Plywood ‘vs OSB (Waferboard)

Sometimes choosing the building materials that suit us is a difficult decision. In the case of plywood ‘vs OSB, it’s a no brainer.

The home is a system, exterior claddings, framing and insulation, and interior claddings that all interact with each other on some level. The most important variables playing into this equation are: Air movement into and out of the building envelope (factoring heavily into heat loss), vapor transmission to and from the building envelope, and strength.

Air movement- how drafty your home will be- is the same for either plywood or OSB, theoretically speaking. The most important thing to remember is to properly nail the plywood on the outside walls and roof. That means extra nails at all edges, and adjusting the depth setting on the framing gun from super deep setting for framing, down to relatively shallow for shooting ply or OSB. Otherwise, your nails will blow most of the way through it and you’ve lost strength.

Speaking of strength, there is a difference between OSB and plywood. I’m no engineer, so let me relay my personal experience. I was fresh out of high school in the early 90′s and learning about framing houses on the coasts of Beverly and Marblehead, Massachusetts. We often had to build to withstand hurricane force winds, incorporating shear walls to resist lateral wind pressure. When we built the walls laying down, we had one of two choices for bracing it against the wind (no complaints about Bob Seger stuck in my head right now). We could use plywood and lay the sheets across the studs and stagger the joints every four feet in each successive course of real plywood and the stength of the plywood would brace the wall; or, we could cut diagonal bracing into the wall studs from top plate to bottom plate before sheathing with cheap-o OSB. Hopefully this information gives some perspective as to strength.

Next is vapor transmission. This is slightly more involved. Wood can absorb moisture just fine and, when allowed to properly dry out, can repeat this cycle indefinitely. It becomes a problem when the wood doesn’t have the opportunity to dry out. Therefore, the drying process is critical.

Your home is built almost entirely out of building components that hold moisture. The amount of moisture that they hold depends on: The types of materials used, the amount of humidity created by inhabitants (about 70% of a home’s humidity is from occupants cooking, showering, watering plants, dog dishes, perspiring, etc.), the relative humidity in the atmosphere, and of course the amount of air flow in and out of the home.

To allow the home to dry out, we must allow for slow vapor transmission through the building materials. This is why we should never use a vapor barrier on the warm side of a wall system. We don’t want to block all vapor diffusion. This would cause the vapor to hit that sheet plastic “vapor barrier” and condense, with no where to go. After condensing, the next step would be mold.

 

What we need to incorporate into walls and attic floors are vapor retarders that simply slow down vapor diffusion. We must allow the building and all of it’s components to dry out. So, we don’t use sheet plastic on the inside of the walls. Fine, but what does all this have to do with plywood versus OSB? Well, I was laying down a little building science theory, necessary for understanding the importance when choosing a wall or roof sheathing.

If we want to allow the building to breathe, which is the better choice? OSB is made up of wood wafers and glue, then treated with a film of wax to buy some time for exposure to the elements during construction. OSB has a perm rating (the means of measuring how much vapor will pass through a material) that remains very low and constant. When the relative humidity rises, it continues to allow the same miniscule amount of vapor to pass through it.

Plywood, on the other hand, has a decent perm rating and is able to ride a very nice curve. It is made of real, unpulverized wood layers that are glued together in opposing grain directions with an exterior glue. Because it is real wood, when the relative humidity rises, it’s ability to allow vapor to pass through it rises in kind. It allows the home to dry nicely under normal conditions.

Next, let’s talk about mold. Mold is a fungus. Fungi need food. Wood can be food, especially when sopping wet and beginning to break down. Additionally, when wood is pulverized, a sugar like byproduct is created. This is sugar like to mold. Oh wait. OSB is made up of pulverized wood and glue. The more and more that wood is pulverized down, the more sugary wonderful it is to mold. That’s why mold will very easily grow on the paper facing of drywall in a moist bathroom. OSB is far more susceptible to mold than plywood.

Let’s round this out by talking about the all important bottom line. OSB for wall sheathing can cost less than half of what plywood is going for. Let’s say we’re building a decent sized two story addition requiring 70 sheets of 1/2″ sheathing for walls and roof. At current rates, it would cost $525 for OSB and $1,330 for plywood. Now knowing this building science, who would choose to save the $805 and go with OSB?

Let’s consider two more things: Interior subfloor application over floor framing, and the introduction of red type (roofs) and green type (walls) super waterproof OSB sheathing.

First, the easy one. I always use OSB for interior subfloor applications. For this, we are inside of the building envelope and vapor is not struggling to pass through it. It also is cheaper and flatter for floors, making it a great choice and a green choice.

Finally, let’s discuss the introduction of super waterproof OSB. This is becoming incredibly popular for builders. I will never use it. First, you’re still not allowing the home to dry out properly via vapor diffusion. Second, it doesn’t adhere to the most fundamental of all building principles: Every single building component should be overlapped by the one directly above it in elevation. This utilizes the physics of gravity to keep the home dry and has been successful for centuries.

Then how does this other system work? First, the OSB is coated with some special magic potion to keep water out. Fine, wonderful, remembering that this also means that it keeps vapor in. What about the joints between sheets? Well, this is the kicker. It means that the carpenters must use a special- and expensive- proprietary tape gun to apply the special tape to the seams. That’s where we no longer overlap materials. You see, with plywood or regular OSB sheathing, we apply a rainscreen over the sheathing, such as tar paper or Tyvek, etc.. We overlap this rainscreen, maintaining the most fundamental principle of building. Conversely, the tape that is applied to the special OSB relies entirely on adhering over the surface of the plywood, a non glossy surface.

If you hang out at the lumber yard long enough, you’ll hear other guys asking what to do when their finicky tape gun no longer works properly. They dread shelling out the big bucks for a new one. Yes, these applicators don’t always apply the tape to create a watertight seal. And what if the sheathing has a film of sawdust on it’s face? I don’t know, you tell me. If you pay attention to addition jobsites, builders that are using this new super OSB with the taped seams on a roof with a finished living space underneath are still tarping the roof until they get the shingles on  it. How much confidence do they seem to have in the product they are using on the roof? When roof plywood, on the other hand, is properly covered with ice and water shield and roof underlayment paper such as synthetic tar paper, the building will stay dry during construction every time.

Why would the lumber industry offer a product like this? Because they want to sell to the ever growing demographic of builders that don’t understand how to properly and permanently water proof a building. The lumber industry is dumbing down the products. Yes, we should all write our representatives in congress.

In conclusion, plywood made of real wood is by far the best thing for your exterior walls and roof. It only needs elementary attention to waterproofing details, and will then allow your home to breathe properly. OSB is the best choice for interior flooring substrate.

This material was largely a result of an education from Joseph Lstiburek, Ph.D., P.Eng., ASHRAE Fellow. He has been widely renowned as an international building science expert for decades. If you want to learn more about building science or cure insomnia I’m just kidding, some of the writings on the site are actually very entertaining), visit his website at http://www.buildingscience.com.

 

By John Bradshaw

 

 

 

Tackle Custom Interior Trim Carpentry in the Winter or Summer?

Our clients have been asking regularly about whether we also do interior work. This tells me it’s time to write a blog post about interior trim work to provide a small sampling of what we can do.

This is an example of woodworking with poplar we did several years ago, during the winter. It has stayed tight with no cracking or bulging. No need to keep caulking and repainting. The “white” lines you see on the crown molding joints are glare from the camera flash.

We’re passionate about all carpentry. To date, our blog posts have been entirely about exterior projects because we feel that sealing water out of the building is job #1. So, when you no longer have water vulnerability, well heck yes we should talk about your interior wish list.

If you’re thinking about moving forward on a custom trim carpentry project, displaying an exquisite vision that showcases magnificently honed carpentry skills is likely one of your goals. That makes perfect sense. And if we’re going to bring your vision to fruition, we’ll want to ensure that it looks and functions flawlessly for a long time to come.

That brings me to the part where we talk about what conditions are ideal for an enduring and beautiful product. First, let’s get the wood species out of the way. If you want a stained product, then choose the species you like. If you want a painted product, then we’ll be selecting poplar to work with, along with cabinet grade plywood and possibly other accessory products. We choose poplar because of it’s availability and affordability. More importantly, we choose poplar because of it’s stability, strength, and straight and smooth grain. Commercially available poplar is not ever suitable for exterior projects in New England though, it turns black and rots easily. But, it’s phenomenal for interior painted woodworking.

Next, wood is hygroscopic, it absorbs or desorbs moisture in effort to reach equilibrium with the relatively humidity. If your home does not have humidity controls here in New England, then we need to understand what the wood is going to do in the future, in response to the relative humidity changes of it’s environment (the home). Barring humidity controls, wood expands as relative humidity rises in the summer, and shrinks when relative humidity drops in the winter. Don’t think that you can fight the “hydraulics” of it, you can’t. You could slow the absorption or desorption by sealing all sides before installing, but this only slows the process. Alright, that’s enough of the technical mumbo jumbo for now…I’m giving myself a headache. Most importantly, proceed with your trim project when the wood’s moisture content and humidity conditions are ideal.

So, as far as ideal conditions, do we want to be building these things during the winter or the summer? Long story short, winter…conscientiously. The wood has already shrunk, in large part, and will not be continuing to shrink. Therefore, you cut, glue, nail and paint the trim during the winter; then, it’s only going to get tighter in the summer. This can be terrific, unless overdone. The wider the boards are, the more they will shrink or expand. If you have boards that are too wide and too dry, they will buckle or cause other components to buckle when they expand in the summer.

Look at this photo. I believe that this trim was installed during the moist summer months, shrank during the dry winter months and exposed a gap at the mitered joint. The gap was filled with caulking and repainted during the winter months when the wood was still contracted. Then, the wood expanded again during the summer months, squeezing the caulking back out of the joint.

How can we help this joint at this point? I don’t know, Bobby-Jo. I suppose I would probably use a better grade of adhesive caulking that stays pliable. I would apply it during the fall, maybe October when the drying has begun. Essentially, shoot for the halfway point between it’s max expansion and max contraction. Then, paint it. You could expect it to be squeezed during the wet summer, but hopefully just form a slight bulge in the paint, and then subside again in the winter, while remaining bonded. The key here is to not use a hard, unflexible product. This would pop out when squeezed during the wet summer months, every time. Hmmm. It’s October now. I think I’ll ask this customer if I may try this experiment and monitor and report back after several changes of seasons.

To wrap it all up, for the sake of your project, please choose to do your custom interior woodworking project in the winter, to ensure tight joinery all year round. For the sake of your wallet also, you’ll want to tackle this during the winter. Why? Well, you may have an opportunity to save a little bit of dough by offering a New England craftsman an interior project to keep him or herself warm during the bitter winter cold. Most elite craftsman are in a minority these days, keeping them in demand year round, but you may have just enough luck to find one looking to fill a winter time slot. Cheers to moving forward with your vision!

 

By John Bradshaw

 

The Most Common Roof Error I See

Photo #1- This home in Portsmouth features an outside corner to the immediate left of the trash barrel that is exactly the scenario most likely to be done wrong. The inside corner that’s just a touch up the roofline is also a common culprit.

Photo #2- If you click on this image to enlarge it, you will see there is a gap between the copper step flashing going up the roof line and the horizontal copper apron flashing.

You may have learned by now that I’ve never been known for brevity. This blog post, however, should buck that trend a little bit. Why? Simple, because this common error is very easy to identify and resolve. So easy, in fact, that it doesn’t make much sense why these things are almost never done properly in the first place.

The scenario is this: Any time an out side wall corner or an inside wall corner comes down to intersect with a roof plane, assume it’s not flashed properly. Now, this does not mean that it leaks, necessarily. It may not leak due to a proper insurance layer of ice and water shield underneath the shingles. Conversely, the lack of water signs inside the corresponding portion of the home does not mean that it is not leaking. Wood, especially real wood as opposed to “particle board”, has the ability to absorb water, and then dry out. This can happen over and over again, as long as it can dry out. Or, The initial plywood and framing components that are coming in contact with the water that is infiltrating are sometimes not able to properly dry out, and thus rot. The components that lay below these “first absorbers” will get wet, but not necessarily beyond their saturation point. If the lower levels of wood are absorbing the water without exceeding their saturation point, the water may never make it to the drywall layer to display to the homeowner that damage is being done. This explains how a home can be taking on water without showing signs of it on the drywall, for instance.

Photo #3- Just a simple piece of lead flashing to transition between the planes.

What is the error at these corner/roof intersections that is causing these problems? Well, the lack of incorporating nice, wonderful, beautiful, malleable lead flashing into the mix. Am I in love with lead? Maybe just a trifle, but hey, what’s not to love? Standard aluminum step flashing runs up the abutting cheek wall that the roof shingles butt into. And aluminum can sometimes also be used to flash the siding down over the roofing at those wall planes that run parallel with the shingles. But what aluminum cannot ever do is to turn that corner seamlessly. Lead, however, has the ability to hit all these multiple planes seamlessly. Mmmmmmmmm.

Photo #4- This is a home in Hampton. Look how easy this stuff is to spot, yet home inspectors rarely pick up on it, even though this home has stained drywall in the garage underneath.

Photo #5- See the close-up of the hole there. Simple to identify a lack of lead.

It’s comical the things that I sometimes see. I will pull apart these layers to correct and end up seeing layer after layer after layer after layer of aluminum flashing. Each one torn right at the crux of the corner, every time. So, the predecessor’s solution for recognizing that the flashing was tearing: Keep throwing more layers at it. Awesome.

These are flaws that are easy to identify, simple to fix yourself if you’re comfortable on a roof, or inexpensive (unless there is widespread rot) to hire a professional to solve. It may only cost $200- $300 to un-weave some shingles, remove a corner board, install some lead flashing, install a new corner board, and weave the shingles back in. The most difficult challenge you may face might be finding the professional that knows how and when to incorporate lead flashing. Here’s a catchy little reminder when vetting a repair contractor: Find someone with a passion for flashin’. No. I won’t be quitting my day job any time soon.

I must leave you with one last tidbit of information. I mentioned that aluminum can usually be a good flashing choice for the planes that don’t need to wrap around a corner. When it’s not a suitable choice is when there is copper in the vicinity. Copper is ranked higher on the Galvanic Scale of Nobility and blah, blah, blah. Anyway, copper and water runoff from copper interact with aluminum and cause the aluminum to rot right out. That’s also why aluminum is no longer acceptable to use as a deck flashing, because pressure treated lumber is treated with copper. Now you’re prepared to go tackle this one, safely of course.

 

 

By John Bradshaw

 

 

Let’s Play Hide the Plywood

 

This is from my initial assessment.

In photo #2- Do you see the same problem I see?

I was called to a home in Rochester (actually I was emailed to visit this home)to examine recently revealed rot and offer solutions.This home has a second story roof overhang that is almost four feet deep. The homeowner started to find some problems when he ripped up the cement pad that was underneath the entire overhang. The columns had entirely rotted at the bottoms – the home was built before pressure treated wood- so we replaced them. We also replaced all of the fascias with pvc trim and installed seamless gutters and leafguards to manage the water. This proactive homeowner also found some soft and punky plywood after removing some of the siding, himself, to see what was going on.

In photo #2 you’ll see that the major problem I identified was the lack of elevation between the finish grade under the overhang and the wood framing, plywood, and siding. The current building code calls for a minimum of 8″ of exposed masonry between finish grade and framing and plywood. This is to protect the home from water damage and insect damage. With the extra wide roof overhang, there is little threat from water, but major threat from insects. The reason there was no prior insect damage was because the concrete pad was poured right up to the siding. When you’ve seen the devastating damage an underground colony of termites can silently wreak on the framing of a home, you evaluate these situations from a different angle.

In this drawing that I scanned and included with my original work proposal you’ll see 2 things: First, the solution that I’ve been using for a few years now to solve this type of problem; and second, my pre-K art skills not so proudly on display! The foam gasket and the bead of caulking work together to try to form an airtight seal to keep the bugs out. Additionally, we’re wrapping the bottom edge of the plywood with ice and water shield in order to hide the plywood from the insects. I’m no entymologist, but I assume that insects will be far less likely to begin nibbling away at petroleum based products like ice and water shield. The foam gasket must be buried under the ice and water shield. I’ve seen carpenter ants eat away an entire wall’s worth of 1/2″ foam board that lay directly under the siding. The only evidence remaining to prove that it ever existed was the foil facing.

Photo #3- We could stand to add some insulation, since the wall was opened up anyhow.

Photo #4- Insulation beefed up and ready to continue.

 

 

 

 

 

 

 

 

 

 

 

Photo #5

In photo #5, we applied the 7/16″ x 1″ wide foam gasket to the bottom edge of the back side of the plywood. We then began to wrap the bottom edge in ice and water shield. What I did was to gently pre-slice the paper on the back side of the ice and water shield so that I could stick it to the back of the ply and to the foam, but leave the protective paper on the part that will wrap up the front face of the ply.

In photo #6 you’ll see the back edge of the ply wrapped with ice and water shield up and over the foam gasket.

Photo #7

In photo # 7 this is the installation of said plywood panels. Notice I ran the ice and water shield past the first piece of ply, so that I could have good overlapping with the next piece. The process calls for smearing the bottom edge of the ply into a thick bead of high grade caulking. In this case, the local lumberyard didn’t carry Lexel brand caulking (in my opinion the most bad posterior caulking available), so that means 2 things: First, we used Phenoseal translucent caulking (goes on white and dries translucent); second, Ricci Lumber needs to think about opening a satellite location in Rochester, or I need to plan ahead and schedule for them to deliver my sundries along with the lumber order next time. Phenoseal is also amazing, it has my 110% confidence.

Photo #8

 

In photo # 8, note that it’s important to nail the bottom edge of the ply approximately every 6″. Also, remember to set the depth of the nail gun so that the nails don’t just blow 1/2 way through the plywood.

 

By John Bradshaw

Photo #9- Peel off the remaining paper and stick the ice and water shield to the face.

 

 

 

 

 

 

Photo #10- Finish the prep with more ice and water shield tucked all the way underneath components that are directly above.

 

 

 

 

 

 

 

Photo #11- Pvc trimboard installation with hidden fasteners. We also installed proper flashing above the trimboard and replaced siding above that. All that’s left now is touch up paint.

 

 

 

 

 

 

 

Finished product from the road. The homeowner can finish his hardscaping now, sorry Rich.

In summary, when repairing rot in New England, or elsewhere, we must evaluate whether standard operating procedure will suffice. In this case, just nailing up plywood would handle the repair, but not protect the home for the long term. Invest the extra effort to think about and evaluate what forces will be working against the project in the future. Then, find or create the solution that will stand against these forces. It’s an investment you will not regret.

 

 

 

 

 

 

 

 

 

 

 

 

What’s the Science Behind Vinyl Siding?

There are lots of pros and cons of vinyl siding. Weighing heavily in the pros list are: It’s very affordable, it installs quickly, it’s virtually maintenance free, it can come re-inforced with foam insulation backer, and there are plenty of colors and styles to choose from. Weighing heavily in the cons column: There is a void of science involved.

Now let’s just get the record straight, I’m no primadonna. I don’t need to build or even repair a home as if it were a baby grand. I don’t utterly refuse to work with some materials out of sheer carpentry snobbery, and I’m equally grateful to be working on a modest single story ranch as I am on an elegant Queen Anne Victorian.

There are, however, plenty of times when I need to lobby for the implementation of fundamental building principles. The cardinal rule of building has always been Water Management. It starts with the selection of a buildable site, is adhered to when determining the elevation of the building, and so on until the end of time.  With regards to this cardinal rule, vinyl siding offers virtually nothing. Does that mean that vinyl siding shouldn’t be used? I don’t feel that way at all. It just means that it’s critical to have a bulletproof “drain plane” behind the vinyl siding. What’s a proper drain plane? It is the proper overlapping of weather resistant building materials (such as tar paper or Tyvek housewrap) to continually carry water out over the face of the home. This technique used to be common knowledge in the trade. Used to be.

Without making this a doctoral dissertation, do I like vinyl siding? Sure. It’s on my first home now. It won’t be the selection for my dream home. But it will be the selection for my final home. Ahh, the circle of life….

 

By John Bradshaw