My Green Home Gets All Sealed Up: Vapor Barrier Installation
I never thought I could get excited about a vapor barrier installation, but as I researched the steps required to make my house truly energy efficient, I soon realized that as an integral part of the system required for a tight house envelope, that layer of translucent plastic had an immense role to play.
In this page, I describe:
how we sealed up the house envelope through vapor barrier installation;
the different kinds of vapor barriers we used; and
the steps we followed in order to achieve as tight a seal as possible.
A Tight House Is A Good House
To achieve optimum tightness, vapor barriers are only one part a system which includes insulation and sealing air leaks (by caulking, for instance). The following aspects have already been covered here:
The air leak sealing part will be treated separately.
Why Use Vapor Barriers?
In cold climates, insulation alone is not enough to keep our costly heat from escaping. Given the slightest opportunity – in the form of a tiny hole, crack or improperly caulked joint, our precious heat will find its way out in an alarming way.
Once we understand the principle that warm air is attracted to cold air, and not the other way around, it should become almost a matter of pride to keep that warm air in!
Although some heat is lost by conduction through the insulation, most of the heat loss in a building is through air movement, or air infiltration, through the walls.
That's where vapour barriers come in. A vapor barrier – also known as vapor retarder – is merely a continuous sheet of impermeable material installed on the inside, i.e. the warm side of the wall, and it's designed to keep the warm air from escaping through or around the insulation and other building components.
There are several reasons for wanting to keep the warm air inside the house through a vapor barrier installation; among them:
Energy efficiency
Heating costs
Preventing the deterioration of the building materials (wood, insulation) by moisture (hence the alternate term moisture barrier). In winter, as the heated air meets the cold air, it creates condensation, which is water of course. The vapor barrier is designed to prevent this phenomenom from taking place.
Health problems caused by molds
Continuous is the keyword here. For the best performance, the vapour barrier installation must be made as air tight as possible by:
taping all the seams with special tape,
using plastic boxes around electrical boxes, and
caulking any penetrations through the vapor barrier by things such as wires and plumbing pipes.
Since vapor barriers are applied by stapling, and I don't like the two little holes that each staple makes, I taped over all staples, too, for good measure.
The reason for the plural is that this project was multifaceted. We installed:
6 mil polyethylene on walls, over low-density spray foam;
6 mil polyethylene on flat ceilings, over low-density spray foam;
6 mil polyethylene on cathedral ceilings, over high-density spray foam;
6 mil polyethylene on one wall, over fiberglass batts;
foil insulation/vapor barrier on walls, over low-density spray foam;
vapor barrier paint will be applied to the bathroom walls and ceilings, partly to test the product, but mostly to compensate for the fact that we're not using the special cement drywall there.
Each area had its own requirements.
This is the standard type of polyethylene vapor barrier:
Polyethylene vapor barrier comes in widths from 12 inches to 20 feet and lengths of 60 to 375 feet. Our local store stocked the 100-foot roll, ten feet wide, which is the most common for this kind of application. This will cover 1,000 square feet, with a minimum of joints.
When buying vapor barrier, look for the gauge stamped on the product, like in the middle photo; .006 (.152 mm) represents what is referred to as "6-mil", which is the desirable thickness.
Any old plastic film will NOT do: the photo on the right has the National Standards of Canada acceptance number and the fire rating stamped on. This is very important.
In order to be effective, the vapor barrier must be completely sealed.
Where pieces join and around windows and other components, a special red tape, called Tuck Tape here, is used. It's quite expensive, but there is no substitute for it.
In corners and other uneven surfaces, acoustic sealant – the black stripes seen here – is used. They tell me there's no substitute for acoustic sealant either, which has the advantage of remaining flexible permanently.
Acoustic sealant is applied with a caulking gun. The large size represents quite an economy, so it's worthwhile investing in a giant caulking gun if you're going to carry out your own vapor barrier installation.
Once the vapor barrier has been applied, provisions must be made for supporting the drywall.
That's where strapping comes in.
Strapping was applied at precise intervals, to correspond with the measurements of the drywall panels.
Measurements must therefore be very exact.
Strapping comes in very long lengths, so it takes two people to put them up on the ceiling. This wall is all strapped and ready to receive drywall.
Although strapping (which is just strips of soft wood measuring 2 inches wide by 3/4 of an inch thick) seems inexpensive, at less than three dollars per 10-foot length, when you're doing a whole house and using hundreds of lengths, the cost quickly adds up to several hundred dollars.
This short piece of wall required about eight lengths, for instance.
Foil Insulation As Vapor Barrier
This close-up shows both sides of the type of foil insulation that we used on the kitchen-dinette walls.
(Foil insulation is also known as foil vapor barrier and radiant barrier insulation.)
This combination of foil-bubble-vinyl is only one of several configurations available.
It's also the one they usually put in the kits for hot water heaters, and that's a nice way of using up leftovers.
One final pass with the steel brush to remove spray foam insulation residue before applying the special vapor barrier to the kitchen-dinette walls. Due to their northern exposure, it was decided that the kitchen-dinette walls would benefit from a different treatment: foil insulation.
Foil insulation is actually a triple product:
an outer foil layer to reflect the heat;
a layer of bubble wrap for extra insulation
a layer of vinyl that acts as a vapor barrier
Altogether, these layers add an extra R-4 of insulation.
Note how it extends beyond the walls at the top and bottom. This is to create an overlap between vapor barriers.
Another way that a combination foil insulation-vapor barrier differs from ordinary ones is in the vapor barrier installation technique:
the product must be stretched very taut;
the strapping is no longer just a support for the drywall: it's an essential part of the strategy because it creates an air space to allow the heat to bounce back towards the room;
the joints must be sealed with aluminum tape in order to preserve the integrity of the reflectivity.
We describe this installation in more detail on our Foil Insulation page.
Cathedral Ceilings Required Special Treatments
The last areas to get their vapor barrier were the two cathedral ceilings.
We started with the kitchen-dinette.
With regular polyethylene vapor barrier, wrinkles don't matter, so the plastic just got stapled to the existing wood supports.
But each staple makes two tiny holes in the plastic, and that had me worried.
This picture shows how all the staples have been covered with tape. Note how the electric box has been sealed as well.
It's crucial to seal every place where the heat could escape. The joint where the wall and ceiling vapor barriers meet was sealed with acoustic sealant, then tape, then a strip of wood.
On this other cathedral ceiling (the sunroom-office one), a different technique was used.
The strapping went first, then the vapor barrier. This ceiling was very uneven so the strapping had to be leveled. Here you see the men inserting shims between the rafters and the strapping.
Technically, the kind of spray foam insulation used here – BASF Walltite (blue foam) – does not require a vapor barrier, but I wasn't taking any chances.
Here's that ceiling, with its precise grid of wood, ready to receive the vapor barrier, then the drywall.
Vapor Barrier In Crawl Space
After all those surfaces were taken care of, only the crawlspace floor remained. Because it's dirt, it's very important to prevent the natural moisture from entering the area, especially since I had gone to the expense of insulating the outer walls (that crawlspace is under the kitchen-dinette, which is exposed to the north winds).
Sealing that floor will also keep the space a bit warmer, which will be good for the water pipes that go through there.
Following the spray foam insulation specialist's advice, the crawlspace vapor barrier was taken up the walls by at least one foot, and it was sealed to the foam insulation by two lines of black acoustic sealant.
Tape alone would not have been sufficient, because of the irregularities in the blue foam insulation. However, tape was used for fastening the edge of the plastic (as well as the seams, as can be seen in the floor portion).
Crawl space vapor barriers vary and your own crawl space vapor barrier installation will no doubt differ from this one; what's important is to understand the principle, which is that a vapor barrier is only effective if it's continuous. Keep that in mind and you can't go wrong.
Sealing Crawl Space
To seal or not to seal the crawlspace? It's pretty obvious that with a vapor barrier installation that is combined with blue spray foam insulation on the crawl space walls and a tightly sealed 6 mil polyethylene vapor barrier on the floor, you're likely to end up with a pretty tight crawl space, impervious to outside air and humidity.
Whether this is desirable or not depends on your climate; therefore, your best bet is to contact your local authority and ask what the recommended practice is for your geographical area.
Attics And Basements
Our Foil Insulation Page has a section on using reflective insulation as an attic vapor barrier and/or as a basement vapor barrier.
The use of vapor barriers in attics and basements depends largely on the climate and you have to be very careful to apply them in the right place. It's important to investigate this and to use the application that is recommended for your area by the local building code.
The "Stimulus Package"
If you've been following this adventure, you will know there are financial incentives attached to increased home energy efficiency in Canada. You can read all about it on Natural Resources Canada's website.
How effective were all the efforts described in these pages? We will know on July 28th of this year (2009), when my second blower door test takes place. (The second energy audit was originally scheduled for April 30th but we were given a 3-month extension due to the shortage of contractors in my region.)
That will conclude the energy audit that was begun in November 2007 with the first door blower test.
In addition to revealing a certain lack of leaks, the results will determine the house's eligibility for government grants totalling a couple of thousand dollars. A mere pittance compared to the actual cost, but in these difficult times, every little bit helps.
Because of my permaculture background, and my desire to stop giving the power company a large percentage of my income, I would have done all this work anyway. But for others, the financial incentives may have added the necessary motivation to take the first step towards a more sustainable lifestyle.
GREEN NOTES
GREEN PLASTIC? I looked, and couldn't find any truly "green" vapor barrier materials.
If you know of any, please let me know and I will gladly list them here.
Meanwhile, the manufacturer claims that his product is reusable and recyclable, and I've read somewhere that "clear polyethylene's content is up to 80 percent "reprocessed" material, which would make it environmentally sustainable."
The cost of this could be uneven quality and poor tear and puncture resistance, so I checked with the manufacturer of the product I used and was assured that their vapor barrier products are made with 100% virgin resins.
You might want to do this kind of checking before proceeding with your own vapor barrier installation.
A good insulation system, which includes a properly done vapor barrier installation, makes a house very energy-efficient so in spite of the materials used, it's hard not to classify this case as one where the end justifies the means.
Therefore, why is it that none the green building books that I looked at mentions their existence? It's no surprise, then, that so many people visit this page every day!
