Previously on controlling the control barriers, we talked about what they were, and how we are handling the water and air barriers. Below is the cross section of how our walls will look for reference. To quote a certain Sci-Fi show…”and now, the conclusion.”
VAPOR
As borrowed from the International Association of Home Inspectors: A vapor barrier is a material, typically a plastic or foil sheet, which resists the diffusion of moisture through ceiling, wall and floor assemblies of a building. Vapor-diffusion retarders are also effective for controlling moisture in basements, crawlspaces, and slab-on-grade foundations. For most of us, we’d call that the plastic we’re used to seeing on the inside of your house right behind the drywall. The reason that is there is because with the way the “traditional” wall assembly was made, that would stop warmer, moisture rich air, inside your house from migrating through the wall and the possibly condensing on the backside of your OSB and turning into water droplets in your wall.
There’s a lot of higher level science around vapor barriers and vapor migration, but the easiest thing to know about it is that a piece of unpainted drywall is a Class 1 barrier. By that I mean the permeance of unpainted drywall is very high, generally between 20 and 90, so it’s not a vapor retarder at all. As soon as you paint it though, it becomes a Class 3 barrier, and it’s permeance drops to between 1 and 10. Here’s a little more science to ponder around that. Over an entire heating season, a piece of painted drywall only allowed through a third of an quart of water. That sounds like a lot, but it works out to less than a can of soda. And that’s over the whole heating season, and doesn’t happen all at once.
What does that mean? it means that the little bit of water vapor that might get through is able to dry and become a non-problem. Now, if you poke a 1″ square hole in the drywall, you will get 30 quarts of water in your wall. That’s right…nearly 100 times more water. So the real moral of the story here is air sealing is more important than worrying about vapor barriers. If you have a good air tight seal inside and out, you won’t have a vapor problem. After we drywall, we plan to use AeroBarrier which comes in and uses an aerosolized product to air seal any gaps in your air barrier up to 1/2″ to become air tight. They do it by placing the house under pressure using a modified blower door. (More on that one later too!)
TEMPERATURE
The temperature control layer is another one that’s really easy to grasp. Are you always cold in the winter and warm in the summer? Congratulations, your house likely doesn’t have enough insulation, the biggest component in controlling temperatures. There are lots of little factors that go into temperature control, but insulation has the largest impact.
We are doing a few things on this front. First of all, remember I mentioned that our Zip system was actually Zip-R? The R is a special version of their Zip that has a built in exterior insulation. Great…what does that mean? Normally, your OSB is nailed directly to the studs inside your wall. That creates what is called a thermal bridge. A point when the exterior temperature (lets say cold) can be transferred from your siding, to your OSB sheathing, and then to your stud, then to your drywall. If you were to use a thermal camera and look at your wall in winter, you’d see the studs in your wall because they are colder. Since the Zip-R has that insulation on the back side (an R-6 value approximately), it helps provide a thermal break between the outside and the studs. It doesn’t completely eliminate the issue of thermal bridging, but it greatly reduces it since there’s insulation between that outside layer, and the studs in the walls.
The second thing that we are doing is doing an R-23 blown in insulation in the walls. Code in our area calls for an R-20 in the walls, or an R13 + 5 which means R-13 interior and an R-5 exterior insulation. As you can see, we’re starting out better than code for just in the wall, and then adding the Zip-R on the exterior. because exterior insulation can be better. In the winter when you go outside, to stay warm what do you do? You put on a jacket…aka…exterior insulation.
The third thing that we are doing is in the attic. Normally your attic is an unconditioned space. Your air barrier is your ceiling drywall, and then you have lots of fluffy insulation above it and your attic lives at basically the same temperature as the outside world. You have vents in your soffit to draw air in, and then roof vents (or a ridge vent) to let the air out. When you do monopoly framing like we are doing, our attic becomes a conditioned attic. That is to say, the attic will be nearly the same as the inside of the house rather than the outside. When you do that, there are a few changes that happen. First is that you now have unvented soffits and no roof vents. The second thing that you do is you put spray foam insulation directly on the underside of your roof deck. We will have 5″ of closed cell spray foam on the underside of our roof. There are two types of spray foam. Closed cell and open cell. Open cell has an R-Value of about 3.5 per inch. Closed cell clocks in at about 6.5 to 7 per inch. So, if you do the math on that, there will be approximately an R-32 to R-35 on the underside of our roof. Now wait a minute, you might say. Aren’t attics supposed to have an R-50? Why yes, they do. When they are unconditioned air spaces. Since ours is conditioned, it’s more accurate to think of our roof as a wall, but built on a slope. That’s the reason we only need to have the R-32 to R-35 and still exceed code requirements.
IN SUMMARY
So what do these two long post actually tell us? Well, first of all, they tell us that I’m a large building science nerd. More importantly, they tell a tale of the four major control layers of a house, and hopefully helped you see how they all interact with each other. You can’t just focus all your attention on one barrier. You might get lucky, and some of the work on that one, bleeds over to one of the others, but that’s not always the case. it’s really best to think of them all together as a system that keeps your home healthy.
Another thing this tells us is that the envelope of your home isn’t something you can think about after. You need to think about it before, during, and after. This you decide about your building envelope have an affect on other things that go into your home build. If you think you want to go down this route, talk about it with your builder and architect/designer early on. Any good builder who cares about building a better house is going ot be familiar with these things, and can help you decide what options and choices will work best in your budget and give you the best return on your investment. It’s a lot harder to change and alter some of the building envelope things after your house is done. It’s very easy though, to swap out some carpet or flooring later.
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