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.

Energy Usage: First 8 Months

September 1, 2013

Are you curious how much energy Midori Haus uses and how it compares with other houses in the area?  The proof is in the utility bill.  When I get the utility bill from Pacific Gas & Electric (PG&E) each month via email I always log in to their site to look at the “My Usage” tab to see how our electricity and natural gas usage compare with similar homes in the area.  Take a look at these screen shots:

Our electriity usage averaged 227kWh per month for the last 6 months (Mar-Aug).  You can see from the graph that our usage (blue line) is pretty low, tracking closely to the efficient homes (green line) in the area.  The house was in construction through January and we started living full time in the house on March 15, 2013 when the blinds for the windows were installed.

Electricity usage was a bit higher in February and March when we had several fans and vaporizers running 24/7 to remove the smell coming from the stains applied to the fiberglass door.  Usage was low in April when we were on vacation for 10 days.

 Our natural gas usage averaged 1.8 Therms per month for the last 6 months (Mar-Aug).  Our gas usage (blue line) is just a fraction of similar homes in the area and even lower than the most efficient homes in the area.

We use a gas boiler as a backup heat source for domestic hot water.  Primary source of heat for the hot water is the sun.  We have a solar thermal system (to be explained in a separate post) that pre-heats the hot water.  We also have a natural gas barbecue grill out on the deck.  That accounts for the little blip in gas usage in the summer — 2 Therms in June, 1 Therm in August.

You may wonder what homes are compared in these graphs.  PG&E does a good job of giving us an apples-to-apples comparison.  The definition of similar homes for us is single family houses with an average of 1572 sq ft using natural gas heat within 0.6 miles.

Our house is technically heated by gas because PG&E does not have a category for homes heated by the sun.  How is our house heated?  When the temperature in the hallway thermostat dips below 68F a pump in the mechanical room turns on to circulate hot water (mostly heated by the sun) through the hydronic coil in the house.  The hydronic coil is used to warm up the air in our ventilation system. For a typical house this amount of heat is not enough to make it comfortable.  But Midori Haus is a passive house that is super-insulated and super-airtight.  So we only need the equivalent energy of half a hairdryer to heat our home in the winter.  Should the temperature in the hot water tank dip below 120F then the backup gas boiler turns on, thus the house is technicall heated by natural gas.  But as you can see from the graph above this doesn’t happen regularly.  The higher usage of natural gas in January and February reflects the start-up condition for the hot water heater.  The storage tank and the backup gas boiler were installed and turned on in January when it was cold outside and the sunlight duration short.  So the gas boiler did bulk of the work to heat up 165 gallons of water to 120F.  It’s amazing that we only used only about quarter of natural gas that similar homes used in January and the house was comfortable.

The amount of energy reflected in the utility bill is not only for heating the house.  A good portion is attributed to appliance choices and our behavior.  At Midori Haus we use electricity for cooking, clothes washing, lighting and handful of gadgets plugged into the wall.  When we lived in the 1300 sq ft condo we had lots of gas appliances — wall furnace, standard hotwater heater, stove and oven.  So I expected our electricity usage to be a bit higher at Midori Haus than at our condo.  And it is a bit higher but not by much.  To compare the energy usage between similar seasons I grabbed a few screen shots of the energy usage at our condo in 2012:

 The monthly electricity usage averaged about 147 kWh for the condo for the same period last year (Mar – Aug 2012). Some of the gadgets consuming electricity are the same — laptop computers, stereo, hair dryer, coffee grinder, etc.  The occupant behavior is somewhat similar too.  The difference in occupant behavior is that I spend far less time at Internet cafes now.  Back in 2012 when I was uncomfortable at the condo (I thought it wasn’t warm enough) I packed up my computer and hung out at the local coffee shops.  Now I don’t do that.

 The gas usage at our condo has an interesting story.  In the winter of 2011-2012 we did an experiment of setting the thermostat for our wall furnace really low and wore layers of sweaters to keep warm.  In our mild climate in Santa Cruz if you set your thermostat at 55F in the winter it hardly turns on.  I thought our gas usage would be pretty flat to track with the summer usage pattern.  But it didn’t.  It was lower than similar homes and the curve rather bumpy. The winter usage went up as if we were turning on the wall furnace but we did not.  Our gas usage (blue line) was even higher than efficient similar homes.  What’s going on?  The likely culprit is the water heater.  The gas water heater at the condo was located in a cabinet next to the refrigerator in the kitchen.  It’s in the conditioned space so it had no insulation around the water heater.  This is OK for most times when the kitchen temperature is between 68F and 72F, but not when the kitchen temperature is at 58F.  So the water heater was using more gas in the heater to keep the water warm.  Interesting, isn’t?

 The similar homes for the condo are all apartments or condos with natural gas heat located within 0.9 miles.  Our condo shared 2 walls with our neighbors so 50% of the walls are well-insulated.  But it is definitely not airtight and I was often cold in the winter.  People in cooler climate may laugh when I complain about it being cold in the winter here but I grew up in Hawaii where it’s nice and warm.

It’s hard to compare the energy bills of homes with different types of appliances (e.g. gas stove top vs. induction cooktop, gas clothes dryer vs. electric condensing clothes dryer, gas wall furnace vs. hydronic coil).  To do an apples-to-apples comparison I took the 6 months data (Mar – Aug 2013) for Midori Haus and the 6 months data for the condo (Mar – Aug 2012) and converted the gas usage (measured in Therms) to equivalent electricity usage (measured in kWh).  1 Therm is equivalent to 29.307 kWh.  Plotting this combined usage data on a graph I found the Midori Haus total energy usage to be much less than the total energy we used at the condo.  It’s the same people with same occupant behavior but  living in a different space.  This means the dwelling itself is much more energy efficient and the space is much more comfortable.  Passive house is amazing.

First Interim Blower Door Test A Success!

Good news!  The first interim blower door test passed the passive house criteria of 0.6 ACH at 50 Pascals!

This was very happy surprise when we stopped by the job site on Wednesday, 8/1/2012, after returning from a long vacation.  Riding our bikes into the fenced area we saw that the “blower door” (the red plastic lining fitted into the door frame with a large fan hugged by the elastic band on the plastic sheet) was placed in the front door.  Just as we were saying, “Hmm… They’re doing the blower door test today, I don’t think we can get in,” we saw the blower door being dismantled by our air sealing consultant, Terry Nordbye.  He was very happy and pleased to tell us that the house passed the blower door test.  Yeah!

Then we saw Taylor Darling of Santa Cruz Green Builders, our general contractor, walking out of the house with a big smile on his face and showed us this picture:

This means that the house, at this mid-construction stage, has met one of the most difficult criteria of passive house certification, air tightness test of 0.6 ACH50 maximum.  Our approach to air sealing is to do few interim blower door test during construction rather than wait until the very end so that air tightness is ensured along the way.  You may ask, what does 0.6 ACH50 maximum mean?  Well, ACH stands for Air Change per Hour.  The target of 0.6 ACH means 60% of the air volume in the house exchanging with the exterior of the house (leaking) each hour.  So, for our house with 1,569 square foot of interior space and 9-feet ceiling, the total volume of air within the house is 14,121 cubic feet.  60% of that is 8,472.6

ft3, which is the maximum allowable air leakage per hour at 50 Pascals (equivalent to having 20 miles per hour wind blowing outside of the house).  Now let’s convert the per hour figure to per minute figure to get to a familiar term of CFM (cubic feet per minute) by dividing by 60 (1 hour = 60 minutes).  This gives us a target of 141 CFM.  As you can see from the above picture (135 CFM at 52.2 Pascals) Taylor and his crew managed to exceed the air tightness criteria!  Excellent job, guys!

Taylor said that in the morning the very first blower door test result came in at 0.96ACH and they knew they knew their work for the day would be a quest for air leaks and patching them with foam and tape.  Fortunately it turned out to be only few large leaks rather than lots of tiny leaks.  One was the drain in the hallway bathtub where it didn’t yet have the p-trap and water in place.  Another was a set of penetration made for solar thermal plumbing.  The third was a penetration in the top plate that wasn’t visible from the bottom that leaked air into the space between two beams in the ceiling.  Here are some photos from Taylor on the examples of air sealing:

Solar thermal plumbing

Sealing around the windows

Foam sealing at the perimeter foundation

EPS foam board under the floor insulation in the crawlspace

Taylor and his crew, in addition to being good builders are air sealing rock stars.  This cannot be emphasized enough because they’ve managed to make this 90-year old house VERY airtight.  I’ve often heard that an average new construction today is about 5~6ACH.  What Taylor and his crew managed to do was to get this old house to be 10 times tighter than an average new construction.  It’s much harder to get to this level of air tightness with a retrofit because we’ve reused many of the existing structure (foundation, framing, roof, floor) from 90 years ago and therefore less control compared to a new construction.  You may also remember that the baseline blower door test for this house was 22 ACH.  We kept the vented attic and vented crawlspace so the air sealing was done at the ceiling and the foam board below the floor joist. 

Advice and help from our air sealing consultant, Terry Nordbye, was very helpful.  At one point during our visit David asked Taylor and Terry about which smoke pens belonged to whom.  After they sorted out their tools I asked Taylor, “So how did those smoke pens work in detecting leaks?”  His reply was, “We didn’t use the smoke pens.  We just listened.”  Apparently they boosted up the blower door fan to 180 Pascals and at that pressure they could hear the air leaks quite clearly.  One thing that leaked air more than expected was the window hardware.  We have lovely triple pane tilt-turn windows from Cascadia and the hardware on the windows leaked air quite a bit.

You may wonder with such air tightness of the house envelope what the indoor air quality will be like.  That will be another post on Heat Recovery Ventilator.

Beginning of Air Sealing

Why Air Sealing?

We want our house to be comfortable, durable and energy efficient.  So air sealing is an important element in meeting those criteria.  You know that air can pass through small cracks, spaces and even pin holes, right?  For example, if you see daylight in the door frame when it’s closed you’ll feel a draft standing next to it when it’s cold outside and warm inside.  Then if you put weather stripping around it to prevent air flow the house feels more comfortable, right?  So, stopping air leakage leads to comfort.

Air sealing also leads to durability of the building because air can transport moisture.  You might ask, what’s the connection to durability?  Well, remember the pictures of rotted skip sheathing due to rain water intrusion?  Prolonged water exposure can cause wood to weaken and also invite mold to grow.  Not good for durability of the structure nor the health of occupants.  Let’s imagine a hypothetical example for illustrating why moist air passing through cracks in the walls is bad for durability.   Say there’s a lot of cooking going on in the kitchen and the indoor temperature is 80 degrees with 50% humidity and the outside temperature is 40 degrees.  The dew point (the temperature that vapor in the air changes to liquid) is 59 degrees.  Warm air can hold more moisture than cold air.  So for a given relative humidity, the the surrounding air temperature will determine if it will stay in the air as moisture or condense to liquid and become water.  For an explanation of using simplified psychrometric chart have a look at this guide on air properties from NebGuide.  In this hypothetical example the warm air leaking through the kitchen wall will to the outside will encounter drop in temperature along its path and when the temperature drops below dew point the moisture vapor will change to liquid water.  If the cold surface happens to be the insulation layer it will get soggy and dampen the wood around it and if it doesn’t dry out over a period of time there will be rot and maybe mold.  So, stopping air leakage leads to durability.

Air sealing is good for energy efficiency.  Imagine driving in your car with the windows open in the winter.  The heater is on in the car but the hot air is escaping through the window.  When you close the window it’s warmer because the heat is no longer escaping through the window and you can use a lower temperature setting to stay comfortable.  Same thing with the house.  If you have the windows open you use more energy to heat the house than if you had the windows closed.  The opening and closing of the windows are something we can do voluntarily to control and minimize the use of energy to heat the house.  The air sealing of small cracks and spaces are like having lots of tiny miniscule windows that we can’t close.  Some of these cracks are buried under layers of building material and homeowners can’t get to them easily.  So we rely on the builders to ensure that these miniscule uncontrollable windows in the house at different stages of construction.

Air Sealing Examples

When we’re talking about air sealing for passive house standard the builder is taking steps to mitigate air leaks from tiny spaces like gaps between two pieces of wood on a top plate, mudsill, etc.  To refresh your memory, we are striving towards Passivhaus certification and the airtightness goal is 0.6 air changes per hour (ACH) at 50 pascals.  If you recall our baseline blower door test came in at 22 ACH.  This means we are targeting the house to be 3600% more airtight than the original house!

By the way, you may want to take a look at this page for a brief overview of Passivhaus.

Air sealing work on the wall framing was done prior to sheathing.  These guys examined the places were unintended airflow could occur, like the junction of mudsill and studs, corners, etc.  Below are some pictures of air sealing examples.

To prevent air leakage in the small pinhole openings the crew shot foam
into the openings, sometimes drilling through the stud to access a spot where
the foam could expand and fill all the crevices, then sealing the outside with
a tape.  The crew tended to this
tedious work diligently towards the goal of 0.6 air change per hour at 50
Here are some examples of air-sealed spots where foam was blown in and covered up with tape.
Sealant from Perma Chink was applied with a bulk loading gun to ensure sealing of the sheathing to the wall studs.

22 ACH – Leaky House

December 9, 2011

As you may know, we are pursuing the Passive House Certification.  One of the criteria is airtightness, targeting 0.6 air changes per hour (ACH) at 50 pascals.  This is pretty darn tight.  I’ve heard that a typical new home built today in the US is measured at about 6 ~ 7 ACH.  So we are trying to get our remodeled home to be 10 times more airtight than that.

So, we know that our house built in 1922 is quite drafty.  How drafty is it?  We decided to find out.  To establish the “before remodel” baseline we did a blower door test on Monday, 11/28/2011.  This was done using a blower door, which is a device used to measure the air leakage rate by pressurizing and depressurizing the building.  We did the pressurized test.

After the technician from Allterra Environmental fitted the frame and flexible panel to the front door he slowly cranked up the fan speed and we watched the numbers on the meter rise up and up and up….

The top number 49, is in pascal, a unit of measure used for pressure measurement.  The bottom number is the volume of air movement measured at cubic feet per minute (CFM).  When the picture was taken there were 5,390 cubic feet of air moving out of the house every minute.

How do you convert 5390 CFM at 49 pascal to air change per house (ACH)?  Well, you first multiply the the CFM number by 60 (because 1 hour has 60 minutes) and divide the total by the volume of the house.  The real estate report showed the house to have an area of 1574 square feet and the average ceiling height of the house is 9 feet, so the volume of the house is 14,166 cubic feet.

So, the ACH is (5,390 x 60) / (1574 x 9) = 22.83

I’ve heard that 50 pascal is equivalent to having 20 mph wind blowing outside.  So this means if we had constant high winds of 20 mph the house would completely exchange the air in the house over 22 times. That’s leaky….. and cold!

You can find out more about the history and the basics of blower door at this link.

Here’s a snapshot from the report: