next year. [Our owner-designed, deep-in-the-woods project is very idiosyncratic; mtg. conventional style stds. was not an issue - thank god 8-) ]

    More info on our bldg. is here: & here:

    BTW, once again The Lemon Residence has appeared at the Annual VT Passive House Symposium; it's becoming a go-to classic. Ours was presented at the symposium, as well as at the Better Bldgs. by Design '14 conference in Burlington, VT, where more info. appears in the PowerPoints.

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