Two Walls

Where we are now on the project, with the roof framing on and decked, most houses would be ready for sheathing or likely sheathing would be finished and the framing pretty much done, but for us there is a second wall to build outside the first one.

Rain

It is always good to get houses dried in and safe from weather in as short a period of time as possible. With less swelling and movement, and less effort to dry the framing after the house is enclosed, houses built with less exposure to rain are favorable. Sometimes the weather and scheduling cooperate for this to happen, this spring has not been a cooperative time. So when we start on the second wall when we could be getting the house in the dry and 3 more inches of rain fall over a weekend my sense of urgency heightens, and I start to regret the complexities of my little house. In a month, typically one of the dryest, where twenty days in we’ve gotten about seven inches of rain, a month that averages three, I force myself to remember the house will be covered soon and it will dry out and the second wall is integral to the whole concept, lots of insulation, little foam. Here’s a spoiler alert, building two walls is about twice as much work as building one. Maybe more.

Twelve Inches of Insulation

The whole purpose of building two walls is to fill the space between them with insulation. In our case it will be twelve inches of cellulose giving us a wall with an R-value somewhere in the low 40’s, but super insulated houses are put together in a variety of ways. A standard stud wall covered in a bunch of foam can yield walls with R values approaching 60, but the building has to be covered with 8 inches of foam to see performance like that, and I have already written about why that approach wouldn’t work for us, mainly we don’t like foam. Another method also starts with a traditional wood frame wall and after sheathing another frame is added. I have done this with a 2×6 wall attached to the sheathing of a 2×6 wall, an even more insulated version attaches a 12 or 14 inch I joist to the sheathing of a framed wall. I can see the value in that approach, especially in time savings, but we would have had to sheath both walls, which I think would cost as much as the time saved, and I joists aren’t cheap. The method we are using does two things that are important when it comes to insulation, first it provides a lot of space. Since the insulation performs generally in a linear relationship to its thickness, more is better. Doubling the R-value of a wall reduces the energy loss by half, with caveat that the rest of the components of the building envelope, namely windows and doors, need to keep pace. Secondly, it separates the interior surface from the exterior. With the two walls, a continuous blanket of insulation rests between the interior framing and the exterior framing. This differs from a standard single frame wall where the stud touches both the interior surface (usually drywall) and the exterior surface (usually osb sheathing) and bleeds energy from one to the other, because wood is not nearly as good an insulator as, well, insulation.

On our site we were severely limited in the footprint available for building. Building to the full extent of the available space we have a structure that is 16 feet by 29ish. The twelve inches of wall thickness versus say a more typical 6 inch wall represents more than 100 square feet lost to insulation. I only say this because for us it was a balance between sufficient insulation without losing too much floor space. Going through the trouble and expense of building two walls, it is likely better to get more insulation out of the prospect; expanding to 16 or 18 inches of thickness would involve the cost of some more insulation and some exterior surface, small expenses relative to the second wall. Again make sure the windows keep up.

Lots of little pieces

Our walls are built with an inner load bearing wall that uses 2×4 studs on the lower floor and 2×6 studs on the upper floor (because of the wall height). The outer wall is 2×4 studs. Because it sits on the foundation wall the outer walls starts about 3 feet higher than the inner wall. The two walls are staggered with the inner wall breaking at the floor system and the outside wall extending 5 feet or so past that point breaking at a plate and then a second level of studs extending to the roof deck.

the lower half of the outer wall is in on the left, both the lower half and the upper section are in on the right. the dividing plate is visible about midway up the wall.

The roof deck ties the outer wall back to the structure at the very top of the building and of course it is tied to the plate on top of the foundation wall. Between those two points we used a lot of 3/4″ plywood gussets spaced 2 foot on center vertically to tie the two walls to one another.

There are a couple nice things about the two wall system. First only one side of each wall receives a finished surface, so if there is a width discrepancy between a stud and the plate they just have to be flush on the finished side, the other side will only see insulation. Similarly when we needed to switch to 2×6 for the upper inside wall we attached an extra band to the floor system and extended the additional stud width into the insulation cavity. We didn’t need to use 2×6 on the lower wall just because the upper wall needed them. Also the walls could be plumbed and straightened independently. So small errors through the floor system, for example, didn’t need to transfer through to the outer wall, we just set the plate in the middle of the outer wall to a string and tied it back to the inner wall that had been plumbed and braced, doing this again at the mid-point of the studs we were able to achieve a very flat outer wall surface.

To tie the outer wall to the inner wall we used 3″ x 11″ x 3/4″ plywood gussets nailed to aligned studs in each wall. There are a lot of gussets. We had form material from the foundation walls we were able to get the gussets from, 800 in all.

The outer wall is finished to the roof deck and we have started the gussets between the two walls. the gray color of the gussets is concrete. the gussets are made from the form material for the foundation walls

This shows the outer wall complete in the back and the many gussets in place to tie the two walls together.

 

With the outer wall completed (except for some blocking) our next framing task is the roof dormer for the stairway access to the roof and the sheathing. The forecast calls for few dry days. Sometime next week we ought to be ready for some roofing, finally a dry house!

Framing the Roof(s)

With the second level walls framed, plumbed and braced it is time to frame the roof or actually roofs. On top of the two different height walls are two separate roofs, framed with different materials. The lower roof will be the planted roof, built up with multiple layers of waterproofing and drainage material, topped with 12″ of soil. The upper roof is the roof that supports the dormer that covers the roof access stairs, with the remaining area, about 8′ x 14′ serving as a patio. The different uses and associated loads of each roof dictated the size of the framing material needed. In the case of the planted roof, covered in saturated soil, the loads required a 16″ I joist, for the other are, just serving as a patio, only a 12″ I joist was needed. The deeper the joist, the more insulation that can be packed in, which is a good thing, and I would have prefered to use 16″ joists on both roofs, but that additional 4″ would have put the rear section of the house beyond the maximum allowable building height. So we have to be satisfied with an R value of about 50 including the foam tapers on the top of the deck for that roof, where the planted roof will be more like R70.

More Cantilevers

Step one: install the LVL’s that will hold up the upper roof and cantilever the lower roof over the bay window

Since the bay windows project from the building without bearing directly on the foundation, their weight being carried by cantilevered flitch plates at their base, they are not able to carry the loads from the planted roof. So to support the load from the portion of the roof that sits above the bay window we employed double 16″ LVL’s cantilevered over the bay. The double LVL’s served as the rim along the entire front and southeast side of the roof.

The cantilevered LVL rim boards

The beams join at a corner of the building that is suggested by the rest of the walls, but not realized until the roof comes together over the bay window. This creates an interesting play of triangles: the two corners of the window projecting from the main building and the roof projecting over the center of the window like a ship’s prow. This will become clearer when the second outer wall increases all three projecting points and even more so when the sheathing solidifies the mass of the building.

We also needed a set of double LVL’s across the transition between the lower roof and the upper roof. Because of the orientation of the dormer for the roof stairs, the joists for the upper roof had to run back to front. This necessitated a beam across the building at the roof transition. The lower roof joists don’t bear on this beam, they run side to side with both ends landing on a wall, with the exception of the joists over the bay that hang from the previously mentioned cantilevered beam.

Getting the lower roof joists into place

Planted Roofs

It is easy to write about the process of building the roofs, placing framing members and revealing the puzzle pieces that will go together to make the whole. In the end this is what building a house is, a series of steps and layers that culminate in a place where people live. It is also easy to get lost in those steps, to substitute process for purpose. In our case I think it is worthwhile to discuss the purpose for the planted roof.

Not everyone loves green roofs, the value that they add to a sustainable design is debated. Engineers tend to point out that the benefits they provide are available through other means. Their contribution to reducing urban heat islands can be accomplished with high reflectivity roof materials. Their mitigation of storm water run off can be handled through infiltration systems on the ground. Because dirt is not a very good insulator, planted roofs don’t really add to the energy efficiency of buildings, except that the plants on the roof, supplied with sufficient water, will provide evaporative cooling through transpiration. Better and cheaper to increase the insulation on the roof to improve the efficiency of the building, and insulation will perform during both the cooling and heating seasons. And using potable water to irrigate plants on the ground is wasteful enough without adding irrigation to the roof.

I agree with all that I just wrote, and there are good studies to back it up, but there are a couple of reasons, general to building and specific to this project, why we decided a planted roof was purposeful and included it in this house.

Water

It is hard to assess how much of building is directed toward managing water, water from the sky, water from the ground, water in the air. Maybe it is just in the Southeast, but I don’t think so and here we are mostly spared the problem of frozen water, thank God. Anyway it is a lot. The conventional wisdom and vernacular architecture in the South embraces steeply pitched roofs to shed rain water quickly off the house. I have found, though, over many years of repairing houses it seems more damage is done by the rain water after it leaves the roof than from the time it spends on the roof. Overflowing gutters, splash from the eaves, flooding foundations, all lead to problems for houses. When I built my first flat-roofed house I never noticed the water that left the roof, it was collected behind a parapet, drained into a downspout, then through underground pipes and finally flowed onto the ground forty feet from the house. The roofing material was a heat sealed membrane that was user-friendly and durable. The one drawback was that the leaves from nearby trees tended to collect at the screens for the roof drains, but at least when you have to clean them you are standing on a (nearly) level surface. So the only thing better: a flat roof where the leaves don’t matter. A filter of soil and the other layers of the planted roof negate the ability of accumulated leaves to clog the outflow for the roof. As opposed to a traditional roof that encourages the rain water to flow off the roof as quickly and directly as possible, dragging with it any debris that has landed on the roof, the planted roof slows the rain, absorbs and retains some of it, and filters it through a broad area before releasing it slowly to the roofing to convey it away from the building. It may seem counter-intuitive, but part of my no maintenance scheme included the planted roof. I was going to garden somewhere, why not accomplish roof maintenance at the same time I was planting or weeding.

The Garden

My musings on water management made the idea attractive, but the reason for the planted roof is the desire for garden space on a piece of land so small its hard to escape our own shadow. We have always had gardens, frequently unruly, sometimes very productive, sometimes less so, a garden is part of who we are and I have nearly arrived at the place that I believe if land isn’t going to be left to its own and return to what here would be a subtropical deciduous jungle, it ought to grow food. The house dominates the lot, add a parking space, walkways, space for our aging dog, consider the north side lost in the shade of the house, bamboo and a struggling pecan tree on the adjacent property shade some of the narrow strip to the south; even so we will plant the front hill with blueberries and strawberries, we will put some blackberries on the fence line. But the roof offered an unobstructed view of the sun, and though small, plenty of space for a generous kitchen garden. The roof is where we will have our cooking herbs and spring lettuce. Every garden we have ever had has been an evolutionary project and this will be as well. I see some years with sweet potato vines cascading down the walls, while beans grow on a trellis above them, maybe our virgin roof top soil will even produce a zucchini before the squash borers kill the vine.

Watering the Roof

One drawback to planted roofs is that they almost certainly need irrigation. Maybe not always and not as much with the carpet of sedums that often make up green roofs, but with plants like tomatoes that are thirsty in the ground, watering will be a necessity on the roof. I didn’t want to take an idea that is borderline sustainable and throw it clearly on the wrong side by watering our roof with potable water. To avoid this we needed to get our irrigation water from somewhere other than the tap. My first choice is stored rainwater, and we have some roof area that will not be planted that can be collected during rain storms and used during the hottest months. Several hundred gallons of stored water can last a few weeks without rain as long as the irrigation is efficient and there is plenty of mulch. We will have a 300 gallon storage tank under the porch with a pump to lift the water to the roof, and a subsurface irrigation system to put the water where it is most needed. The water keeps the plants alive and makes the garden viable, but during the hottest months it also provides the one energy benefit of a planted roof, evaporative cooling.

We did get the roof framing done and moved on to building the outside wall, things are wet now, but next week looks clear. A good week’s work and we’ll be close to the end of the framing

Here the roof is fully decked, ready for the rooftop dormer and the completion of the second wall

 

Going Vertical

With the floor system finished and the subfloor (and finished ceiling) installed it is time to make this thing look like a house, or at least look like the house that it is.

Two wall heights, Three ceiling heights

As I have said earlier, raising walls is one of the absolute joys of construction. Tilting a wall up, looking at the top plate overhead and a cloudless blue sky beyond, is a singular moment, emblematic of the entire effort, and makes me feel that I am lucky to have a job where I get to work outside. And makes me forget about carrying buckets of gravel in the rain a month before.

Our plan is to get the second floor interior walls up and straightened before working on the outer walls. There are two different wall heights on the second level, 10′ 4 1/2″ in the front and 14′ 10 1/2″ in the rear. This is for a couple of reasons. First the higher wall in the back is needed to allow for the loft. With 7′ being the minimum ceiling height allowed, 14′ 10 1/2″ gives us enough for two levels plus a 2×6 floor system and a little extra for ceiling covering and such. Because the planted roof is on the front section and that overall roof package, framing, insulation, and planted roof paraphernalia, is 30″ thick, the wall height below it needed to be lower. The stairs from the loft to the roof also open on to the front section so having it lower than the back requires fewer steps to get up onto the roof.

Here is a section showing the floor levels, roof levels, and stairs between them.

A vertical section of the house, showing the stairs, floor levels, loft, and roof access.

There was one difficulty in building the tallest walls on the deck and raising them into place, the walls are about as tall as the building is wide. And once one side was in place there wasn’t enough room for the other side to be laid flat. The last of the tall walls we framed were at least not long so we built them where we could, stood them up, and wrestled them into place.

First wall up

The three front walls in place

The first rear wall up

everything up, windows framed in

 Old Methods, New Face

We worked last Saturday because the weather was good and we hadn’t gotten much done on site for much of the week. I worked the full day, everybody else split half shifts to preserve some of their weekend. In the afternoon Woody came in after we had gotten the rest of the second level walls up and started to plumb corners in the AM. He and I finished plumbing corners and framed in window rough openings. At some point during the day, in the course of conversation, he pointed out that one of the things he liked about the house was that though it is a fully modern design it uses old framing methods to get there. It was something I hadn’t thought about, but he is right, let in 1×4 bracing disappeared with the advent of plywood, and balloon framing with fire codes and pre cut framing and small trees, and we’re using both in this house, and I don’t think I have ever used either method before, mostly because I’m not old enough.

I have built another double stud wall house, but that one was sheathed twice, once to brace the walls and then again to hold the siding. I didn’t want to do that and so needed another way to brace the interior wall and 1×4 braces let in to the studs satisfied. We can insulate around them so the stud cavity between the two walls is uninterrupted, they brace the walls against lateral loads, and they use a whole lot less material than plywood sheathing. And we are going to fully sheath the exterior wall with 1/2″ CDX plywood, mostly to hold siding, but also the exterior wall has a full foundation under it and though it won’t carry the bearing loads of the building, it will be subjected to other forces on the building like wind loads and it will act in concert with the interior wall (because they are fully tied together) to handle these loads.

The balloon framing is really only used on the loft framing, though the double wall acts a little like a balloon framed wall. For the loft floor we are going to use a let in 1×4 on the back wall of the house that the loft floor joists will rest on. The joists will also be nailed into an adjacent stud. Balloon framing multi story houses disappeared from common building practice long before framers quit using let in bracing, I guess because the studs had to be so long. It made sense for the loft floor because it simplified the framing of the back wall by allowing the entire rear section of the house to be framed to the same height, and again serves to allow the insulation to flow fully through the framing cavity. The main floor system was platform framed and we placed foam blocks against the band joist so the amount of insulation wouldn’t lessen at the floor system. That won’t be needed at the loft floor which won’t create an obstruction within the wall.

Balloon framing does create the need to address a common problem of the past, the potential for a fire within the wall to jump floors because the framing doesn’t create a full barrier between the floors. This is the aspect of the double stud wall that behaves like a balloon framed wall. The goal of the double stud construction is to provide a continuous insulation layer, with no bridges of framing for heat to move through. In order to achieve this there is no framed fire block between the floors, a block that would also be a thermal bridge. Instead the wall cavity is completely filled with dense pack cellulose, which in addition to the many other wonders of cellulose serves as a fire block, partly due to the fire retardants in the insulation (borates mostly, not like fire retardants in pillows and furniture) and also due to the lack of air flow through the material.

So fully modern, yes, but leaning on older techniques that were properly suited to accomplishing our overall goal

Getting off the ground

Sometimes it seems like the whole construction effort up to the point where we are now is a long slog just to get up and out of the dirt. In multi story projects once you get the first floor system above the slab completed, the next bit of framing, raising walls, is as good as construction gets. As a friend said, who saw us after most of the second level walls were in place, ” It’s like you build the whole house in a day,” and it kind of feels that way. Before we got there though we had to frame the second level floor.

Framing as finish

A few things about the floor system that are atypical: first, we are not going to cover the floor joists underneath with a ceiling making the joists and subfloor the finished ceiling for the first level, second, there is a significantly cantilevered corner, and last we are going to pour a 1-1/2″ thick concrete slab on the subfloor to serve as the finished floor for the second level. I am going to admit I am not sure that having the joists and subfloor as the ceiling will be the greatest idea. It will provide a lot of spider habitat, which makes for much dewebbing or a cobwebby house. Both the joists and the subfloor are southern yellow pine which even clear finished will be a relatively dark surface, on top of the recesses of the joist bays being shadowy anyway. On the plus side since the interior walls and ceilings are going to be a 1/2″ formaldahyde-free birch plywood, leaving the downstairs ceiling open saves both the plywood and the labor that would be needed to cover it, and I have always liked southern yellow pine, and not just the old stuff, as a finished surface. Here is another opportunity for me to show one of my laminated detail drawings.

this shows the framing details for the transition between the first and second levels

This shows the 2x 12 blocking used at the wall plates to close the joist cavity and provide a transition between the wall plywood and the subfloor. I filled the space between the floor system band joist and the blocking with 2″ of sheet foam before we sealed that cavity so that we would not reduce the insulation at the floor system.

The 2×12 floor system with blocking and foam

As pretty and even as the drawing looks, framing lumber can confound the best of intentions and teaches us why we cover it when we try to leave it exposed. The joists varied from 11″ to 11 5/16″ which would challenge even a drywall ceiling. In the end we culled a few, planed a few, and ripped blocking to keep it consistent. And reminded ourselves often that the intent was to leave the framing exposed. The one place where we will cover the ceiling is the downstairs bathroom. I haven’t decided what will cover it yet, but I am sure I don’t want either floor joists or birch plywood to serve as the shower ceiling. It is not going to be tile either, too sanitorium, so we’ll figure that out in a couple months.

The Cantilever (and the notch)

installing the beams that support the cantilever

There are a few things that happen when you design two-story structures. One is that once you have designed one of the floors you have to come up with a way to design the other floor using the same area and shape. In larger houses this can frequently end up with trying to find some use for the extra square footage dictated by the space above or below. In smaller houses where space is more of a premium using all the area available is easier especially when the plan is opened up, on one level specific uses are partitioned and defined, on the other the same area and space is used in a single open room comprising several uses. That is somewhat how we got to our design, and it is the notch out of the lowest level that dictated how the different levels would be used, the notch that I added early in design to get at least one parking space off street. Our plan is inverted, the bedroom is on the (almost) entry-level, with the kitchen, living, dining above, and this was driven in large part because the private portion of the house was more suited to less square footage than the public portion, and with the notch making the lower level about 30 square feet smaller than the upper it made sense to put the bedroom on the lower level. There was a cascade of serendipity that followed this decision. The loft above the second level was accessible and connected to the main living area, the kitchen could inhabit a space with a low ceiling under the loft, the roof which will serve as both kitchen garden and outdoor dining is closer to the kitchen than the bedroom with this configuration. The notch though required the cantilever if we were going to regain the space for the second level and keep the parking area free of a support post for the walls above. This is an instance where an engineered wood beam served well. Though we are limited in how low the beam can be placed (no lower than flush with the bottom of the floor system) and still maintain headroom under the cantilever for the parking area, we are not limited in how high the top of the beam can be as it can extend up into the second level inner wall as much as necessary. Expecting that a double 16″ LVL might be needed this was exactly my plan, to allow the beam to occupy the bottom 4″ of the second level wall, the engineering only required a double 12″ LVL, which very conveniently allowed the subfloor to finish nearly flush with the top of the beam and the bottom plate for the second level wall to be placed on the top of the LVL’s and the subfloor with no stud length adjustment. The fully loaded deflection for the cantilever is a tiny 0.08″. There is one downside to the cantilever: it provides a thermal bridge. The joists and beams are a direct path for heat transfer between the interior and the exterior, something the framing plan for the rest of the house was designed to avoid. This occurs at the bay cantilevers as well. In both cases I will use sheet foam on the undersides of the cantilevers to provide a thermal break. Again more foam, but again a limited amount in an area that would be hard to insulate another way.

Upstairs Concrete

This was not my idea actually. Mary proposed this and I was resistant, cost and difficulty mostly, I had planned on the subfloor being the finished floor. But the design is intended to capture passive solar in the winter, and for that the added thermal mass of the concrete floor would be a benefit. It will weigh more than a wood finished floor, though not as much as one might think. Lightweight concrete weighs about 120 pounds per cubic foot so pouring the upstairs floor 2″ thick will add 20 pounds per square foot to the floor system, something the 2×12’s spanning 14 feet can easily handle. There are still a few details to work out around the stairwell where the edge of the concrete will be visible, but we won’t want to see the edge of the subfloor. This gets especially sticky where the landing to the loft sits on the second level floor. I had always planned on the screened porch floor being poured concrete, part of the no maintenance plan. That is a much more difficult prospect than pouring the concrete inside, even more so considering that I am planning a plywood formed structural slab; a subject of a future post I am sure.