Passive House Principles: The Envelope

The next 5 blog entries will be a series about the Passive house principles.   Today I'll explore some details about "The Envelope".

How does one create a home that uses less energy?  Well, its fairly common knowledge that adding insulation helps reduce energy gain/loss.  How much do we need to add?  Well, typical code built walls are stated to be R20.  They are not actually R20!!!  Studs in the walls act as a thermal bridge to the outside which effectively reduces the R-value significantly.   This next picture was taken with a thermal imaging camera.  You can see every stud in the house! and look at all that heat passing through the windows!



Now look at the next image.  Can you figure out which one was retrofitted to the Passive House standard?


The one in the middle!  The exterior temperature of the building is the same as the vehicles parked on the street.  The other buildings are leaking heat like a sinking boat!

Any penetration to the outside, like cantilevered decks, HRV ducts, etc all lead to a sort of thermal short circuit that enables energy to flow from inside to outside or vice versa.  Once thermal bridging is taken into account, the R-value is effectively R15 to R17.  Heat flow doesn't scale linearly with R-value.  Instead the U-value is more descriptive: U=1/R.  When the wall is R20, the U-value is 0.05.  This means that the reduction in heat flow is 95%.  When the wall is effective R17 the U-value is 0.059. So the reduction in heat flow is about 94%.

So what happens when we double the insulation.  Let's say we go from R20 to R40.  For R20 the U-value is 0.05 or a thermal resistance of 95%.  When the R-value is doubled to R40, the U-value is 0.025 which would correspond to a reduction of 97.5%.  So doubling R-value doesn't get us a huge bang for our buck!  We have only increased our resistance to heat flow by 2.5% by doubling the thickness of the walls.  However, energy is energy and a percent here or there does matter.  Sometimes adding insulation is a cheap option so more may be better and the long term payback may not be too long.  Keep in mind that once the building is paid off, the savings are perpetual!

Lets look at windows.  Typical double pane glass windows are about R2.  This corresponds to a U-value of .50.  So typical windows only reduce heat flow by 50%!  Now we know where a lot of our energy is going...literally right out the window!!  Triple glazed windows are about R8.  This gives a U-value of  0.125, or an 87.5% reduction in heat flow!  so adding another pane of glass increases the thermal efficiency of the window by 37.5%!  So triple glazed windows are the obvious choice if you don't want to double the thickness of your walls.

There are other benefits to super thick, high performance walls.  There can be many layers.  This is a great article from Joe Lstiburek (https://buildingscience.com/documents/insights/bsi-001-the-perfect-wall) about the construction of a perfect wall. Each wall assembly needs to be able to protect the structure from bulk water,  water vapour,  bulk air transport and heat loss.  So each of these layers can be used in some way to promote air tightness, insulation, and protection from the elements.

There is a huge correlation between the air tightness of a building and heating/cooling cost.  If cold/warm air infiltrates the envelope, it's air that needs to be heated/cooled.  If air exfiltrates the envelope its the air needs to be heated or cooled.  So having an airtight envelope is important if you want to be able to predict your heat bill.  Ill talk a little more about this when I get to the balanced ventilation part of the series.

There are lots of walls that are Passive House compatible, each has it's pros and cons.  See some examples below.  In each of these walls there is never a straight way for heat to flow to the outside so the walls have minimized thermal bridging.

Image credits:  Hammer and Hand

Head on over the Green Building Advisor to see more examples: http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/five-different-high-r-walls



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