Do I need to install a vapour barrier if I am using rigid foam insulation (extruded polystyrene)?

May 14

I have an old semi in Toronto with no insulation on the second floor. I am removing the plaster in the middle bedroom and replacing it with drywall and am trying to decide whether it is worth insulating just that one room (equal to just a third of that exterior wall) and what the best method would be. I am thinking of using extruded polystyrene rigid foam boards but I don't know the best way to install it and if it needs a vapour barrier if it's just going to be that one section.

Thank you.

Couple things here. Adding insulation is always a good idea, even if it's just a third of an exterior wall. Next, extruded polystyrene (XPS) can act as a vapour barrier provided it's thick enough. Usually 1" is enough, but check with the manufacturer. Common ones are Owens Corning or Dow Chemical. To make it a true vapour barrier, it would have to be tape and continuous. Next, tar paper is not a vapour barrier but a moisture barrier, no good here.
Next, 9.25.4.2(1)in the Ontario Building Code reads:
Vapour barriers shall have a permeance not greater than 60 ng(Pa●s●m²)(0.78 PERM INS.), measured in accordance with ASTM E96, "Water Vapor Transmission of Materials", using the desiccant method (dry cup).
If 4 mil polyethylene meets this then it's fine, but I don't believe it does, and you would require a 6 mil polyethylene sheet.

Next, do not insulate between studs with a rigid styrofoam board. There will be gaps between the board and the studs allowing heat loss. Does this matter? yes it does.
Next, use a batt insulation like owens corning, Johns Manville, Roxul, OFI, CertainTeed, Knauf, etc etc.
For further questions, contact manufacturers. Their reps would know best.

I need to insulate my garage door. What should I use?

Filed under: Rigid Foam Insulation

6 Responses to “Do I need to install a vapour barrier if I am using rigid foam insulation (extruded polystyrene)?”

  1. donktheclown, on May 14th, 2009 at 10:48 am Said:

    Yes, if it is an exterior wall, vapor barrier it. Shouldn't cost more than a few dollars to do so. Foam boards do not stop moisture around the edges.
    References :

  2. jentrie2, on May 14th, 2009 at 11:02 am Said:

    Maybe Tyvek like on homes

    Ask at a builders store like Home Depot or Lowes
    References :

  3. minz, on May 14th, 2009 at 11:12 am Said:

    duno
    References :

  4. chicagirl51, on May 14th, 2009 at 11:58 am Said:

    You need a vapor barrier on all exterior walls. Get a roll of 15 lb. roofing felt and apply that to the exterior wall. If there is already a vapor barrier on the outside wall then just ut your unfaced insulation batts against it and drywall over it.
    References :

  5. premiumtommy, on May 14th, 2009 at 12:16 pm Said:

    yes you need it. the 2" poly insul will be an r-value of 10 and you would still have 1.5" of space left in a 2×4 wall. you can buy 2×4 fiberglass insul w 13 r-value. the fiberglass is easier to do in wood frame structures as the poly is usually prescored at 16" to place between metal studs. so cutting it at 14.5" for wood studs can be a pain. DO NOT use 15lb felt paper. it is not an approved vapor barrier it is a moisture barrier and you would have to either tape the seams or cut to break on the studs. just use 4mil clear plastic that comes in rolls from home depot.

    regardless wether or not there is tyvek or another brand of home wrap on the exterior, you need plastic wrap over the insulation. exterior home wrap is designed to let the wood behind it breath, not to stop air from infiltrating the home.
    References :
    15 yrs carpentry experience

  6. Iain, on May 14th, 2009 at 12:43 pm Said:

    Couple things here. Adding insulation is always a good idea, even if it's just a third of an exterior wall. Next, extruded polystyrene (XPS) can act as a vapour barrier provided it's thick enough. Usually 1" is enough, but check with the manufacturer. Common ones are Owens Corning or Dow Chemical. To make it a true vapour barrier, it would have to be tape and continuous. Next, tar paper is not a vapour barrier but a moisture barrier, no good here.
    Next, 9.25.4.2(1)in the Ontario Building Code reads:
    Vapour barriers shall have a permeance not greater than 60 ng(Pa●s●m²)(0.78 PERM INS.), measured in accordance with ASTM E96, "Water Vapor Transmission of Materials", using the desiccant method (dry cup).
    If 4 mil polyethylene meets this then it's fine, but I don't believe it does, and you would require a 6 mil polyethylene sheet.

    Next, do not insulate between studs with a rigid styrofoam board. There will be gaps between the board and the studs allowing heat loss. Does this matter? yes it does.
    Next, use a batt insulation like owens corning, Johns Manville, Roxul, OFI, CertainTeed, Knauf, etc etc.
    For further questions, contact manufacturers. Their reps would know best.
    References :

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