Tuesday, February 24, 2009

Fat Walls

As a follow up to our previous post regarding Passive Houses, we would like to explore some options for making a wall more insulated. Code allows for the wall to be less insulated than the rest of the home. While a roof is R38 (all figures are for the Northwest) and floors are R30, walls are only required to be R21. This is 33% worse than floors and only half as good as roofs. On top of this, walls have all these nasty holes called 'windows' that reduce the actual value down to R13 at best. This makes the walls a full 2/3 worse than the roof.

"Well that's okay, because heat rises, right?"

No. No, and no. Hotter air rises above colder air but heat moves any direction from hot to cold. Ideally a house should be equally insulated on all planes. This means we need to get walls up to the R30 to R40 range.

First option: Standard wall of actual R13 plus 4" of XPS rigid foam board gets you to R33. Not bad, but 4" of foam really messes with window openings and requires some extra detailing.

Second option: Advanced frame 2x6 wall with actual value of R18 plus 2" of same foam gets up to R28. Not bad, but not enough.

Third option: Add U=0.20 windows (R5) to this wall and start touching R30.

As you can see, there's not much left to do except...

Fourth option: Increase wall to 2x8 advanced framing (R24 actual) with 2" foam to get R34.

Our current favorite option: Two separate 2x4 walls with a 1" air gap in the middle and 2" of XPS foam on the outside, U=0.20 windows. This gets up to an R40 and is easy to frame. One wall is built to standard advanced framing, then the second wall is built with a minimum of lumber. All it has to do is hold gypsum in place. The gap at windows and doors is bridged with plywood gussets. This means the window openings will have to be 1/2" bigger on each side. Every extra inch of gap you'd like to add will increase the insulation value by another R3-R4.

"But I'm going to lose floor space in the house!"

Really? How much will you really lose? A standard 40x40' house with a 20x20' garage in the corner has 160' of perimeter. Two and one-half extra inches of wall reduces your floor space by 33sf. We're talking powder room or walk-in closet. And besides, if you're more worried about the little amount of floor space over an energy efficient home, we'd like to have a talk about priorities.

Besides, Passive House promotes 12". We're just advocating little steps.

If you'd like to hear more about these systems, drop us a line or visit istockhouseplans website.

No comments: