Efficiency by Design - Walls

Walls are so misunderstood. In a world of "More wood is better!" it's hard to know what to cut. Efficiency by Design can optimize your walls to help you know what's what and what to cut.

What's in a wall? There are three basic parts: First are the studs that stand straight up. These are held in place by plates at the top and bottom. Finally all of it is tied together with sheathing. How do they work?

Unbuckling Your Walls

Pop Quiz:

Why does wall sheathing buckle?

If you answered something like studs at 24" o.c., I'm sorry to disappoint you.  The most common cause of wall sheathing buckling is because it wasn't properly gapped.  We've hounded on this before.  But now the APA has developed a mobile tool that will help educate builders on some of the most common building issues.

See this and many more tips at the APA website.  Tambien en español!

• Prevent Buckling with Proper Spacing includes spacing recommendations for APA Rated Sheathing, APA Rated Sturd-I-Floor®, and APA 303 Siding. (Form M300, now available in Spanish)
• Construct a Solid, Squeak-Free Floor System describes how to prevent floor complaints and callbacks with proper floor sheathing installation. (Form Q300, now available in Spanish)
• Minimize Nail Pops describes how to reduce nail pops through recommended fastener selection and installation. (Form S300, now available in Spanish)
• Storage and Handling of APA Trademarked Panels provides guidelines to help protect panels from damage in storage, during shipment, and on the job site. (Form U450)
• APA Panels for Soffit Applications provides information on recommended panels and spans for open and closed soffits. (Form N330)
• Finishing APA Rated Siding describes recommended finishes and application recommendations for APA Rated Siding. (Form Q350)
• Proper Storage and Handling of Glulam Beams provides recommendations for storage and handling of glulam beams. (Form R540)
• Minimize Glulam Checking Through Proper Storage and Handling provides tips for preventing glulam checking. (Form F455)
• Proper Installation of APA Rated Sheathing for Roof Applications provides step-by-step instructions for roof sheathing installation. (Form N335)
• Proper Selection and Installation of APA Plywood Underlayment includes information on selection, handling, installation and fastening APA Underlayment panels. (Form R340)

Advanced Framing Techniques in Video

The APA (Y'know, the plywood people) recently unveiled a new video outlining advanced framing and how easy it is to achieve in your building.  If you're still building at 16" o.c. with redundant studs at corners, windows, and T-walls, see this video.  These techniques actually ask you to do LESS in your building while achieving cheaper costs, a more comfortable home, environmental friendliness.  If you still balk then I won't stop you from building substandard home.  But for the future of your income, please at least consider staging these techniques into your repertoire of framing practices.

By way of reminder, it's not the number of studs that keep your house from blowing down or siding from warping, it's the use of plywood gapped per manufacturer's specs that achieves strength and durability.

Heat Load Calculator

A while back we posted a series on the mechanics of calculating the heat load of your home.  At the end we promised to offer up an Excel file that is set up for you to do your own calculations without getting a headache or hand cramp

The calculator comes pre-filled with info from our Houston 2448.  Everything in light yellow can be modified.  The file or the rest of the cells are not locked.  This should be considered open-source, AKA modify at your own risk.  If you enter any values into white cells, you may destroy formulas.  There are also no fail safes or error checking in here.  Double check your work.

There are 7 components listed: slab, floor, walls above and below grade, windows, doors, and roof.  Each of these has inputs for area and R-value.  Note that windows should be input as U-value.  When inputting wall area, don't take windows or doors into account.  They are automatically deducted from the wall area in the calculations.  Outside design temperature can be modified for the first four items; remaining values are derived from those.

All the work is shown on the following columns.  The UA value, Δt and Btu/hr values are shown.  Indoor temp can be changed to your desired setpoint.  To the right is a little table with all sorts of nerdy calculations in it.  Percent of load tells you which component is losing the most heat.  Cost/hr tells you how much it costs.  In the example you can see that more than half the heat loss in this house is through the walls.  Of course!  There is only R-15 in the walls!  You can also see that increasing to R-21 doesn't do much for that factor.  Increase the walls to R-30 and you can get that component down to about 1/3 of the heat load.  Still high.  Note that the rest of the load percentages change as you change the area or R-value of an item.

Lower down on the page is a place to take leakiness of the house into account.  Input your target or measured ACH50 as well as volume of the home.  You should only change the HC if you know what you're doing.

The final input is for number of bedrooms or potential bedrooms.  This little calc will determine internal gains from humans.  It takes the number of bedrooms and adds 1 person per ASHRAE standards.  If there will only be two people living in your 3000sf house, enter one bedroom for kicks.

Total peak heating load is given near the bottom of the sheet.  The final table gives an idea of how much of what types of heat is needed to keep the house comfortable.  A forced air unit size and efficiency can be entered.  As you can see, even this is WAY too big for the house.  Even 2 1kW cadet heaters will do fine.  In this case we would recommend a 500W heater in each of the bedrooms and bathrooms with a 1kW in the great room.  Still a bit much but at least reasonable.  Perhaps a mini-split heat pump would do for efficiency as well as adding some cooling if you are in the South.

This calculator should be used for entertainment purposes only.  No guarantees about the results or performance of this tool are made or implied.  If you break it, you bought it.  If you find errors, please feel free to let us know.  If somebody who knows javascript is bored, we would be thrilled to turn this into an online tool.

Energy Efficient Wall Systems

You may remember our post a couple of years ago promoting Fat Walls.  In their 11/11 monthly newsletter, Energy Design Update recently reported on 15 different wall assemblies modeled through TRNSYS software.  The walls were simulated in the climates typical to Atlanta, Pittsburgh, and Phoenix.  Of the 15 walls, three tied in first place for an overall value of R-43.  One of these walls fills a 2x6 cavity with closed cell polyurethane spray foam for a rather high price tag.  The second wall involves 10" thick SIPS.  The third wall is our option number four from the previously mentioned post with 2" more of foam.  That is, a 2x6 wall with blow-in and 4" of outboard XPS foam.  As we mentioned back when we wrote the initial post, this makes window detailing a bit of a bear.  Attachment issues come into play as well.  The advantage of this system is the standard wall framing and no loss of floor space inside the house.

A reasonable compromise might be 3" of foam.  This allows the use of true 2x4 for bucking out windows while allowing 1/2" air space.  Half inch furring strips can then be used over the foam for attachment as well as a rainscreen.

They also modeled a similar wall as our top choice, double 2x4, total 8" thick with 2" of outboard foam.  Our results?  R-40 with U-0.20 windows.  Their results were R-38 with U-0.25 windows.  As you should know, U-0.20 windows are slightly better than U-0.25 resulting in a slightly higher total wall R-value.

So apparently we know what we're doing!

You're fired!

In searching for some material on fireblocking, we ran across this thread on the DIY Chatroom.  Indispensible material.  This is generally the bane of do-it-yourselfers and code officials alike.

http://www.diychatroom.com/f98/how-fireblock-framing-37190/

Istockhouseplans is currently working on trying to fireblock a double 2x4 common wall with raised heel trusses.  We'd like to rock the wall all the way up and then hang the trusses but are unsure that the hanger would achieve strength through two layers of 5/8" type X drywall.  A more viable option would be to nail a 2x4 ledger through the sheetrock into the walls studs.  This would require 1.5" + 5/8" + 5/8" + 1" embedment = 3.75" nails.  While 18d nails might not be common, this is going to require a bunch of hand driven 20d nails.   Those won't exactly fit into a power nailer.  The other option is multiple 2x16 blocking between trusses.  Not really an option though.  Maybe stacking 2 pieces of 4x8 would do it?  Does anybody have input?

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.