I am enclosing a second story porch. The floor is made of pressure treated lumber. The joists are 2×8 pressure treated as well and are easily accessable from the first floor as well. I want to insulate the floor and walls of the porch. I plan on putting 3/4″plywood subfloor which will be covered with a PERGO floor. What is the best way to insulate? Do I need a vapor barrier? Where should the vapor barrier be located? Additionally, do I need to put any type of barrier under the insulation which I plan on putting in the joists, which will be covered with plywood? This will be the ceiling of the first floor porch.
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It depends upon wherew you live--I'm assuming a cold (in winter) climate? To get started, you can look at http://www.buildingscience.com for some answers.
Thank you. I will read the article.
Installation of a vapor barrier assumes that you have an area of generally higher ABSOLUTE humidity and an area of generally lower ABSOLUTE humidity, separated by an insulated wall. The vapor barrier goes on the side with the higher absolute humidity. A vapor barrier is needed when, without it, and given the way that moisture migrates through walls, the dew point would occur somewhere in the middle of the insulation.
If an enclosed porch is not fully "integrated" with the rest of the house (not normally connected to house forced air heating, and with doors/windows separating that are not expected to be open in the coldest part of the year), it's unlikely to have a level of humidity sufficiently high to be a concern.
On the bottom, under the insulation, I'd suggest that you install a housewrap (not a vapor barrier), tightly taped around the edges and at any seams. Preventing airflow into the floor from outside is the single biggest thing you can do to improve the "climate" in the room.
Also housewrap any insulated walls you build, and seal tighly around windows.
The porch is enclosed on three sides by the rest of the house. However, I do plan on using the room in the winter as an exercise room so it will be heated if it is too cold. There are large windows and a French door from our bedroom to the porch.
If you are sheathing the floor with minimum 5/8" t&g ply, that will be construed as a "vapour barrier.
( check your local code)
Edited 10/8/2006 3:05 pm ET by alrightythen