building sciences Installing Windows in Insulating Sheathing Openings A B C D E A) Insulating sheathing installed over frame wall; B) Install beveled wood siding in frame wall opening at sill to create slope; C) Install formable flashing at sill; D) Install window plumb, level and square; E) Install flashing tape at jambs. tapes “stick” better; they stick unbelievably well. The acrylic tapes are often used to “terminate” the top edge of a butyl tape. This is necessary as butyl tapes can fishmouth (their top edge peels away or rolls away over time), so their top edge needs to be “terminated.” This type of fishmouthing does not occur with acrylic tapes. Although most flashing tapes are butyl, advances in acrylic technology have allowed a limited number of acrylic tapes to function as both flashing tapes and sheathing tapes. Today’s tapes are resistant to UV and heat and rain. How do we know? We stick them outside for years and watch them, and we also take apart older buildings that have used them to see how well they did (Photo 10). Nothing beats real-world exposure (Photos 11, 12 and 13). The real-world exposure testing is used to develop (“calibrate”) short term “accelerated aging tests.”** The term “acrylic” is deceptively simple and does not give justice to the evolution of the materials science behind it. Acrylic adhesives are based on acrylic polymer chemistry just like many high performance caulks and paints. The polymerization is “tunable” **I am old school. It is hard to beat taking a building apart after 15 or 20 years and having a peek. We have been doing this stuff for around 20 years and that is why I am now comfortable with the technology. The two decades of experience “tunes” the accelerated aging stuff and system evaluation stuff to the point where I now believe it. See the testing protocols and evaluation protocols in the Bibliography section of this column. Photo 10: Fifteen-Year-Old Tape. This is my house. I took things apart at a few locations last summer to have a look at how things were working out. The tape, which is acrylic based, looks brand new and is sticking like it is brand new. The wood furring has aged, but not the tape. I am a believer. Wasn’t that a Monkees’ hit recorded in 1966? Photo 11: Building Science Corporation “Skunkworks” Exposure Facility. To test tape resistance to UV and heat and rain, we stick them outside for years and watch them. The real-world exposure testing is used to develop (“calibrate”) short-term “accelerated aging tests.” Photo 12: Proprietary Florida Exposure Test Site. Not one of our sites. Lots of folks are testing things this way. Especially the folks who make things to sell. Nothing beats real world exposure. Photo 13: Horizon Homes Test Panel. You would be surprised at the number of homebuilders who are testing the stuff they plan to use to see how it works. Our rule of thumb is that if it works exposed like this for two years you are pretty much good to go. 60 ASHRAE Journal ashrae.org February 2013