I want to insulate my unfinished stairway ceiling with ESP (Expanded Polystyrene), but have some questions..?

My house is a restoration that was never finished. The place was constructed very well so far. The stairway ceiling is framed in 2 x 6's with its cross bracing done in 2 x 4's, leaving space for air to travel up for ventilation. I have the stairway closed off in the winter because the second floor isn't finished. The stairway still gets warm enough to melt the snow above causing ice damn's. I have a lot of 2" ESP foam sheets. I want to cut and place them in between the rafters, seal them with spray foam and then put batts of rolled insulation with the foil towards the inside.

****To avoid condensation, my question is should I leave a 1" airspace between the ESP foam sheets and the roof plywood sheathing? The supplier seemed to thing the "dead airspace" would act as an insulator itself. I don't think they realized that I could properly vent the space so the air would not be "dead." I could leave it dead if it would be better. I'm worried about condensation. I can put a soffit vent at the bottom of the stairway for fresh air to flow to the top, which feeds into the vented attic. If I can put the ESP sheets up against the plywood roof sheathing I don't need to put a soffit at all. The ESP has an R value of 4 per inch so mine would be 2" = R-8, plus the batts of insulation too!

*****Should I put a plastic vapor barrier on the inside of all the insulation or will the ESP sheets be enough of a vapor barrier? Thanks!


Instead of buying the egg carton studd I can just use 1" scraps of my foam, but is it wrong to put the ESP foam sheets right up against the plywood roof? I did some research and found tests show venting really doesn't make the rood cooler. Read the end of this article: http://www.applegateinsulation.com/Product-Info/Te...

2 Answers

  • Rob G
    Lv 7
    1 decade ago
    Best Answer

    You never want a dead air space on an exterior/ interior wall or ceiling, or roof deck in this case. It is ok to have a dead air space as long as that space is totally within the living space envelope, this way, you will not have a temperature differential on either side of the wall as you would with an exterior wall, which will lead to condensation.

    Spacing the foam down with scraps is fine,and a good idea (insures uniform spacing and uses the scrap material)

    I don't know what tests you saw, but venting the underside of a roof deck is always a good idea, it will keep the living space below cooler, allow your roof to last longer, because it doesn't have that "oven" of super heated air just below it.

    Also any time you can install a vapor barrier, do so. The plastic will be much more effective than you could ever be with just trying to "seal" the foam panels.

    For all intents and purposes you are creating a "mini attic" so it should be treated AS one with air spaces, ventilation, and vapor barriers

  • 1 decade ago

    Purchase some styrene insulation chutes. They are made from egg carton like styrofoam and come in sizes for 24" spacing and 16" spacing. Put them end to end right up against the roof decking. Then install the ESP sheets, then the fiberglass batts.Don't forget the sides of the stairwall walls if they are not insulated.

    While the framing is exposed I would add a ceiling light & smoke detector if not already there.

    Good Luck !

    Source(s): General Contractor 30yrs. and counting
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