The Many Layers of the Planted Roof
After the roofing membrane was completed we got busy putting in place the multiple components that make up the green roof. This started with 2″ of expanded polystyrene. The insulation board of choice for this application is extruded polystyrene, think rigid pink foam insulation at a big box store. But the truth is I had the EPS as a leftover and the only down side is it absorbs more water than XPS which reduces its insulation capacity. Without the added layer of foam that roof is already approaching R-70 so I was willing to sacrifice a little R value to save the money in this case.
The next thing to go down is a drainage layer. This is to provide a free-flowing area for the water that drains through the soil to exit the roof without much obstruction. For this I chose a product called Enka Drain, made by a North Carolina company. The particular drain mat we used consisted of an open fiber mat with an egg carton kind of profile topped by a water retentive layer that was like a thick wooly moving blanket. The wooly blanket holds about 1/4″ of rain to slow the desiccation of the roof soil, a problem for roof soils in hot climates, for example Georgia.
The drain mat we used didn’t come with an integral soil barrier cloth so that is the next layer. Here we used a very heavy non-woven soil barrier that can move a lot of water, 150 gallons per minute per square foot. That is well beyond what the roof could shed or what would ever fall out of the sky. The soil barrier will prevent the roof soil from clogging the drain mat so the roof remains free-flowing. We brought the soil cloth up the edges of the parapet to serve as a protection layer between the roof membrane and the gravel edging that would end up around the perimeter of the roof.
It is common to use gravel as an edge material for green roofs, but it requires something to keep the gravel from mixing with the roof soil. Commercial edgings are often perforated aluminum sections adhered to the roof keeping the gravel in place, but allowing water to drain. Those edgings are pretty expensive so I was looking for some alternative that would be simple and cheap and readily available. After kicking it around for a long time, I came back to an idea I had early on and rejected. Beside my shed I had a pile of header block left from a recent job building a garage, these are 8″ concrete blocks with a 4″x4″ notch out of the top side commonly used as a form for a concrete slab poured into the top a block wall. I realized though that I could quickly cut the block in such a way that once the gravel and soil were in place only a thin line of the top of the remaining block would be visible creating a clean 1 3/8″ concrete separation. And if we placed the block on a bed of gravel it would allow for water to drain freely between the soil and the gravel edge. We also could use the gravel bed to place the block level allowing for the roof soil to end up level, which is preferable. We used silicone to glue the individual blocks together into a continuous edging.
As a light weight filler we broke up scraps of styrofoam and filled the space between the blocks and the parapet to within a couple of inches of the top edge of the block. It keeps it from the landfill and it serves as a good and light drainage medium. On top of the foam we placed a strip of the soil cloth and then pea gravel to the top of the block. All the gravel used on the roof is round stone for the obvious reason that sharp edge stone is a hazard for roof membranes. In the picture above (center top) is a perforated aluminum box we made on site to keep the drainage scupper clear of stone and debris. There is a removable lid to facilitate cleaning the area around the scupper if it ever proves necessary. Around the aluminum box we used a larger 2″ round stone, to make the area around the scupper drain even more readily.
With all these pieces in place we are ready for the soil, something that will wait until construction is nearly completed. Even so we have had a lot of rain this summer to test the roof package as it is now and it has drained very well (even during the violent storm that dropped 3″ of rain in about 30 minutes and uprooted three trees across the street). There is a noticeable delay from the beginning of the rain to when water starts coming out of the scupper, a good thing that will only get better, that means a longer delay, with the soil installed.
Seven Thousand Pounds of Insulation
With the house in the dry, we were free to move ahead on the interior. Our first step was to make sure that all the penetrations through our air barrier (the plywood sheathing) were sealed before we covered them with insulation and made them inaccessible. With that done we insulated. There are many things that we do on our houses that push well beyond common practices for the area. For much of those, we do them ourselves, but there are some things we choose not to do or are unable to do, and rely on subcontractors, often asking them to move beyond their experience and sometimes comfort to accomplish our end goal. I will admit that this doesn’t always go well. This is sounding very ominous so let me say now that the insulation went fine, but dense packing 12″ walls with the quantity of insulation I needed is not something they had done before. I was nervous about this and considered trying to get a subcontractor from Atlanta that may have had some experience with what I wanted. Finally I decided to use the local contractor I had used for years and with whom I had a good relationship, giving them fair warning, though, that I would be spending a lot of time over their shoulders making sure they understood what was required.
So to keep dense packed cellulose from settling in a tall wall you apparently need to pack it to a density of 3 pounds per square foot. That is really dense, think a full belly after a large meal kind of dense. And obstructions in the wall don’t help, as the insulation fills so fast it will pile up against an obstacle and compact and refuse to flow into a deeper cavity. So having an idea of where the cavities that can’t be seen are in the wall is important to ensure that insulation gets into them . This involves much filling and pushing with hands, really until no more cellulose can be pushed into the hole. I realized during the insulation install that I need to be more careful with making sure that framing components are placed to allow free access around them, making the insulators job easier and eliminating extraneous wood at the same time.
The total volume of insulation in the house is around 2400 square feet, the insulators put 7200 pounds of cellulose in the house, pretty much our target of 3 pounds per square foot. I will do infrared imaging after we get some climate control going to see check how well-distributed it all is.
Plywood Walls
We made a choice to cover the interior walls with plywood. I am going to say up front this was not an inexpensive decision. I bought a large quantity of a decent formaldehyde free 1/2″ maple plywood, and got a great price and even so it probably cost 3 times what drywall would have been. Though it did relieve the worry of having to pick paint colors.
Cabinet plywood comes a half-inch oversize in both dimensions, so our first step is to true the sides and cut them to an accurate 48″x96″. This is fast and precise with our track guided circular saw and a jig we make to dimension the sheets. Creating very precise, square sheets lets us butt the boards to one another with virtually no gap between them, making for a largely seamless installation.
All the panels are face screwed with trim screws. Concerned about the potential of cellulose dust filtering through between the panels, we put a small bead of clear caulk along the edge of each sheet before placing the next sheet. With all the plywood in and the detailing around windows and doors finished we are moving on to cabinetry and storage.
And tomorrow the solar goes in!
small house small lot 