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How To Install Poly Vapor Barrier

Installing polyethylene (poly) to the warm side of a exterior wall is very important. This is generally referred to as installing vapour barrier because the main function is to stop the warm moist air in your home from entering the cold interior space inside of your wall cavity.

This will cause condensation inside the wall cavity which can cause deterioration of building materials and even mold to form. The vapour barrier always goes on the warm side of the wall between the framing and the drywall. The secondary function is that it stops the infiltration of cold air from coming through the wall cavity into your home.

Poly should be used on the warm side of any building surface of which is insulated such as insulated ceilings,walls and floors.

Difficulty Level
3 / 12

Tools Required

Materials Required

Vapour Barrier / Poly Installation Steps

There are different thicknesses of poly that can be bought at your local building supply stores on various sized rolls but there is one thing that a vapour barrier grade poly needs and that is to be approved for use as a vapour barrier. In Canada it should be C.G.S.B. approved. If you use a non-approved poly, it does not last and will deteriorate over time and crumble leaving you unprotected.

Installing poly is not that hard but you must properly seal all joints around windows, doors, electrical outlets and seams. Before starting the poly be sure that everything is complete in the areas that you are going to cover with the poly so you do not have to open it up again later. This includes electrical, blocking, insulation etc.

TIP: All electrical boxes in these areas should have be installed with a vapour box already around them. They are basically a preformed poly cup that goes on around the electrical box and has a flange to which you can seal your vapour barrier to without too much trouble. These vapour boxes come in different shapes and sizes depending on the type of electrical box you are using. You could also use plastic electrical boxes that do not require the extra vapour box they are made with the sealing flange already attached to them.

Here is a photo of a vapour barrier box installed behind a receptacle box
electrical box vapour barrier behind

Before you begin to poly, first clean the room out of anything you will not need or that is in the way. Then sweep and or vacuum the room. It sounds dumb, but I always vacuum because as soon as you start with your first sheet of poly and it drags across the dirty floor once, the static in the sheet picks up a lot of dust and dirt and before long it is all over you and in your eyes. So after you vacuum you are ready to start hanging the poly.

To start installing poly it does not really matter if you do the walls or ceiling first but I prefer to do the walls in a room first. So to do the walls start by deciding how big of a piece you can handle and cut if from the roll. I would start with about a 8 foot piece until you do one to make it easier. Also you want the poly to be wide enough that it reaches the top of the wall at the ceiling and leaves at least 3 extra inches at the floor.

TIP: You will probably want someone to help you with hanging the poly. An extra set of hands is really helpful with this.

If you and your helper each take one top corner and line them up where the wall meets the ceiling and stretch the poly out between you both so there are no sags or wrinkles. Now place a row of staples about 16 inches apart along the top edge to hold it up. Once that is done you smooth it out from the top edge that you did, to the bottom about 2 feet at a time and place a staple in each framing member as you go. Do not worry about cutting out around windows or electrical yet. Next cut another length of poly and this time and start by over lapping it onto the end of the last one you completed. Lap it far enough that the edge is over a framing member in the wall, this will make it easier to seal later. Continue the process around the room until you have covered the entire wall area that you needed to.

Now that you have the walls done it is time cut out around any windows and doors. Cut the poly sheeting about 1/2 inch outside the perimeter of the windows and doors then use the sheathing tape to seal that area to the window and door jambs all the way around them. Next cut out around the electrical boxes enough to allow you to once again use the tape to seal the poly to the vapour boxes. Next at all the over laps tape them as well.

Finally down at floor level pull the poly up off the floor enough to allow you to place about a 1/4 inch bead of acoustical caulking right in the corner where the wall meets the floor. As you do this then fold the poly down again and press the poly into the bead of acoustical caulking to make the seal.

TIP: Acoustical caulking can be very messy to work with and it is almost for certain no matter how careful you are you will get some on you or some where that you step in it and track it around so be very careful while using it.You need basically paint thinner to clean it up properly if you do get it on something.

Now that you have done the walls the ceiling is really the same principle. Just make sure to allow enough poly (about 4-6 inches) to hang down onto the walls so that you can run some tape around to seal it up. I never use acoustical on the ceiling because for sure you will have it on you or in your hair so just use tape! You may also find that you need a few more staples in the ceiling poly to keep it from sagging too much, that is fine to add a few but remember your ceiling finish (probably drywall) will soon be on there to help out.

Here is a photo of a kitchen ceiling with pot lights and all the poly vapour barrier is installed and sealed:
kitchen ceiling vapor barrier poly

So as you can see this is not a tough or costly job but just remember vapour barrier is a very important component of your house structure and should be sealed as good as possible, so take your time and do it right the first time.

See also: Fiberglass Batt Insulation Installation Instructions