Fixing a Leaky Log Home
Rustic and charming as they may be, log homes pose unique challenges for air sealing and insulation
Early settlers who felled their own trees to build log homes were probably so grateful to be out of the weather they didn't worry about air leaks or cold walls. But when your heating bills are $500 a month, it's a different story altogether.
That's the situation facing ADK Homeowner, as he explains in a Q&A post at Green Building Advisor.
"Built the dream log home and now face the heating nightmare in Climate Zone 7," ADK writes. "A masonry heater reduces the heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load. of my in-floor heating system by half but without it my heating bills [average] $500 a month."
ADK has a plan. On the inside of his log walls, he wants to install between 1/2 inch and 1 inch of rigid foam insulation, followed by log siding installed on the interior (to preserve the aesthetic he liked so much to start with).
"I am thinking the rigid foam provides a thermal break and the log siding maintains the log look," he writes. "The insulation and log siding will not be cheap, but my heating season goes from late October through March or early April, and at least three of those months have many sub-zero days. I can afford to lose the 2 inches to 2 1/2 inches of space per room and the labor would be mine."
Will it work? Or is this plan just plain crazy? That's the subject of this Q&A Spotlight.
You need more foam
"The easiest thing to do is to sell your home and buy a home with tight walls," counsels GBAGreenBuildingAdvisor.com senior editor Martin Holladay. "It's very hard to fix log homes."
Holladay's suggestion may be tongue-in-cheek, but he's certain that ADK's proposed foam layer isn't nearly thick enough. He should really be looking for a way to add 4 inches of rigid foam insulation — either on the outside or the inside.
If he elects to install foam on the inside, he'll face several challenges, Holladay says. One is that log houses typically settle after they are built. The second challenge is making sure the foam is continuous between floors That is, it spans the area where floor joists or roof rafters meet the walls.
"If you decide to go ahead and install interior rigid foam, pay close attention to air sealing," Holladay says "Multiple layers of rigid foam will be better than a single layer; make sure that the second layer of rigid foam has staggered seams, and seal the seams of every layer of foam with high-quality tape."
He notes that most of his friends who own log homes have installed drywall on the interior, and to Holladay that makes more sense than installing log siding over foam on the inside.
Do your best, but don't sell
Howard Gentler is familiar with ADK's problems. His son owns a 24-year-old log home down the road, and while Gentler and his son can cut all the firewood they need to heat the place, "that doesn't change the fact that it takes a lot of heat (and a lot of work)," Gentler writes.
Gentler says they are considering adding 1 1/2 inch of foam, evidently on the interior side, and sealing it with drywall. "More foam would be better," he adds, "but the space taken from the smallish bedroom we are starting with would be a factor. Our thinking is this will add R and help very much with air leakage through seams."
As to Martin's sell-it-and-get-out advice, Gentler adds, that's a little extreme. "Do what you can to make it as efficient as possible, then consider it one of life's tradeoffs," he says. "As an energy-use issue, the new owners would still have to use more energy than a tighter designed house."
Add a Larsen truss wall on the outside
Rather than adding an insulating and air-sealing layer on the inside, Dana Dorsett suggests moving to the outside of the house.
First, Dorsett would seal the exterior logs with 1 inch of closed-cell spray polyurethane foam. That would be followed by a Larsen truss wall between 8 inches and 16 inches deep, depending on what will fit beneath the roof overhangs. Blow that wall full of cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection., then cap the wall with conventional siding or log siding over a rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. , he says.
"That puts all of the structural wood inside the thermal and moisture boundary of the structure, keeping it much better bounded in both temperature and moisture content over the seasons, reducing the amount of dimensional changes that wreak havoc with air-sealing efforts on log houses," Dorsett writes. "It also doesn't eat up living space, or force you to live in a construction zone while it's going on."
A Larsen truss works on a flat surface, TJ Elder replies, and in this case ADK would be better off with a row of studs over the exterior logs. In any event, the basic plan of insulating on the outside rather than the inside makes sense, Elder says. "Insulating the exterior makes it possible to cover everything from the foundation to the roof," he writes, "and to add considerable thickness for some real R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. .
Shimming the trusses to the logs to create a co-planar surface wouldn't be hard, Dorsett responds, but they would have to be well anchored to the wall to support the weight of the cellulose.
"There will be a lot of detailing to attend to to get the window flashing done right with any exterior insulation approach," Dorsett says, "but it's really the only way to turn the current 3-legged dog thermal performance into a thoroughbred performance building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials.."
Conduction, not air leakage, is the big problem
ADK likes what he hears, except that that Dorsett's plan is out of his price range. Nor would his wife consider covering the inside of the walls with drywall.
Besides, he adds, an energy auditEnergy audit that also includes inspections and tests to assess moisture flow, combustion safety, thermal comfort, indoor air quality, and durability. conducted on the house showed only two minor air leakage concerns, both of which were addressed at the time.
"I have misplaced the report, but the blower-door results were well within tolerances," he says. "My non-scientific observations lead me to believe the primary cause of my heat loss is conduction through the 8-inch milled kit logs... I think the heat is passing through like a hot knife through butter — current outside temperature of 12 degrees and logs range from 52 to 54 degrees on interior."
Fitch Plate doubts that any home built from milled logs could be adequately air sealed without a second wall, and he urges ADK to dig up the energy audit and post it at GBA for review. "I have not seen a milled log house unfinished on the inside for decades," Plate says. "Yes, lots of seasonal cottages and camps, but never a primary residence. Where are you that you can get a Certificate of Occupancy with this structure?"
He would go for a second wall, plus chinking all joints with ChemLink caulk, which he says won't dry out and has "amazing elasticity" to move with the logs.
A slight change in plans
The conversation has ADK thinking of modifying his plans so that he'll be adding 2 inches of rigid foam to the outside walls and then tongue-and-groove log siding. He has sufficient roof overhangs to make that happen, and while he thinks he'll lose passive solar heating on the south wall, that will be offset by improved heat retention.
(In a followup email, ADK said he could do the work on the outside of his house for about $7,500.)
One interesting footnote: the problems that ADK is experiencing were never brought up by owners of log homes he visited before building, or by his local building inspector. "Nor was this an issue during multiple seminars at log home expos or with my specific log package provider," ADK says.
Our expert's opinion
Here's how GBA technical director Peter Yost sees it:
It’s really hard to believe that air leakage is not a significant part of the problem. Definitely dig up your energy audit and confirm just what air leakage numbers you got, and if that report has no infrared imaging, you might consider getting that done to convince yourself just what the problem is: air leakage, conduction, or most likely, both.
You should also check out the only log home retrofit included in the Deep Energy Retrofit Thousand Home Challenge, by Turner Building Science.
Some key points from that log home case study are:
- They took a whole-house approach because the building’s problems were NOT just about the stacked-log exterior walls.
- They focused on moisture and thermal comfort issues as they worked on energy.
- They did not want to re-skin as part of their retrofit so they used a one-part foam, PUR Black to seal gaps between logs.
Several years ago, I did some building science training for Precision Craft Log and Timber Homes so I decided to check in with them on this issue of log home energy retrofit. I spoke with Phil Mattison: “We have good systems for energy efficiency in new log and timber homes, but retrofitting without re-skinning is far from easy," he told me. "We generally recommend taking a look at Sashco sealing products and in particular their Conceal product for those who want the exterior sealant to blend in.”
The problem is probably not just the logs, but other likely thermal bypasses. For more information, see the Energy Star Thermal Bypass Checklist.
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