Keeping Cool During Heat Wave

We continue to be very happy with our Passive House.

Last week we had a bit of heat wave here.  It was unusual for Santa Cruz to have 3 consecutive days of temperatures in the mid to high nineties.  The chart below show the daily high and low temperatures (in Fahrenheit) in our area.  The Weather Cat weather station is located just 2 miles away from Midori Haus and is in a similar residential area so it provides good representative historical weather data for us.

What was really unusual about the heat wave last week was that Santa Cruz was about 10 degrees warmer than San Jose.  Normally it’s the opposite.  Have a look at this map below.  Even if Santa Cruz is south of San Jose the cool ocean temperature keeps the area mild and comfortable.  So I was surprised on Thursday last week when we drove back from Berkeley to see the temperature sensor on my car showing 96 degrees in Santa Cruz when it was only 85 degrees in San Jose. 

When we got home and stepped into the house it felt comfortable.  And we have no air conditioning.  Because of the super insulation and air tightness of the house the temperature inside the house stayed in the mid seventies during the entire time.  Here is an example of the temperature reading inside the house showing 23-28 degrees cooler than the outside.

Plotting the periodic temperature readings on this graph you’ll notice that the internal temperature stays in a narrow band while the external temperature swings wildly.

While passive house dramatically retards the heat transfer from the outside to inside (during summer) and from the inside to the outside (during winter) it does take a little bit of conscious action by the homeowner to optimize the comfort.  Let me show you what I mean.

First is shading.  Our deck that extends to the back yard faces south.  This is great in the winter because the 2 rooms facing south receive lots of good solar heat gain when it’s cold and the sun angle is low.  During the summer we want to keep the sun out.  So on Wednesday evening Kurt took out the canvas shade cloth and installed them over the arbor.  Originally we had intended to grow some plants to provide natural shading, perhaps grapes or kiwi.  But we chose not to go down that path because the shade cloth provides us with more flexibility.  After the first summer we decided to keep these shade cloth as a permanent seasonal solution rather than rely on plants because it’s easier to maintain.  Here’s a picture of that.

 Another part of the shading is inside the house.  We have these roller shades installed over the windows.  The ones on the south side of the house are all made of light-blocking thicker material we got from Advanced Blind & Shades,  and they are manufactured locally.

Then there is the setting on the heat recovery ventilator (HRV).  To minimize bringing in excess heat during the heat wave we set the control on the Zehnder ComfoAir 350 HRV to “A” during the day to reduce the ventilation flow down to 23.5 cfm from the normal 95 cfm at “2” setting.

The Zehnder ComfoAir 350, as with other heat recovery ventilators, has the ability to perform passive night time cooling in climates where hot daytime summer temperatures are followed by cool evening temperatures. These are climates in which one would normally open the windows at night. 
Using the HRV “Summer Bypass Mode” allows all the benefit of night time cooling with the windows the added benefit of retaining the filtration of air introduced into the home interior. It allows for a cooling to a precise user selected set point and then resumes its temporarily defeated heat exchange function.

Finally, we open the windows and let the cool air in during early morning.  As you’ve seen from the daily low temperature in from the Weather Cat table above it gets nice and cool overnight.  Typically in the mid to high fifties even during the heat wave.  This really helps to reset the internal temperature before the day heats up again.

By the way, this night or early morning cooling works because the ocean temperature is pretty constant and cool throughout the year.  The table below is the average ocean water temperature from NOAA.  You can see from the table that the water temperature just a mile away from our house is abut 56 degrees Fahrenheit, plus or minus 3 degrees.  Once the sun goes down the cool ocean water cools the air so the overnight temperature is consistently cool.  That’s why homes in Santa Cruz don’t have air conditioning.

Because we don’t have air conditioning we don’t use extra electricity during hot weather.  Our electricity usage continues to stay pretty low during the heat wave of May 13-15, 2014.  Below is the screen shot from PG&E, our local utility, showing our electricity usage for the current month. 

We are happy to be comfortable in our Passive House that uses very little energy.

More Insulation

September 4, 2013

Today we added 10 more inches of blown-in cellulose insulation in the attic.  This will help keep the house a bit more cooler in the summer and warmer in the winter.  It’s been on the warm side lately so this extra insulation will help.  Most houses in Santa Cruz does not have air conditioning because we have such temperate climate here so some homes will get toasty warm when we don’t have the morning coastal fog.  Even on these hot days the thermostat in the hallway never rises above 75F (23.9C), so it’s still comfortable inside the house.

Before:  Blown in cellulose in the attic on 7-Aug-2012

After:  Additional cellulose in the attic on 4-Sep-2013
Before:  14″ of Blown-in cellulose

After:  24″ of Blown-in cellulose


October 2, 2012

We want to be comfortable in the house.  There is a narrow band of temperature and humidity range we human beings are comfortable at.  The temperature-humidity chart below show the comfort zone to be in the low 70’s to 80 degrees Fahrenheit, which is about the ambient temperature in Hawaii.  Since less than 1% of the population of the U.S.A. live in the 50th State most of us encounter climates where the outdoor temperature is either too warm or too cold.

So to keep the inside of the house comfortable the shell of the house would need to slow down the transfer of heat between inside of the house and outside of the house.  This (either keeping the interior cool when it’s blazing hot outside or keeping the interior warm when it’s frigid outside) is what insulation does.  What type of material to use for insulation and how much to use depends on where you live and the goal you’re trying to achieve (performance and cost).

US EPA’s Energy Star site has this insulation map and accompanying table that shows different climate zones and the recommended level of insulation.  You’ll notice numbers preceded by “R” on the table such as R30, R25, R60, etc.  The R-value is the unit of measure for resistance to heat flow.  The higher the R-value the less heat flows between the inside and outside of the house. Different materials performance as insulators and you can find a table of R-value for different materials in this Wikipedia article.

Rather than use a generic table of recommended R-values we had our Passive House consultant and designer, Graham Irwin, perform the analysis and calculation using the Passive House Planning Package (PHPP) from Passive House Institute.  When all the details of the house (climate data for our zip code, house orientation, materials used etc.) are entered into the PHPP software it will calculate the amount of energy needed to heat and cool the house.  We have specific energy target we want to achieve for heating the house to meet passive house standard: 15 kWh/square-meter/year or 4.75 kBTU/square-foot/year. The same target exists for cooling demand for the year but since we live in a temperate climate that doesn’t require air conditioning it’s not a big concern for us.

Using the insulation specifications below our heating demand for the year to keep the house at comfortable temperature of 68-degrees Fahrenheit (20-degrees Celsius) is 8.75 kWh/square-meter/year.

Attic – 12-inches of blown-in cellulose; foam was applied to the tight corners along the wall-roof line (~R35)

Floor – 5.5-inches of fiberglass batts in the floor joist cavities and 2-inches of expanded polystyrene (EPS) board under that. (~R26)

Walls – 3.5-inches of dense pack cellulose (wet spray) in stud bays and 3.25-inches of rigid mineral wool (Roxul TopRock DD) on the exterior wall (~R28)

We had many discussions with the insulation experts, contractors, energy efficiency specialists and architects before deciding on the materials we chose.  One key learning for us was when we heard Alex Wilson speak at PG&E’s Pacific Energy Center in San Francisco.  He shared a story of a well-intentioned homeowner wanting to save energy and reduce greenhouse gas emission was horrified to learn that the spray foam product he used to insulate his house would take 60-years to payback — this is not financial payback but the time it would take his energy savings to offset the amount of greenhouse gas released into the air from the blowing agent used for his spray foam product.  So we decided to stay away from spray foam products.  You can find one of Alex Wilson’s article on insulation at Green Building Advisor.  The chart below is from that article.


Here are some before and after picture of our attic.

Before:  Very little insulation in the attic

You can see from this “before photo” that we did have some attic insulation, if only to barely cover the ceiling joist.

After:  over 12 inches of blown in cellulose

In this “after photo” you can see the measuring tag sticking up from the sea of cellulose to indicate that between 12 and 14 inches of cellulose filled  the space.

The insulation sub-contractor, Ponzini Insulation, did the cellulose insulation in the attic and in the wall cavity.  Applying blown in cellulose in the attic.  They had a big truck with special attachments to pump the cellulose through the long hose to apply the insulation.

Insulation Truck

Installer applying blown in cellulose into the attic


Crew from Santa Cruz Green Builders did the insulation and air sealing below the floor in this cozy crawlspace.  Fiberglass batts filled the 5.5-inch deep cavity between the floor joists.  Then sheets of 2-inch thick EPS board was applied under that.  The seams of the EPS boards were meticulously taped using Siga tapes.  Because the EPS board serves dual purpose of insulation and air barrier the edges of the board coming in contact with other material (wood, concrete) were foamed to prevent air leakage.

Crawl space view of the floor after completion of insulation 


Our 90-year old house did not have any wall insulation!

Before:  No insulation between stud bays

As you can see from this deconstruction photo, the space between the exterior skip sheathing (horizontal board) and the interior wall made of lath (small strips of wood) and plaster was empty.

After:  wet spray cellulose filled the wall stud bays

Wet spray insulation was applied to the open stud bays in the interior wall cavity.  This took almost one week to dry fully but it allows for visual inspection of cellulose application.  Speaking of inspection, we had Quality Insulation Inspection (QII) performed by Bright Green Strategies for the California Advanced Homes rebate program.

Mineral wool material applied to exterior sheathing

Sheets of rigid mineral wool were applied to the exterior wall.

We used TopRock DD from Roxul