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steveel 10-05-2010 10:23 AM

Insulating a full height walk up attic stairway
In my old house in Pennsylvania, I have a full sized stairs leading up to the unheated attic. The access door is a fullsize door in a 2nd floor bedroom wall.

Option 1 is to weatherstrip the door as an exterior door, make the thermal envelope zigzag thru the stairwell, with insulation under the treads and in the sides of the stairwell (treating them as kneewalls)

Option 2 is to think of the stairway as a bedroom closet, by building an R30(or more) weatherstripped stairwell hatch, and allow convection thru the strairway "closet" via a large gap at the base of the door and a new vent cut in the upper panel

I'm leaning toward option 2. We'll be insulating the attic floor to the same R value later on.

Comments anybody?

Gary in WA 10-05-2010 01:29 PM

Depends on how often it is used, I'd go with option #2. If using fiberglass, I wouldn't store anything in the attic attracting glass fibers or moisture from the outside air ventilated through. Air seal the attic especially at the wall/ceiling taped drywall joint many forget how thin that is. Use whatever is required and remember the basics:


Ron6519 10-05-2010 01:58 PM

I don't understand why you would put in an insulated hatch and cut holes in the door for convection. What does that accomplish?
I have the same scenario. A full staircase going to the attic through a closet. I just put 6" of foam board on the doors at the top of the stairs.

steveel 10-05-2010 02:25 PM

Thanks Gary, and for option 2 we'll use 6 layers of R5 XPS foam sandwhiched between a couple layers of plywood or OSB.

Ron, in my case I do not have a door at the top of the attic stairs. What I have is a door in a bedroom wall. If you peek in thru that door you look up into the attic. Option 2 lets us keep the thermal envelope flat all the way across the attic, without crazy dips and zigzags down the stairwell. But that means creating a dead air pocket on the stairs. The door would be at the bottom of the pocket, the stairs would be the floor of the pocket, and the new attic hatch is the roof of the pocket.

My understanding is that in winter inside water vapor still gets into nooks and crannies even if inside heated air doesn't get there. That means if we seal up that air pocket, that space will be colder than the rest of the house (one side is an exterior wall). Relative humidity might be 45% in our heated space, but high enough to grow mold and mildew in that pocket due to the lower temperature in there. This is the same reason some closets on exterior walls grow mildew down by the baseboard or behind thickly loaded clothing racks. By allowing convection through the door, we'd be joining that air pocket to the rest of the airspace in the bedroom. Since we'd be enlarging that airspace the only down side I forsee is it might impact a Manual D duct sizing calculation down the road.

If I talk like I know what I'm talking about, then I fooled you. ha ha. This is 90% book knowledge, not real world experience.

All comments welcome

Steve El

Ron6519 10-05-2010 03:48 PM

Steve, I've never had an issue with moisture in the closet. It's an interior door, not hermetically sealed by any stretch of the imagination. It's been that way since I put the insulation at the top of the stairs in 1992. The closet is cooler then the room, but that's true of every closet in the house(built in 1949).

steveel 10-05-2010 04:44 PM

Glad to hear you've never had a problem Ron. I could certainly be overthinking.

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