AIR TIGHT BUILDING ENVELOPES

                                                                                                                                         

    As we move ahead in our green church project, the next step is attempting to make the interior of the church as air tight as possible, no small feat in itself. We fully realize that we will not get every opening, rather we are attempting to seal up each level, crawl space, main floor and attic as best we can. The term building envelope is sometimes used indiscriminately as a catch all term, in reality it’s not. A building envelope is what separates the occupied space from the outside elements. This envelope is a blend of the structure and its parts, exterior walls, windows, doors and any other openings like a kitchen stove vent or dryer vent for example, each one being an area that needs to be sealed with various air seal products to eliminate air loss thru or around this opening. A thermal envelope is the installation of a complete insulation and air barrier around the inside of the home.

      The discussion about vapour and air barriers is a complex and often confusing element in home construction and by no means will I answer this in one column, or many columns for that matter. What we are attempting to do is reduce the air leakage from the inside of the church, escaping to the outside. There are enough studies to fill a small room on the differences between air and vapour barriers. I look at it, as do many professional builders as two separate parts of the building envelope. I agree with a statement made in SAB….” Building scientists estimate that as much as 100 times more water vapour can be carried thru walls by air leakage than will be carried out by vapour diffusion….this makes air barriers 100 times more important than vapour barriers. We all know that excessive water vapour captured within in the building structure spells disaster to the structural integrity over time.

     In our part of the country, we must stay warm in the winter and increasingly we air condition our homes due to the heat of the summer, again a climate change due in the most part from global warming. Living in an insulated, sealed home can make life more comfortable, however all of the water vapour we create by showers, cooking and even breathing is absorbed into the air of the home and this air will attempt to exhaust the home by diffusion and air movement. This is where a properly installed mechanical ventilation system becomes a necessity, hencemthe recommendation of a heat recovery ventilation system for any new or extensively renovated home, such as our church.

    We realized when we began this project that adding an air barrier on the exterior, that’s the wrap you see on new home, the originator was called Tyvex and there are many brands today, was cost prohibitive. We then decided to make an extra effort to reduce the internal air leakage and accepted the fact we will have some vapour diffusion.

    This is a time consuming process and if you have been following our progress you will know we have assembled a completely new internal steel studded, insulated wall and angled ceiling. As we have progressed, sealing the steel studs to the floor, adding spray foam in the voids of the old plaster and along the floor seam is all part of this air envelope. One thing I had to stress with our crew was, installing an air barrier is not just covering the walls and ceiling with 6 mil plastic, it’s an entire process, starting long before the plastic goes up and Cathy, Spar and John quickly caught on so to say that construction workers can’t change and advance their thinking is wrong as long as they understand the reason why they are doing something. In fact our crew started to pick up some areas that might have been missed had they not fully understood the reasons behind an air barrier envelope.

    In many cases, when the 6 mil is applied, acoustical seal is applied at to the top and bottom plate, a few studs to keep it flat and then the plastic is stretched over the external walls. My feeling is that each cavity, the space between the studs is an air pocket and our crew applied sealant on every stud and plate. I then had them take the plastic, cut to sections and roll up in a piece of ABS pipe and place it against the wall and unroll this plastic like wallpaper, stretching and pressing the plastic to the sealant, making a snug fit.  While time consuming, we now have sealed sections in each stud area.  

Cathy was charged with taping all the openings for water lines, electrical boxes and vents. Far too often I see two or three pieces of Tuck-Tape quickly fitted around these openings, hardly an air seal. Cathy cut the plastic that was already fitted around the electrical boxes and taped the edges to the plastic. We then used low pressure foam to fill in behind the boxes where there may be any air remaining. Then she began a layering of the Tuck-Tape, carefully pressing each layer until she was 6-10 inches from the actual opening. While some may say this is a waste of time and tape, remember the original comment about air leakage and voids in the walls are high on the air leakage list.

    Due to the angled ceiling and some openings in the walls for electrical and plumbing, we created a “chase” of kind and these larger openings were filled with spray foam along with the space between the wall and the angled ceiling. As discussed last week, spray foam gives us both the insulation and the air barrier we need.

    Each week my lady Donna puts these columns up on our web site, www.alltechgreenchurch.ca so if you wish to see more pictures or have missed a column you can catch up on our progress. On a personal note, “Thanks” to the many readers who have sent comments, glad to see everyone is enjoying the chance to see firsthand how a renovation this extensive can be done.