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.

Tuesday, February 10, 2009

Passive House

Last post we discussed some interesting ways to increase the R-value of your walls while still maintaining normal and acceptable building practices. But let's say you want to take it a step further. Welcome the Passive House, a remarkable building process started in Germany and quickly becoming a popular method of building homes. Passive houses require no heating source but are generally required to have one per code.

"Wait, no heating source? No furnace, no baseboard heaters, no heat pump, no radiators?" Yup. I mean, nope, none of that. The homes are built so tight and insulated so well that they get heated every time you use a hair dryer or brew coffee. In fact you could keep the home heated all winter with a few well placed candles.

"But if the house is built tight and you're burning candles, won't you die of carbon monoxide inhalation? Shouldn't the house breathe?" Oh for the love of Pete, if your house 'breathed' you would be making monay selling tickets to an authentic haunted house. Houses don't need to breathe. The occupants inside need to breathe and yes you need to cycle fresh air through.

Performance issues first. R-values need to be 44 or better. This does not mean using R-44 insulation. This means the average value of your whole wall, including energy sucking R-3 windows, needs to be accounted for. Floors and ceilings need to achieve the same values. Currently these values are in the R-10 range for walls (using R-21 insulation), and R-30 for floors and ceilings assuming no penetrations. So the solution for floors and ceilings is simple, increase the value to R-49 in both and call it good. The walls are a little more difficult. How would you achieve an R-50 wall? One method is to use 9.5" I-joists for studs and fill the cavities with either spray foam or high-density blow-in. This gets you to R-38, and then add 3" of EPS rigid foam board to the outside. Typical windows are as good as R-3 (U=0.32), but R-7 (U=0.14) windows can be used.

Typical houses leak at a rate of 10 or so air changes per hour when you suck out 50 CFM of air. An Energy Star home cannot exceed 7 and 2 or 3 is generally considered darned good. But the Passive House must not exceed 0.6 ACH at CFM50.

"Oh the cost! This would be a fortune!". How much do you think it really is? Since R-5 windows are about the limit available in the U.S., you are looking at roughly an $8000 increase for a modest 2000sf home (foam is cheap). Not terrible, but here's the kicker. You can skip the HVAC system, so there's a $4000 savings. Instead you'll install a small HRV system to provide fresh air to the home without it adversely affecting temperature. Add about $2000. You'll also want to provide a couple of small cadet heaters so add another $1000. So you're in it for $7000 extra. But when you sell the house, market it well and you'll gain an extra $20,000 over similar homes. Why? Because the heating/cooling bills will be reduced from $200/mo to less than $50/mo. The extra $20k will finance at $120/mo. Math says that your buyers will save a few bucks a month or break even. That assumes that energy costs will stay stable. If you believe this, I've got a barn to sell you. So consider that this becomes a hedge against future energy costs and your buyer breaks even and gets a more comfortable home.

If you'd like your next house to be aggresively passive, we would be happy to detail it for you.