CREATING AN INTERNAL AIR BARRIER

     If you have been following this series, you will know we are embarking on a major renovation of a heritage church to bring it to an EnerGuide 80 home.  To bring you up to date, go to www.alltechgreenchurch.ca and click on columns. This is number four in our series. This week we look at what is probably the most important part of the renovation, upgrading the interior envelope to attain the energy efficiency level we are striving for. 

     The church has a rubble stone foundation crawl space, which will be spray foamed with 2 lb. closed cell foam. The attic will be blown in with cellulose to an R60 level. The more difficult part is the walls and an angled portion of the ceiling. The church is wood frame, full 2 inch by 4 inch walls and 2 inch by 6 inch rafters. The  walls, angled ceiling and attic portion are filled with wood shavings. 

                                                                                                                                                                                                                           

     Based upon calculations by our energy auditor, we currently have an R12 wall.            This is far from the R30 we are hoping for, which means we must increase the insulation values by introducing a significant thickness of insulation. 

     


     One option would be the installation of 6 inches of exterior rigid foam board, but it would mean removing the siding, which is a costly process and puts the air barrier on the exterior wall, something I am not a fan of doing.

.     The only other option to get to where we want to go is to create a complete internal wall and ceiling, continuous air envelope system. Fortunately, the open area of a church makes this idea feasible. We discussed a number of scenarios and each one meant reducing the interior space considerably. Because this is not a large church, we have opted to build up the new wall system as thin as possible, starting with a steel studded wall filled with Roxul insulation.  Roxul insulation is made from stone wool, a creation using volcanic rock and recycled slag and there is an application that is manufactured specifically for steel studding. We will delve into Roxul next week.

    The main concern we have is creating an air barrier over the existing wall and adding to the insulation values, trying to reach the desired R values. By using Roxul R14, if we can make a 3 5/8 wall work, we still need another R4, plus some form of air barrier. 6 mil plastic is most commonly used. The other issue is the angled ceiling, which is 8 feet wide and runs down both sides of the church; check the photo gallery on the web site for a better view.

     First we will address the wall and, after doing some extensive research, with the desire to add foil radiance to the air barrier, we decided to look into what is known as EPS rigid foam. This foam board comes with a foil backer on either one or both sides. There are a number of benefits to this material, over and above the radiance benefit. It is available in different thicknesses with ratings, depending upon the manufacture, ranging from R 3.6 to 6.5, per inch. When we add these insulation values to our wall, a one inch or 1.5 inch foam board gets us to our desired level of insulation. By using foil tape on the joints and closed cell spray foam around any opening; this also gives us an acceptable air barrier.

     There is one catch here, something called fire rating and this usually necessitates the installation of ½” drywall over the foam. In our attempt to bring this renovation in within the budget, we would like to eliminate this added cost.  Back to the drawing board, as they say, and further research came up with a foil board called Thermax made by Dow Corning. The catch, its “very” expensive and only sold in the USA. Trying to reach the OBC fire ratings and our insulation values, within reasonable cost, means we must scrap the foil board and use conventional metal studs, increase our wall depth to 6 inches and use the Roxul R24 batts on 24” centres, then use the 6 mil plastic and drywall the walls.

     This left us with something of an issue, how do we get the angled portion to the same level as the attic, given the fact this raftered area is an R18.  Here again, after some considerable research and cost evaluations, using the steel studding, making up wall sections and screwing them to the wood angled wall and then using the same batts made the most sense. We now have R 42 in this angled area. Faced with the fact we now must drywall the walls and angled area, the addition of John Mansville AP foil faced 2” inch rigid foam board on the angled area under the drywall, adds an R13 factor, bringing us to a total R value of R55, which is much closer to the R 60 of the attic area. We have accepted the additional foil board cost here in order to get the “R” value to the level we are looking for.

    While I would have really liked to use the Thermax sheathing, the question of acceptance of fire rating in Canada coupled with the cost, eliminated it. Standard EPS foil board has to be covered in drywall now so when we factored in the cost, increasing the steel stud wall thickness to 6 inch and using Roxul was the only answer for the largest areas, the walls.  Once we start the actual project in May, we will be detailing every step. Careful attention to sealing every window and door opening, electrical box and any other wall openings, can and will make or break this extensive envelope addition.  Stay tuned…