House in Progress Diary

Kraft paper is just "brown paper" -- the stuff supermarket bags are made of. You can buy it in rolls for wrapping packages, although I don't know if that's the same weight you would want for a home-building project.

The most effective vapor would be something like a 4 mil plastic. And it's very cheap. Creating a good vapor barrier would be to create a complete envelope around the entire space. So, yes, cover everything. And overlap your seems at least 12 (or more is better)inches.

Take care to make things are also well sealed around your electical boxes, etc..

Marty

This seems to me to be a very important decision. Why don't you ask your air conditioning man &/or a knowledgeable construction guy....POPS"30"

I agree with Pops~ whoever planned your HVAC system should tell you what to do here. They'll not only have general knowledge, but will also know exactly what they're talking about re:YOUR system. They shouldn't charge you fo the advice, either~ this is part of system design.
Depending on what they say (of course), you might want to put your insulation or VB on the exterior side of the ductwork. I don't know for sure, but it seems to me you might retain some efficency in the system that way, as opposed to carrying your conditioned air outside the thermal barrier of your home. Just imagine all that nice warm air travelling 10 feet essentially outdoors in a Chicago January~ might not be so toasty warm by the time it gets to you!

Not only heat loss. I would be concerned about condensation. Air conditioning lines can cause condensation. Inside closed, dark walls, this can lead to mould problems....POPS"30"

Just to clarify Brian W.'s point, kraft paper is the same paper that the butcher uses for wrapping meat, so it does have some vapour barrier properties.

Personally, I don't like it. I agree with Marty that a poly vapour barrer would be better. Make it completely air tight by taping all seams with Tuck Tape (the red stuff...it will stick to anything).

The duct work itself should be insulated. You can buy the stuff made specifically for ducts that'll slide over the duct like a sock.

You will definitely want to get insulation in the wall cavity especially between the duct and the outside wall, to prevent heat loss and condensation.

Hope this helps. I also highly recommend www.joneakes.com for more information about insulation and ventilation.

Cheers!

Here in the Pacific Northwest (in BC), we have an epidemic of what is called the "leaky condo syndrome".

So many houses have rotted from moisture trapped within walls that the province-run housing insurance company went bankrupt from paying out to fix rotten, mouldy walls.

The result was a lot of investment by the government (the Barret commission) to determine *exactly what* building practices caused these problems and how to prevent them in the future.

I took the resulting Building Envelope course (40+ hours) and was shocked at how much damage poly vapour barrier can do to an older (retrofitted) structure.

The water gets into the wall and, if there's poly in the way, can't get out. Like leaving a tuna sandwich in a plastic bag for a year -- yuck!

Their advice was:
- don't use poly
- make sure you create an "air barrier" on the outside (cold) side of your walls. In this case (where you can't get at the outside face of the wall), caulk/foam the joints of the exterior siding from the inside.
- install insulation (kraft paper optional)
- install wallboard
- seal *all four* edges of the walls (especially around windows, along ceilings and floor joints) with foam/caulking.
- paint the wallboard with "vapour barrier" paint.

The official notes state:

"Vapour barriers typically stop between 10 and 30% of the water vapour that moves from inside a building into insulated cavities, the rest is stopped by the air barrier.

"About 0.3 litres of water vapour will pass through one square metre of drywall by diffusion over a heating season.

"In contrast, many *gallons* of water vapour will pass through a 1/2" square opening by air leakage over a winter. The actual amount will depend on specific site conditions."

So, if you're using a vapour barrier, you *must* guarantee that there is no movement of air into the wall.

Don't mean to scare you -- just passing on what we learned (and saw first-hand).

.../j

jch - Thanks for sharing. Needless to say I'm somewhat baffled by this topic as it seems even professionals struggle. That said, after re-reading your posting and refering to this video explaination several times I think I'm catching on.

One distinction I'm seeing between your notes and the link above is that the video suggests the air barrier can go anywhere in the wall at all as long as air currents cannot get through the wall". Since your cautions seem to be specific to using a poly barrier on the exterior/cold side I'm wondering if an air barrier on the warm/interior side isn't as problematic--the moist air would never get in to the wall and thus wouldn't be trapped inside.

On a related note, we're also planning on a plaster skimcoat over the wallboard so I'm guessing that can serve the same function as the vapor barrier paint you reference.

Aaron,

That video was a nice find :-) It sums it up nicely.

In a cold climate, you should *never* install a vapour barrier on the outside (cold side) of a wall.

My caution was against using poly unless you specifically make the wall absolutely air-tight.

In old houses, making the wall air-tight is impossible -- there are too many cracks between studs and the sheathing, cracks in the sheathing itself.

And, because your house is stucco clad (like ours), the problem is 10 times worse.

In our course, stucco houses kept coming up again and again as examples of houses that have rotted from the inside out.

Stucco (like any masonry) acts like a giant sponge. It has a tremendous capacity to hold water (very slow to dry), cracks easily, and wicks moisture vertically up to 8 inches(!).

So if you have stucco on the outside of an old house, you must *assume* that water *will* end up in the wall cavity at some point.

Old (hollow, uninsulated) walls dry out during the summertime. During the winter, heat from the house easily reaches the outside surface of the wall and helps dry it out too.

That's why these mouldy, rotten walls are only found in renovated/insulated houses. Old hollow walls dry out before they get a chance to rot.

But as soon as you add insulation to an old wall, you're preventing that heat from reaching the outside sheathing. The sheathing now becomes a cold surface which any leaking air (say, through outlet boxes) will condense on.

It gets even worse if there is a poly vapour barrier on the inside (warm) side of the wall. Moisture now cannot escape the wet wall into the house.

Plaster skimcoat is not much of a vapour barrier (I don't have my chart of permeabilities nearby for the exact numbers). Which, in your case, is good.

My advice would be:

1) for an older house (especially one with stucco) skip the vapour barrier.

2) Instead, focus on preventing any air leakage into/out of the wall at all penetrations (windows, outlets, baseboard, sill plates).

3) And before installing insulation, look for any signs of water staining on the exterior sheathing.

4) If there are any, you absolutely must fix the leakage (with foam/caulking) before you insulate. Otherwise, you're going to see a *lot* of rot down the road--it only takes a couple of years to set in.

Note: If there are air leaks in your walls, you will only feel them on really windy days, or in the basement of your house. The stack effect (warmer air rising and trying to squeeze *out* of upper-floor cracks) will mean you will never *feel* drafts on your upper stories, even though they are taking warm moist air into your walls where they condense on your cold sheathing.

Hope this helps,
.../j