You can go to
http://susdesign.com/overhang/index.php
and play around with the calculator.

You can also draw a rough sketch of the house in Google Sketchup and then you can rotate it, watch the sun move at different times of the year and see the shadows, etc.

Yes, I'm using SketchUp Pro for schematic design and will be moving into Revit for design development which is more precise.

The calculator at susdesign.com is a very simple model that has you visualizing it in a few seconds as opposed to putting an entire schematic in. I highly recommend it as a starting point.

Pick the latest reasonable date in the Summer, before which you do NOT want sun coming in. For up in the PNW, I usually choose 11 am and sometime in late August.

Keep in mind that the vertical plane of the window is part of the "overhang" so that if you have thick walls and the windows are set back in, you would add that amount to the overhang.

I don't see how East and West overhangs can do anything for you.  When the Sun is either due East or due West, it is so low that it is going to see in no matter what.

John, make sure you are properly following the latitude/longitude sign convention that the software requires. You should not have any problem designing an effective passive solar roof overhang to control the irradiance entering your south wall fenestration at 37.78N. ICFHybrid’s guidance/recommendations are good. The susdesign.com tool is likely the best available tool for quickly visualizing what is going on and Chris is also a great guy to work with too. It is usually best to minimize the windows on the east and north sides as they only result in increased heat loss in winter months. If possible, it is best to eliminate windows on west side for the same reason and to also avoid late afternoon overheating in summer months. Of course, views and building architecture often require some design tradeoffs.

If you want to go deeper down the rabbit hole to accurately determine climatic and clear sky irradiance BTUs to bounce against your building heat loss analysis BTUs to allow optimizing fenestration area/SHGC and perhaps design thermal mass too, check out the DIY passive solar software on our website.

Borst DIY Passive Solar Design Software

Posted By ICFHybrid on 20 Jun 2013 11:17 AM
...snip...
I don't see how East and West overhangs can do anything for you.  When the Sun is either due East or due West, it is so low that it is going to see in no matter what.

I would add the qualifier to this statement "I don't see how East and West overhangs can do anything for you in terms of passive solar gain." Certainly overhangs help reduce the chances of water penetration from the outside through siding and windows, and probably extend the life of both (as well as paint) by reducing weathering.

Good point Lee. Again, depends on building architecture, but for a full hip roof design we like to retain the same overhang used on the south facing wall for all the wall faces and like to locate/size the fenestration to allow targeting between 3-4’ of overhang for all the benefits you mentioned. The downside of doing this is the additional initial roof cost and some additional engineering to allow constructing the building to handle the higher wind loading resulting from the larger overhangs.

Yes, I have a full hip roof design with 2' overhangs on all sides, except porches on part of the front and back that increase the overhang in those locations. The 2' overhang for the south side was the result of using the tool at http://www.susdesign.com/overhang/ along with the window sizes and locations that are standard for this production house. A 3' overhang would be even nicer for protection of the windows and siding, but would not work for the desired passive solar heating dates at this latitude and with the standard window sizes and locations.

We also like to use 6/12 attic trusses with a large energy heel. This truss in combination with the large overhang creates significant real estate that can be initially used to obtain high insulation R-value and attic storage space, and also used in the future to easily create habitable space. The additional cost of using attic trusses with a large energy heel is relatively small.

Another factor to consider is whether the south facing side of the home has any projections from the building itself or 2nd floor balconies. These can cast shadows at different times of the day during the spring through fall sun depending on how much projection they have. Of course any buildings next door can cast shadows and provide shade. The same goes for trees.

Recessing the windows from the exterior wall is another way to achieve more summer solar shade without having a larger overhang. On a thick wall like a 12" ICF wall, setting the windows back 6" or what they have termed an "innie" window install gives a 24" overhang a 30" value. Innie windows also help with protecting them from the elements and winds, some have even recorded higher R-Values with innie window installs. See Article

As always, there is a line between a perfect passive design and design aesthetics. Not having any windows on the west side of the home does hinder design and practicality. If you have bedrooms facing west, windows may be required by fire code and sometimes ones views face west, in which case some energy loss in heat gain during summer is a trade off. A good window with a low SHGC (<0.30) are best suited for west facing windows. Also good thermal shades will help during the summer sun. Out here in Phx they usually recommend a 0.26 SHGC on all windows.

A question about that software program for shading: Let's say a south facing window during summer solstice is NOT 100% shaded. With the sun being high in the sky, will that window receive the full SHG or only in the winter when the sun is facing directly at it on the horizon?

"Let's say a south facing window during summer solstice is NOT 100% shaded. With the sun being high in the sky, will that window receive the full SHG or only in the winter when the sun is facing directly at it on the horizon?"

The reflectivity of glass is a function of angle of incidence. At angles of incidence greater than 50 deg., the reflectivity increases quickly, so the transport of heat into the room decreases.

I would add the qualifier to this statement "I don't see how East and West overhangs can do anything for you in terms of passive solar gain.

Yeah, all my comments on overhangs have to do with passive solar gain.

Lee is correct. Our software takes into account the solar angle of incidence, the % shading at each minute of the day from the overhang, the SHGC of glass, any local terrain obstacles (e.g., trees, etc.), and several other factors when determining the solar heat gain BTUs.

Posted By sailawayrb on 21 Jun 2013 08:06 AM
Lee is correct. Our software takes into account the solar angle of incidence, the % shading at each minute of the day from the overhang, the SHGC of glass, any local terrain obstacles (e.g., trees, etc.), and several other factors when determining the solar heat gain BTUs.

So with a window that cannot be shaded during summer and is rated at 0.37 SHGC, would you estimate that the window sees about 1/2 of that SHGC or less during summer solstice (south facing)?

In my reading of Passive Solar homes, I have observed that quite a few people are having issues with "overheating" during swing seasons. That they have to turn on the A/C during fall or spring to cool down the home while other non-passive solar homes nearby don't have to run A/C during those same days. It seems as though spring and fall is a "loss" in terms of energy when the home overheats due to the passive solar design. Of course that is offset by the winter gain. Do you agree?

Fenestration with a SHGC value of 1.0 will transmit 100% of the solar irradiance that it receives. Fenestration with a SHGC value of 0 won't transmit any of the solar irradiance it receives. When you use an overhang to control the solar fenestration exposure, you typically want to use the highest SHGC fenestration that you can obtain while also considering U-factor so as minimize heat loss/gain too. Using the highest SHGC fenestration you can obtain minimizes the required fenestration area (and the associated fenestration cost) that is required to achieve the passive solar heat gain design objective. Double pane glazing fenestration without low emissivity coatings can be reasonably obtained that have a total SHGC rating in the 0.63 range and a total U-factor in the 0.45 range. A SHGC of 0.63 will allow 63% of the incident total irradiance striking the exposed fenestration area to enter the building. So a SHGC of 0.37 would only allow 37% of the incident total irradiance striking the exposed fenestration area to enter the building...so about 41% less...but less all the time, not just during the summer. In the summer we would use the overhang to minimize our solar fenestration from being exposed to the incident total irradiance. If we had any west facing fenestration we would certainly want the lowest SHGC we could reasonably obtain because the overhang would not limit the incident total irradiance exposure.

Just for completeness, the aforementioned incident total irradiance is the sum of the incident beam irradiance, the incident diffuse irradiance and the incident ground reflected irradiance. So the overhang doesn't do much to keep the fenestration from getting exposed by the incident ground reflected irradiance. So terrain reflectivity must be properly considered to correctly determine the solar heat gain from this source. Terrain reflectivity can often be put to good advantage for both increasing solar heat gain in the winter months while reducing solar heat gain in the summer months.

Yes, you are indeed correct that many people have issues with overheating during the spring and fall months. Some really unfortunate people even have overheating problems in the winter and summer month too. The short answer for why this happens is because the passive solar design was NOT done properly.

A common mistake made by novice passive solar designers is that they use the AVERAGE CLIMATIC solar heat gain to determine their passive solar design parameters instead of properly using the MAXIMUM solar heat gain. AVERAGE CLIMATIC solar heat gain is based on historical solar data in a given location and factors in cloudy days, etc. MAXIMUM solar heat gain is what occasionally occurs on clear sky hours/days in a given location. I'll explain more about this later.

Another common mistake made by novice passive solar designers is that they use grossly inaccurate design rules of thumb such as these:

1) Passive solar buildings should have a total south wall passive solar fenestration area between 7% and 12% of the total building floor area.
2) For every square foot of south wall passive solar fenestration area in excess of the above 7% design rule of thumb, a passive solar building should have 5.5 square feet of 4 inch thick thermal mass material.

Relative to 1), there are many factors besides the actual fenestration area that have a significant effect on the passive solar heat gain. For example and perhaps most obviously, the amount of irradiance that actually enters the fenestration has a significant effect on the actual solar heat gain. The amount of irradiance that actually enters the fenestration depends on the building's local latitude, local atmospheric optical transparency quality, local climatic sunshine availability, local terrain obstacles, actual Solar Heat Gain Coefficient (SHGC) of the fenestration, and the actual hourly utilized fenestration area as governed by the actual passive solar roof overhang. And perhaps more importantly, the maximum amount of hourly building solar heat gain should always be equal to or less than the hourly building heat loss as determined by performing a building heat loss analysis.

Relative to 2), there are many factors that can significantly affect the thermal mass performance. For example, the maximum temperature that the thermal mass will reach during the daily irradiance time period depends on the initial temperature of the thermal mass, the hourly irradiance magnitude (BTU/hour), the daily irradiance time period (Hours/Day), the absorptivity of the thermal mass material, the specific heat capacity of the thermal mass material, the actual mass of the thermal mass, and the exposed floor heat loss. The heat gain provided by the thermal mass during the night time hours depends on this maximum temperature, the surface area of the thermal mass, the emissivity of the thermal mass material, the R-value of the thermal mass material, the convective heat transfer coefficient of the surrounding air, the specific heat capacity of the thermal mass material, the actual mass of the thermal mass, and the exposed floor heat loss. The thermal mass will also release heat during the daily irradiance time period when the thermal mass temperature exceeds the room temperature. Consequently, the passive solar heat gain that occurs during the daily irradiance time period can result from both the irradiance that enters the building that is NOT absorbed by the thermal mass PLUS any heat gain that is released by the thermal mass during the daily irradiance time period.

In short, a competent passive solar building designer will NOT use design rules of thumb and will instead properly determine and properly align the heat loss and heat gain BTUs/hour. A competent designer will first estimate using historical climate data what the building MINIMUM heat gain requirements will be for every hour of every day of the year. Then the designer will use an accurate solar irradiance model to determine exactly what the MAXIMUM solar heat gain will be for every hour of every day of the year for the overhang, fenestration area/SHGC, and thermal mass initial design parameters. Then the designer will iterate on these design parameters until the MAXIMUM solar heat gain (BTUs/hour) at most only just equals and never exceeds the MINIMUM heat gain requirements (BTUs/hour) for any hour of every day of the year. If the design is accomplished in this manner, the risk of ever overheating the building is nil. We actually use a computer algorithm to accomplish this very quickly so this is not as complicated or difficult as it may sound.

It is only after properly doing the passive solar design in this manner that the designer then uses the AVERAGE CLIMATIC solar heat gain data in conjunction with heating degree day data to estimate what % of the total monthly heating requirements will likely be provided by this passive solar design to allow sizing any supplemental heating system that may be required.

Now all of the aforementioned is just for a "pure" passive solar heating system design. If you also put a hydronic radiant floor heating system in the building, properly zone it to integrate with the thermal mass irradiance area, and provide hydronic heat rejection capability, you can get much more aggressive in capturing solar heat gain without any risk of building overheating.

Everything I summarized here is explained in much more detail in the passive solar design instructions on our website. Happy summer solstice!

In my reading of Passive Solar homes, I have observed that quite a few people are having issues with "overheating" during swing seasons. That they have to turn on the A/C during fall or spring to cool down

Despite claims of "my designs never suffer from that", it's a limitation you have to live with unless you go to an active solar design. Some type of automated whole house ventilation is less expensive to run than AC.

If the ambient temperature is 86F and you are overheating, there might not be any answer except air conditioning. However, if the outside temperature is low enough to use for cooling, and it's too hot inside, then you've made a mistake somewhere.

Every passive solar home should utilize a cooling stack effect because it is essentially FREE cooling.

Overhangs and window shading is your #1 line of defense. Don't let it get inside in the first place.

I totally agree ICFHybrid.

If you are located in a climate where the temp or the humidly is continuously above your comfort level, you will most certainly need AC. I guess I thought this obvious, but I suppose this would be another common mistake that could be made by a novice passive solar designer that I should add to my list. Namely NOT understanding basic passive solar heating/cooling climate limitations and NOT first performing a heating/cooling load analysis for the building location BEFORE recommending the heating/cooling system strategy that should be used!

Passive solar heating and cooling are more applicable and appropriate in locations that don't have this type of continuous high temp/humidity climate. Even in southern OR where we are based, the temp can easily reach a dry 100F during the summer days. However, the temp will drop to 50F during the night allowing for passive cooling stack effect or forced ventilation cooling. A well constructed/insulated building in conjunction with a "pure" passive solar heating/cooling design can easily provide upwards of 50% of your annual heating requirements with little risk of ever overheating the building in this type of diurnal temp climate if properly designed.

If you also integrate the passive solar heating system with a hydronic radiant floor heating system like we often do, the % of the total annual heating that can be achieved by capturing solar irradiance is only limited by the actual climatic solar irradiance available at the specific location and the client's economic assessment of how far they want to pursue this design objective. Again, there is little risk of ever overheating the building in this type of diurnal temp climate if properly designed.

Relative to passive cooling stack effect versus forced ventilation cooling, we always first work with the building's architecture design to take advantage of passive cooling stack effect to maximum extent possible. However, some energy efficiency conscience clients rightly avoid multi-story buildings, high ceiling styles and vertical corridors to minimize heating/cooling requirements. In any event, we still prefer and we always recommend forced ventilation cooling even when we can take full advantage of passive cooling stack effect. While we have successfully designed automated window systems and automated drape systems at client request, it is really hard to beat the simplicity and reliability of an autonomous/integrated forced ventilation system and a properly designed passive solar roof overhang. The energy usage and associated cost for operating a forced ventilation system for the small amount of time it is actually needed to cool the building in this diurnal temp climate is nil. Furthermore, most well constructed/insulated buildings will require forced ventilation anyhow and preferably an ERV or HRV system will be used too.

Bottom line, if you are located in a non-diurnal climate where the temp or the humidly is continuously above your comfort level, you will still most certainly need AC and passive cooling stack effect or forced ventilation cooling will NOT suffice to keep the indoor temp/humidity within your comfort level.

On a 2-story open plan design with an HRV/ERV can one utilize a passive cool stack design without having clerestory windows?

I assume with a cool stack design ( 2 story with open floor plan), one is basically just evacuating the interior air at night via an HRV/ERV or opening the windows and replacing it with cool outside air? Or is it more complicated than that?

Posted By Lbear on 22 Jun 2013 05:05 PM
On a 2-story open plan design with an HRV/ERV can one utilize a passive cool stack design without having clerestory windows?

I assume with a cool stack design ( 2 story with open floor plan), one is basically just evacuating the interior air at night via an HRV/ERV or opening the windows and replacing it with cool outside air? Or is it more complicated than that?

For a 2-story plan, you would probably want to open some lower story windows and then EITHER open the upper story clerestory windows (for passive cooling stack effect) or use forced exhaust ventilation located in the upper story (e.g., a whole house ventilation fan). In short, you need a way to get the hot air in the upper story out of the house and replace it with the cooler air entering the house from the open lower story windows. A HRV/ERV system designed with this in mind could get the job done too, but this would ultimately depend on the size of the building (and heat capacity of the furnishings in the building) and the HRV/ERV system air exchange capacity.

BTW, if you go with a whole house ventilation fan, you need to take the necessary additional steps to ensure it seals closed and is well insulated when it is NOT operating. Otherwise, you risk creating a significant infiltration heat loss problem.