Emerson W has acquired his first free-standing home, a Cape built in Maryland in 1952, and in no time he’s compiled a long list of upgrades the house will need — everything from a new heating system to dealing with vented, unconditioned crawl spaces.
In a lengthy post in the Q&A Forum, Emerson lays out the details of current conditions and a tentative plan for making the house more energy-efficient and more comfortable. His post covers a half-dozen major areas of concern. That’s too many for a single Spotlight, so we’ll look at just a couple — how to approach insulation for a a slate roof, and what to do about a 24-year-old oil-burning furnace.
First, the roof. Emerson has apparently spoken with a roofing contractor who believes the slate roof at the front of the house might need replacement in eight years. At that time, Emerson could add rigid foam insulation on top of the existing roof deck, but replacement isn’t in the cards now. On the rear of the house, also roofed in slate, external foam doesn’t look like an option. For those reasons, Emerson thinks an insulate-from-within strategy might be best.
Then there’s the HVAC system. The 4,400-square-foot house is heated with a 24-year-old oil burner with a two-zone central air conditioning system.
“I was leaning toward geothermal,” Emerson writes, “contingent on the 30% tax credit returning, but having read GBA on this topic, I am now thoroughly confused. The house layout would be great for minisplits — it looks like a rancher with a partial second floor. Question: What is current thinking on geothermal vs. ductless minisplits?”
Despite the lack of upgrades, the Climate Zone 4 house sits on a “great piece of property” and seems to have loads of potential. “Other than these issues,” Emerson says, “the house is a model of green efficiency.”
Don’t be hasty with replacing slate
Slate roofs are common in some parts of the Northeast where there are, or were, plenty of local quarries. In areas of western Vermont and eastern New York state, for example, even chicken coops and other outbuildings often had slate roofs. When an old barn fell down, the slates could be removed, reused on another building, and still last another couple of decades. These days, slate is an extremely expensive roofing option.
“My first reaction, as a former roofer, is that you must have been given bad advice about the slate roof,” says GBA editor Martin Holladay. “A slate roof is a rare and wonderful asset. Unless the original roofer made major blunders, there is no way that slate installed in 1952 ‘has 5-10 years of life left.’ What does that even mean? A slate roof can last 150 or 200 years.”
Holladay acknowledges that might be painting too rosy a picture, but he adds that he’s seen perfectly good slate roofs torn off by “ignorant roofers (or knowledgeable roofers who are greedy and want the slate).”
“Some roofers advise replacing a slate roof simply because they are inexperienced at flashing replacement,” he adds. “It’s true that some less expensive types of slate can eventually deteriorate due to freeze/thaw damage — although Maryland isn’t a particularly harsh climate — and it’s possible that your slates are deteriorating. I would start by asking your roofer: Exactly why are you recommending that I remove the existing slate?”
Emerson says he has received “conflicting guidance” on the slate roof. “Our inspector said 20+ years left (with a 75-year life expectancy),” he says. “A slate roofer (who sell roofs) said 5-8 years on the front, and some spots on the back need major repair or possible replacement sooner.”
Insulating a slate roof
The dilemma is not only when to replace the slate, but how to insulate the roof. At the moment, the attic has a scant 2 inches of blown-in fiberglass on the floor. Emerson is aware of the potential problems of spraying closed-cell foam on the underside of the roof.
A slate roof is vapor-permeable, Holladay says, and dries readily to the exterior. Slate is often applied over skip sheathing — that is, strips of sheathing applied so there’s a gap between the boards.
“If you want to insulate the rafter bays under a slate roof, here are the rules,” Holladay says:
“1. Never install spray foam insulation to the underside of the skip sheathing. You don’t want to glue the slates to the roof assembly.
“2. The best job will include a ventilation space (at least one inch deep) below the skip sheathing, created with site-built ventilation baffles made of plywood or rigid foam.
“3. As with any type of retrofit attempt to insulate rafter bays, the basic problem is that it’s hard to get enough R-value when you are restricted to the depth of the existing rafters.”
In Emerson’s case, the slate appears to have been installed over asphalt felt and solid plank sheathing, not skip sheathing. Does that change the insulation strategy?
Yes, Holladay replies.
“That means (in theory) that you could have closed-cell spray foam installed on the underside of the sheathing if you wanted,” he says. “In practice, however, I would want to verify that the slate is installed on skip sheathing above the asphalt felt. If that’s the way the slate is installed, it’s an excellent installation.
“However, if the slate is nailed directly to the solid board sheathing (through the asphalt felt), there isn’t any air space under the slate, and the spray foam installation would be unwise. All of this advice assumes that you care about the slate. In my mind, you should. Slate roofs are a valuable element that add to the historic character of our neighborhoods. Every time a slate roof is lost, the entire neighborhood loses something valuable.”
Delay a decision on HVAC
Although a 24-year-old furnace may be getting a little long in the tooth, Dana Dorsett suggests that Emerson delay the decision on his HVAC system for now and concentrate on air-sealing and otherwise weatherizing the house. How much fuel he uses over the winter becomes a “firm upper-bound” on sizing a replacement system.
When the time comes for a replacement system, Dorsett says, Emerson may find that ductless minisplits sized correctly for cooling will also be able to handle the heating load. “But,” he says, “we don’t really know that yet.”
“A fuel-use heat load calculation wouldn’t tell you the room loads, but it would establish the upper limit of the whole-house load,” Dorsett explains. “It’s possible that the heating ducts could be used for cooling, using a right-sized heat pump, at a lower operating cost than heating with oil, at a much lower installed cost than a full-on ductless solution. Right now it’s all an open question.”
What to do about ducts in the attic
Emerson is leaning away from turning the attic into a conditioned space, so that means dealing with heating and cooling ducts in a way that doesn’t court disaster. After reading an article by Building Science Corporation’s Joe Lstiburek, Emerson sees three options:
(1) Leave the leaky and partially insulated ducts alone, and blow R-49 worth of insulation on top of them. When the HVAC system is updated in the future, the air handler could be moved into conditioned space and new ducts installed under the second floor — inside the thermal envelope.
(2) Air seal and insulate the existing ducts, then bury them in R-49 of insulation.
(3) Use the existing ductwork with a new HVAC system.
“Hard-piped tight and insulated new ducts in the attic will usually be safe to bury in the attic insulation; leaky ducts, not so much,” Dorsett says. “If you’re going to use the attic ducts even for one season it’s worth making the effort to seal them. Whether the money for new, improved ducts would be better applied to a ductless solution requires more analysis. To be sure, air sealing the attic floor is going to be a lot easier and more reliable over the long term if you can get rid of the dozen or more duct penetrations.”
Ductless vs. ground-source heat pump
Options for new heating and cooling equipment include ductless minisplits and a ground-source heat pump, and a major factor to Emerson is the 30% income tax credit available with the ground-source system as well as low operating costs.
“Don’t count the geo-tax-break chickens before they hatch,” Dorsett suggests, “but also vet the geothermal contractors extremely thoroughly.”
Ground-source heat pumps are efficient, with relatively high Coefficients of Performance, but Dorsett points out that the stated COP doesn’t include the energy it takes to pump the heat exchange fluid through the earth loop, and that can be significant.
“That additional pumping power adds up, and is in the control of the system designer, some of whom are competent; others, well… ” he says. “Typical solutions will deliver an all-in COP of about 3.5 (=HSPF 12-ish) , even though best-case geothermal solutions can be in the ~4 (HSPF 13.5) range, but hitting the 5s requires hiring the Tooth Fairy and his trusty unicorn for the job (though it can be done with very carefully designed ultra-low temperature radiant floors, etc.). “
In Climate Zone 4, he adds, a correctly sized ductless system will probably top the performance of a ground-source heat pump on the basis of annual power consumption, although a ground-source heat pump probably would do better than a ducted air source heat pump.
“In my area (U.S. climate zone 5A, southern New England, on the edge of Zones 4 and 5 for heating hours) the substantial cost difference is often large enough to pay for enough rooftop PV to cover the full heating energy use of heating with ductless (not just the energy use difference!)” Dorsett says. “But geothermal is also more expensive here than in many areas; thus your mileage may vary.”
One other advantage may be cost: “In general, I’m a fan of ductless minisplits,” Holladay writes. “Abandoning your attic ductwork and installing minisplits may be less expensive that creating a conditioned attic or moving your ductwork to a new location inside your home’s thermal envelope.
“That said, some people don’t like the look of minisplits, and some minisplit systems introduce the problem of room-to-room temperature variations,” He adds. “Both of these problems can be addressed by using ducted minisplits instead of ductless minisplits.”
Our expert’s opinion
Here’s what GBA technical director Peter Yost thinks:
Since Emerson has spent a lot of time on GBA and Building Science Corporation’s websites, I probably don’t need to say this, but I will anyway because it’s so important: Always prioritize according to the list below.
- Bulk water management
- Drying potential (vapor profiles)
- Thermal continuity
In other words, eliminate leaks or wicking, then air seal, make sure building assemblies can dry if they get wet, and insulate well. And for #3, that includes making sure that you have household sources of moisture under control (for more on that, read “Moisture Sources, Relative Humidity, and Mold.”)
Emerson’s slate roof: Our home here in Vermont is over 100 years old and is roofed with slate. Our local roofing expert tells me two things. One, there is a broad range of slate and that can mean the difference between 100 and 200 years of service life. Two, the slates are only as good as the fasteners so assessing a slate roof means learning how the fasteners are holding up. I have never met a slate roof expert who can tell me if the slates have eight or 20 years left. I would talk to at least three references for your roofer in order feel a bit more certain of his (or her) slate “quals.” Or, provide GBA blog-lifers with detailed photos of your slates; maybe seeing your slates up close and personal will help us out on this issue.
I also checked in with our local leading roofing company, , on this topic. To start, I asked Brian Knowles about this InspectAPedia web resource, .
Brian said, “I think the information in the article says it all about evaluating slate condition. There is a lot to know and most of it revolves around where the slate comes from.
“For a homeowner, the best way to determine the condition of a slate roof is to look at the slates that have fallen off. Are they mushy, soft, brittle, flakey? Have they broken and fallen off, or are the fasteners failing? If you are getting whole (unbroken) tiles then it is likely there’s a problem with the fasteners. A professional roofer can remove a few tiles and examine the fasteners to see if they are corroding. If the slates are coming off in pieces then you have to decide whether it is a symptom of damage from ice and snow, or whether the slates are just no longer stable.
“Vermont slates are usually very long-lasting; most of our slate roofs in Brattleboro usually go bad because of the old steel fasteners, not because the slate is bad. That said, I have a client who has a 14-year old slate roof that was quarried in Vermont from a bad vein with lots of pyrite inclusions. This is the mineral substance that the reference article talks about.
“This makes his slates very brittle and flaky, thus requiring extensive repairs every spring. I educated him on this by comparing a high-quality north county black slate tile from New England Slate Company with the stuff from his roof. The difference is obvious. You would have a tough time breaking the good tile over your knee, and it would likely leave a bruise. The bad stuff can be broken in your hand with little to no effort.
“The ‘touch and feel’ evaluation is best but is not always easy to perform for the entire roof. You can tell immediately with little to no training weather the slate is good. If it’s bad, it feels bad: it’s brittle, flaky, mushy, or soft. Good slate is what it is — rock. It’s solid and sound; it doesn’t break or crush easily.
“It is also important to look at all of the slate on the roof to decide whether it is all the same material. Repairs over time usually get made with material that is different than the original slate used. You don’t want to cast judgment on an entire roof based on the condition of one or two bad slates that were installed by a repair crew who had a bad batch of repair tiles.”
Emerson’s HVAC: Definitely get your enclosure where it needs to be (see priority list above) and then right-size whatever system you choose. My personal opinion on HVAC options:
- Minisplits: the best duct is no duct in my book. Minisplits have a proven track record and they are both simple and quiet compared to just about any ducted system.
- Ducts and equipment in conditioned space: I know it is possible to seal and bury ducts, but it’s just hard to do well, and then a mess if you have a problem down the road.
- Air-source vs ground-source: Ground source can be a good fit for larger buildings but it’s just not a good fit for small, single-family detached homes in my book. And it is not easy to find expert installers — ones who understand soils, geology, and HVAC.