Many things we have done on this project were simply unknown to us before we started;  we are learning all about traditional plasters, we’ve built our first flat roof … and I now know way more about tire sizes than I ever hoped to!

Surprisingly (to me) specifying and ordering the glass has turned into another learning experience.  Surprising because we have specified and ordered plenty of windows for past projects.  But, just before ordering we found ourselves scratching our heads over tempered glass, one-in-ten wind speeds for the Kamloops region and building code requirements for windows in a residential building.

Normally (at least in all of our past building projects) I draw up a set of plans showing window location and sizes.   The plans are passed on to the window and door manufacturer working on the project, who then assembles a package of windows compliant with the Building Code.  This process is relatively simple and has always been fairly straight forward.  Of course in this project we are building most of the windows ourselves and the majority of the glass is going into a sloped green house wall, things we have never done before …

The British Columbia Building Code (2006) has a fair bit to say about the windows in a residential building. There is a basic requirement for a minimum square footage of glazing in rooms that is specified as a percentage of the floor space of the room.  Living rooms and dining rooms require ten percent glazing and bedrooms must have at least 5% glazing.  So a 100 square foot bedroom would require at least 5 sqft of glazing.  These specifications are considered the bare minimum for passive solar lighting of a room during daylight hours.

The Building Code also has safety requirements for glazing relating to proximity to doors, the ground or the floor and the shower.  These safety requirements exist to prevent you from accidentally breaking the glass from falling through it  or shattering it by slamming a door, and if you do break a window from hurting yourself on the shattered glass.

The residential Building Code does not have much to say about glass in a green house.  However, it does specify that overhead glazing (like a skylight) must be made of safety glass so that large shards of glass cannot rain down on your head if the window breaks. Tempered glass meets this requirement since if it breaks it crumbles into little pieces (think car glass).

The specification in Earthship Volume I for the fixed greenhouse glazing units calls for ‘… double-paned, insulated glass …’ and ‘… stock glass units, 1″ thick …’.  Further, you need to ensure that the glass is not low E or tinted.  Many of the processes that improve the insulation of the glass retard the solar gain needed to make plants grow and to heat your building.  However, I do not remember any discussion of using tempered or safety glass for the sloped green house glazing. Can anybody clarify this based on their reading?

I searched the internet and did not find any definite specifications calling for tempered glass for the south wall earthship glazing.  However, most of what I read on the subject of sloped glass, overhead glass and green house glass consistently recommended using tempered glass for these applications.  From a website about construction and renovation issues,

…  my opinion as a professional inspector … if there’s any chance that based-on the position of these windows that someone inside the home (usually small or young children) or outside the home (gardener, you yourself, a careless drunk walking-by, etc.) could accidentally (or otherwise) impact the glazing, put in safety glass. It’s the best modern construction practice, period …the disclaimer: All information provided above is based-on contemporary NATIONAL building codes and practices. Your mileage may vary …

and I did find a few references to using tempered glass specifically in an eartship,

What about hail? Won’t the slanted glass break? … Wherever large sheets of glass are exposed to wind, ‘small’ projectiles and the elements, they should be tempered glass and in most places, it’s required.  Tempered glass is very difficult to break and when it does, it doesn’t break into shards of cutting pieces, but rather thousands of small, almost ‘granular’ pieces.  The slanted glass indicated by many passive-solar designs must be tempered glass.  Our window array has withstood 2-1/2” diameter hail, so far [crosses fingers, knocks on wood].

We have chosen to use tempered glass units for all of our green house glazing.  This glass is about 25% more expensive than standard units, but will hopefully minimize any issues down the road.

Since we are building the fixed windows ourselves, I also did some research on the ability of these units to successfully resist wind loadi ng in our area (in other words not shatter due to a wind storm).  It turns out that hourly wind speeds that have one chance in ten of being exceeded in a given year are used to specify glazing units.  In the Kamloops region our one-in-ten hourly wind pressure is 0.3 kPa according to Appendix C of the 1998 building code (I am using the 1998 values as these tables are no longer specified in the 2006 version of the code in favor of a separate document).   According to Table A-9.7.3.2.(1)A of the current BC Building Code (2006) 5 mm tempered glass works for our largest window size.

So, all of this results in the following specification for our fixed windows …

The fixed windows will be sealed, double-paned, insulated glass glazing units (1” thick) (tempered, regular 5mm glass).

Seems like a lot of work for that one line doesn’t it?

Our operable units will be standard tan vinyl sliders with a 3.5″ jamb to match our window framing.

We have not specifed our skylights yet, but it looks like we will use triple-paned units (made of safety glass) with a low E coating on one of the panes.  We do not want heat gain through these units.