What is Behind the Paint?
11/11/2019
Now that we’ve gone through how a home is documented, how all the parties collaborate during Construction, and even how aspects of the project are Specified, we will delve into some construction detailing. I am proud to present the seventh post for the Video + Blog series “What the Hell is Architecture?” A series that will present Case Studies, Code Commentaries, How-To Videos, and anything else that comes to mind to try and get the word out on what Architecture, Engineering and Construction ACTUALLY is. (Something the industry has done a poor job of enlightening the public on.)
This week we will “crack open” a theoretical wall and see how much building science happens behind walls, which most people are only concerned with the paint color. We’ll also poke our heads “above the ceiling” to learn what happens above and below us. For this post we will describe the detailing for a two-story wood framed home with board and batten Siding, CMU foundation wall and a pitched asphalt roof. Just like the construction of such a wall itself, we’ll start with the foundation.
- *Disclaimer*
- This is not the order in which such an assembly would be constructed, simply an easy progression for this post.
Excavation, Site Work and the Footers
We will fast forward past all the excavation and site work that must happen on a construction site prior to the building. We will go into further detail on that in a future post. To very shortly summarize, a significant amount of time (and money) must be spent to excavate the site, install all underground utilities and piping, and then place all soil to the proper grading required. (shape and levels of the land)
During the design of the project, the frost depth (we will explain shortly) will be determined, as well as a Geo-Technical report will show the condition of the soils below. This report will show how far below the earth the foundation must be placed so the building is stable and will not settle significantly. This depth will vary by building size and the conditions of the soil. A recent project I was on required 80-foot steel piles to be driven below all the foundation to bypass all the poor soil conditions to reach a depth of stable ground. At construction, excavation will dig down far enough that the concrete footing can be poured, this is where the foundation walls will sit. The purpose of this is to help distribute the weight of all the walls that will be built above. No matter how small a structure or addition you want to build is, if it is built without a properly designed footer or foundation system, its going to eventually sink into the ground. Worse, if it does so at an uneven amount throughout, its going to crack apart and eventually fall apart.
How Far Down?
We mentioned frost depth earlier in the above paragraph, as temperature fluctuates, so does the ground itself. Particularly in winter, the ground will freeze and thaw, causing it to expand and shrink. The footer needs to be built BELOW a certain depth where the ground no longer experiences this temperature movement, known as the frost depth. This also varies by region, somewhere like Waco, Texas only needs to be built 10 inches below the soil. The project in this example however was built in lovely Upstate New York, so it needed to be built roughly four feet below ground.
Foundation Walls
After the footer has cured (drying process for concrete), the foundation walls are installed. For this project, it was a reinforced concrete masonry unit wall, also known as CMU. While the CMU blocks are being installed, vertical and horizontal reinforcing bars are installed as well, also known as rebar. Once the blocks and reinforcing are installed, the wall is fully grouted, with galvanized anchor bolts anchored in the grout. These bolts serve the purpose of attaching the wood framed construction above to the foundation walls, these bolts are bent so after the concrete cures, they cannot be “pulled out” by wind or other forces.
Next, insulation is adhered to the exterior of the foundation wall. We can spend an entire post discussing the Energy Code that buildings need to meet, and in the future we will, but for this post we will just say that the insulation must run the full height of the wall and is required to be R-20. R Values are used to explain how well something can resist heat flow through it, the higher the value the better. The insulation is installed below ground to avoid something known as “thermal bridging.” Uninsulated elements can easily transfer any heat or cold that it comes in contact with, so an uninsulated concrete wall will pretty much let all heat or cold right through it, which can then travel through the slab and into the building. Depending on the situation, this can also require insulation below the Ground Slab, in this case it is not.
The final step before all this expensive and time-consuming effort is buried, is to install a system for drainage. A lot of water is below ground, and if there is no drainage provided, the Hydrostatic pressure will build at the base of the foundation wall and eventually cause damage or even failure. For this project, we will install a perforated pipe around the perimeter of the footer, surrounded with larger stones meant for drainage, wrapped with a fabric meant for filtering. This allows for any water to enter the pipe and be removed away from the building, preventing any pressure.
Burying the Wall
After all that, the foundation walls can finally be “backfilled,” however regular soil isn’t just going to be put back, in this case a 6” layer of compacted structural fill will be placed below the slab, and granular fill below that to help with drainage. More detail on those items in the future, when we discuss Civil Engineering to a greater extent. Now on both the exterior and interior sides, the backfill is not brought up to the top of the foundation walls, this is intentional. On the exterior side, it is left six inches below, as the exterior siding we are going to use for the building should not be in contact with soil or water. For the moment the insulation is left exposed, but we will handle that later. On the interior side, it will be left four inches below, as we have a Concrete Slab to install.
The Slab
Before we can install the concrete slab, we must protect it against moisture. We accomplish this with a 15-Mil (Mil is equal to 1/1000 of an inch) polyethylene vapor barrier beneath the slab, which is a special plastic membrane to keep water out of the slab, and eventually the building. Since it is just a layer of plastic, it needs to be protected as well. (Installing plastic between rocks and concrete will surely rip it and make it useless.) We can protect the plastic with a very fine layer of sand placed on the structural fill, making it safer for the plastic to be installed. At our slab edge, we must create an expansion joint, which is just what it sounds like, a flexible material that can allow some movement in the slab without hitting the foundation walls. Finally, we can place our rebar and pour our slab… We have made it to the ground level and can start building above ground.
Sill Plate
Now that we’ve finished our foundation, we need to create a connection point between the foundation and the framed walls above it. This is done with a sill plate, which is the term used for a pressure treated 2×6 (actual size is 1-1/2” x 5-1/2”) piece of lumber attached to the galvanized anchor bolts we installed earlier. Now as always, we can’t just install this plate without some moisture protection first, so we install “flashing” on the foundation wall. This allows any water that gets in the wall to be moved away from the foundation wall. We also must place some foam sill sealer beneath this plate, this reduces air leakage out of the building.
Framing for Fenestration (Openings If You Didn’t Read the Other Posts)
Now we frame the wall, again with 2×6 studs. These studs are almost always spaced at 16 inches on center, we will explain later why this is important. We will start with all the first-floor framing, including windows. All vertical studs that run the full height are placed at 16 inches on center, excluding those at the window, which are installed at a height to the bottom of the window opening. (A height dictated by accessibility guidelines, in this case 2’-9” above finished floor.) A double sill plate is installed below the window opening, with the top board being a 2×8 piece of lumber. A single 2×8 stud is then installed at the sides of the window, known as jambs. Finally, a 2×8 is installed at the top of the window, called the head. The last few sentences were not typos, even though the studs are 2×6, the window jamb, sill and head are framed with 2×8 pieces of lumber, this allows for the insulation to have lumber to be secured to and not be left exposed.
Header
Above the window head, we will need to create the Header, two terms that are used interchangeably despite being very different. The purpose of a header is to transfer any weight that would be brought down on a window or opening and then transfer it to the adjacent studs, allowing the weight to be transferred down to the foundation. Glass is not nearly as flexible as most building materials, so any kind of movement at all will always result in glass cracking or breaking. The header is constructed of three 2×8 vertical pieces of lumber, with a half inch piece of plywood between each piece. The plywood not only strengthens the header, it makes up for the width difference in the 5-1/2 inch studs and the vertical header only being 4-1/2 inches without the plywood. We will go into structure in a future post, but for now its worth mentioning that the taller an element is, the more resistant to bending it is. (If you take a 2×4 in your hand, you can most likely bend it in either direction of the “shorter” side. But if you turned it to the taller dimension, you could most likely stand on it without it moving.) Finally, the top of the header is framed to the top stud.
Platform or Balloon (Not the Opening to A Riddle)
Before we move on to framing the second floor, we need to construct our ceiling and floor. This is known as “platform framing,” which is basically the concept of building the walls, then the floor, then the next wall, etc. Older buildings were built with “balloon framing,” which is framing the entire height of the exterior wall. This has been phased out mostly, as its not as easy for the laborers and provides fire concerns with full height shafts in the stud cavities for fire to travel between floors easily.
What Came First, The Ceiling or The Floor?
On top of all the framing we have built, we place the exterior rim joist. A tall piece of lumber that spans the height of the floor and ceiling, it is aligned with the exterior face of the wall framing. There is then an interior rim joist that is the same as the exterior joist, except it is aligned with the interior face. These joists provide not only something for the next floor’s walls to bear on, it also allows insulation to be placed to prevent thermal bridging. In this case we will provide spray foam insulation between the joists. (Architects do not control means and methods of contractors, however it is worth explaining here that the insulation needs to be installed before the interior joist…on more than one occasion when both are installed without insulation it is not uncommon for the insulation to just be “forgotten.”)
For this specific occasion, a one-hour fire rated assembly was needed between floors. First, 14” TJI joists are secured to the rim joists at the same spacing as the studs, 16 inches OC. A TJI joist resembles a wooden beam comprised of engineered plywood between two pieces of lumber. To create the ceiling for the first floor, ½” resilient channels are secured to the bottom of the joists, then two layers of 5/8” gypsum board sheathing are secured to these channels. These channels not only provide a way to secure the insulation that will be installed later, but it also provides significant reduction in sound travel between floors. (By allowing the gypsum to be secured to the channel, and then the channel being secured to the joist prevents any direct connection between the joists and Gypsum, providing a reduction in noise travel.)
As we discussed a paragraph prior, a certain fire rating is required between floors, however the gypsum board satisfies this requirement. In this case however we will also use 3-1/2” thick sound attenuated batt insulation to further reduce sound travel between floors. Now it is time to close the assembly and install the flooring. Secured to the joists will be ¾” tongue and groove decking. This provides a secure, flat surface for our underlayment (3/4” self-leveling gypsum in this case) Once all construction is complete, the “finish flooring” can be installed. An important note here, the decking is laid over the entire floor area, including over the rim joists. The underlayment and finish flooring however, are installed up to the wall only, thus making the wall framing necessary to be completed first.
Second Floor…More of the Same
Now that we have framed our “platform” we can frame the second floor as well, and as the sub heading above claims, its pretty much the same process in which we’ve discussed already. The only difference being that due to the height of the roof, there isn’t enough room to place studs above the header, so we will just use straight runs of lumber on top.
The Roof…The Roof…The Roof is Hopefully Not on Fire
The pitched roof for this project is going to be accomplished with gable trusses. These trusses are placed on the top plate of the wall assembly at a spacing recommended by the manufacturer. Roofing itself could fill multiple posts, so we will dive into it in detail in the future. For simplicity in this post however, we will simply state that “a lot of roofing things” are placed on these trusses. To close the exposed area on the exterior side of the wall we will frame it with horizontal studs spaced at 16” O.C. and simply securing vented cement fiber soffit panels. Our future roofing post will explain why it needs to be vented.)
Prior to the roof trusses being placed on the top plate, a continuous polyethylene air barrier (fancy plastic) will be wrapped over the top plate, and then secured to the underside of the truss. (It will need to overlap the exterior weather barrier, so it won’t be secured on that face of the wall yet.) This air barrier is installed for the exact reason you would think, to prevent air movement between the second-floor tenants and the attic. It also provides something to hold in the loose fill insulation that will be placed in the entire attic space. This insulation is not only held up by this air barrier, but we will also install resilient channels and a gypsum board ceiling below these trusses.
Insulation
As we said, the attic must be insulated, in this case with R-49 loose fill insulation. However, in order to save cost, we don’t need to insulate the entire attic cavity, as there is no need to place it on the exterior side of the wall. To box in the insulation, we will frame a mini wall between each truss. This is done simply with 2×4 framing at the top plate and a few inches above the height of the insulation required. The wall itself will simply be the engineered structural sheathing panel system we are about to discuss, brought up above the insulation and secured to the 2×4 framing.
Exterior Sheathing and Wall Enclosing
We have continuous insulation at the foundation walls, and we will have insulation at the attic. In order to keep the continuous insulation at the entire perimeter required by the Energy Code, we will need to continuously insulate the entire exterior wall. We will accomplish this with an engineered structural sheathing panel system. (Particularly one with an integral weather barrier.) This panel is secured to all the stud framing completed earlier, as well as above the trusses to enclose the attic insulation.
At this point we will then install batt insulation in all the stud cavities, the R-Value required (as well as all building energy requirements) is dictated by the Energy Code, in this case R-20 is used. At this point the insulation and wall can be closed up on the interior side and 5/8” gypsum is secured to the studs. Almost all gypsum boards are created at 4-foot wide modules, thus creating the necessity for the 16” spacing for our studs mentioned earlier. There is significant spackling, sanding and painting of gypsum to finish it, for this post we will skip over that.
Windows
Now we will install our windows in the openings, however as you’ve probably guessed after the last few paragraphs, we need to provide some moisture protection. This is done with some flashing installed at the entire window perimeter, overlapping the weather barrier provided by the sheathing system we just discussed. Now we can place our windows, as well as also place 1×4 wood sills and aprons for each of the window openings.
Even More Moisture Management
We will install a vertical rainscreen system for this wall. A rainscreen is simply a gap or cavity in the wall which allows any moisture that penetrates to travel downward and out through a provided system. This may seem odd as many assume water can’t get in if it’s a solid wall…however there is nothing to ever stop water from entering a wall, and if there isn’t a method to allow it to dry out or leave, then some serious problems will occur with rot and mold. Some very small wood strips are installed vertically, creating the cavities for water to travel in, in this case we will use a special fabric product that helps move water downward towards the aluminum flashing we will install. (You may not realize it, but any brick wall you look at it, you are seeing dozens of “weep holes’ that let water drip out. They are on the bottom course of masonry and look like somebody forgot to grout all the vertical joints between bricks.)
Time to Pretty Up the Exterior
All our assemblies are insulated, managing moisture, structurally sound and are closed. So now we are done right? No, as most people don’t care for the appearance of an exposed wall system. For this project we are using a cement fiber board and batten system. You are most likely familiar with this on sheds or garages, it is simply a siding system of large horizontal boards with smaller vertical battens in between. First, we go back to the exposed insulation we left above grade at the beginning of the post. Exposed insulation tends to fail very quickly, so we will install a fiberglass protection panel to cover this and tuck it in behind the flashing installed previously.
Secured to the vertical wood strips we just installed, is the exterior cladding of cement fiber panels. At the “bottom” of the wall, (not truly the bottom, as it is held 6” above grade) at the top plate and halfway up the wall we will install a 1×12 (3/4” x 11-1/4” actual size) cement fiber panel at the entire perimeter of the wall. We place it at the top and bottom to provide the smaller battens two surfaces to terminate into. We place one at the middle as there are height maximums for this cement fiber cladding and we want to hide the seams between panels. We then install 1×6 (3/4” x 5-1/2” actual size) panels at all the window perimeters. Finally, we install the smaller vertical battens throughout the entire perimeter of the building.
Closing
So, you’ve now gone through the construction of a corner of building. To say that there are quite a few other aspects of building construction that we have glossed over in this post is an understatement. It is also worth mentioning that very few buildings only have one wall, ceiling, roof and floor assemblies and thus require significant detailing for the intersection of all these.
This post covered ONE version (of countless variations) of ONE wall type (there are thousands, as well as unique assemblies for each project) with ONE set of building materials. (of endless material selections, with more created each day) ANY construction you undertake needs the guidance of an Architect to meet Energy Codes, moisture management methods and current building science trends.
If this post imparts only one takeaway, it is hopefully that Architects are not simply drawing nice images of wall colors and furniture choices. They are juggling every design decision with prior material knowledge, building science, construction methods and trends, building codes and thousands of other factors not listed here.
If you feel I missed the mark or am misinformed on any aspect of what I wrote, please let me know.
If you did enjoy the post or have any ideas or questions you would like to see or hear about in the upcoming posts and videos in this series, please let me know that as well.
Thank you all and have a great day.
- Bryan Toepfer, AIA, NCARB, CAPM
- Principal – Architect
- TOEPFER Architecture, PLLC
- Direct: 518.443.9366
- www.toepferarchitecture.com
- btoepfer@toepferarchitecture.com
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