Sunday, August 5, 2012

Air Sealing: Step 02 - Wrap it Up

Intello'd ceiling from ground level
Now that we're membraning, there are so many great 'Wrapping' puns flying around on site its almost hard to keep them all straight. 

Yes, its finally time to start sealing up our cabin. We got signed off on our Rough Framing, Electric and Plumbing inspection on Tuesday (applause, applause - thank you very much) and so now it's time to close the walls up and get to insulating. As I've said before, the air-sealing on this project is critically important, we're super-insulating and we bought good windows and we oriented glass to the winter sun and all that other nice stuff - but the most important thing we're doing is fully controlling the air that moves into and out of the house and salvaging as much heat as we can from the exhausted air with our ERV. 

Intello on the loft cathedral ceiling

To seal the inside of the house envelope completely - we're using Pro-Clima's Air Barrier, Vapor Retarder & Cellulose Netting membrane called Intello. It's a semi translucent white (to help see as we're blowing in insulation) high-performance membrane made of a plastic called Polyofin with Polypropylene reinforcement. The membrane is stapled to the inside face of the studs and then seams are taped with Pro-Clima's tapes. The rolls are light, big (about 5' wide) and cover a lot of ground real fast, just like the Solitex membranes on the exterior.

Zoom of the Intello - the grid you see is the reinforcement polypropylene strands

Once the entire interior surface is covered with membrane and taped up, we'll start filling the walls with dense-pack cellulose insulation - using the Intello to hold the insulation in the stud bays. One great thing about this membrane as opposed to standard cellulose netting is that thanks to the reinforcement we need many fewer staples. Most netting requires staples every 1/8", two rows per stud. This basically coats the studs in metal making installation of finish materials a real pain in the neck. The Intello only requires one row of staples, spaced every two inches. So thats much easier to install and will make our lives a lot easier when we go to install the interior woodwork. 

Intello, 2" OC staple, parallel to the stud

And even better than all that - it glows white. So now it feels like your inside of a giant marshmallow when you go into the house.

Intello ceiling, view from the loft looking south

But why all this attention to air sealing? Well, most houses let fresh air flow in through cracks around windows, under doors and around walls. In addition, if you have things like electric outlets, ducts or especially, recessed light fixtures in your ceiling - its a good bet air is flowing in and out of your house around those pieces of equipment.

All this air movement is a bummer for two big reasons. One is because you're loosing all the air you spent a bunch of energy (and money) on to heat or cool until it feels comfortable. This tempered air is just flowing out into the world and mixing with unconditioned air (either too cold or too hot) that is flowing in, which you then need to use more energy on to condition all over again. Most homes change all the air in and out somewhere between 4 and 12 times an hour (whats called ACH - air changes / hour). On a certified Passive house you need to be below 0.6 ACH at 50 pascals (thats a prescribed pressure to measure ACH at) - although most folks try and do much better than that. So if we can limit the amount of uncontrolled air that leaks into and out of our house, and then use a machine called and ERV to bring fresh air into and out of the house, we can drastically cut the amount of energy needed to heat or cool our space.

The other reason we care so much about air-sealing has to do not with air, but with water. Since our building assembly is so tight and our insulation so complete, we need to be very careful and attentive to how water vapor moves into and then (more importantly) out of our walls, ceilings and floors. In a drafty, poorly insulated (or uninsulated) wall - air moves all over the place, so if water somehow gets inside a wall (through vapor diffusion and condensation, or during the construction phase before everything is water-tight), that water vapor can easily dry out and get carried away by the air moving through the wall.

In a super-tight wall, however, there is the real possibility that if we're not careful, water vapor might get trapped inside, condensing on cold surfaces, causing rot, attracting insects and doing all the other terrible, horrible, very bad things that water does to houses.

Our intello wrap is going to function as a vapor retarder - that is, it'll stop any vapor pressure-diffusion that would drive moisture into the wall assembly from the interior. This is required by code (though air barriers are not) and most builders include some type of membrane (usually polyethylene sheeting or paper-faced batt insulation) that performs this function even in the cheapest types of construction.

But, and this is where the building codes are a bit mixed up - air sealing is, in fact, MUCH more important for preventing vapor movement than installing just a vapor barrier. Now: sure - vapor and air are different and just cus' you have an air barrier doesn't mean it's a vapor barrier (un-painted drywall is a good example of this). But - as the famous diagram from BSC bellow illustrates so well - air transported vapor is a much bigger culprit than vapor pressure-diffusion for moving water into wall assemblies.

For a great article on why air-barriers are more important than vapor barriers, see this one by our favorite master of building science over at GBA.

So- the Intello is a vapor retarder, thats fine - but more importantly it's also an air barrier - and we seal it tightly around each and every penetration and intersection. And since our wall's outer layers are vapor permeable - if any moisture does get in- it can easily dry to the outside and get vented away under our rain-screen facade.

In order to figure out if our assembly will function ok given all the issues listed above - we create whats called a Vapor Profile (for more on this - check out another article at GBA). We assign a value to all the materials in the wall assembly, (called a Perm value) and this way we can see which direction, depending on the season, vapor will be moving. 

In our Vapor Profile above, you can see that we expect exterior drying to occur during summer and winter thanks to the very high perm-rating of our sheathing and House-wrap.

The Intello will act as a vapor retarder and so any vapor hitting it from either side will be unable to pass through. However - Intello is what is know as a smart-wrap, that is, it actually changes its performance characteristics when conditions change. For us, that means when its hot and humid - the Intello actually changes its molecular structure and modifies its vapor permeability from 0.17 (very close to impermeable) to 13.2 perms! This means that in the summer, the assembly can dry to both sides. Very cool technology and even though not strictly necessary given our assemblies ability to dry outwards - it's still very good to allow drying both directions as much as possible.

Jason and Carrie should have the rest of the shell finished up soon, then on to air-sealing our windows and penetrations - so look for a post on those soon.