Christian Rodriguez has taken an important first step in improving the energy efficiency and comfort of his 1880s home by arranging for an energy audit. With the results in hand, his first step was to air-seal the attic and add 20 inches of cellulose insulation.
“This made quite a difference both in comfort and heating bills,” he writes in a Q&A post at Green Building Advisor. Now comes a difficult decision: what to do next.
The audit has identified four other steps: insulate the walls of the house with dense-packed cellulose; insulated the crawl space with closed-cell spray polyurethane foam; replace four drafty windows that are original to the house; and install a high-efficiency air-source heat pump.
“Unfortunately, there is only enough [money in the] budget for one of the four options (all estimates within $200-$300 of each other),” Rodriguez says. “According to the energy assessment, our 10-year-old furnace is fairly efficient and a heat pump would strictly be to supplement and even out low winter temps. We also supplement with a wood stove.
“Does it make more sense to invest in insulation or a heat pump?”
That’s the topic for this Q&A Spotlight.
First things first
To William Heiden, the choice is simple: Start with improvements to the building envelope before spending any money on upgrades to the mechanical systems.
“I would recommend improv[ing] the envelope first before trying to fix the comfort issue through mechanicals,” he writes. “Air-sealing and insulating the attic is a great first step. Anything that you can do to lower the heat loss will help you to reduce the size of the mechanical equipment needed to heat the home in the future.”
If the windows are truly original, he adds, they will have a single pane of glass. A less expensive option than replacing them would be to make removable interior storm windows, “an easy enough DIY project.”
After that, Heiden recommends tackling the crawl space, which would have a bigger payoff than insulating the walls.
GBA senior editor Martin Holladay also recommends the same basic route: improvements to the building envelope rather than an immediate upgrade in the heating system.
“If your energy auditor listed the recommended measures in the usual order — starting with the ‘biggest bang for your buck’ measure and then listing other measures from the most cost-effective to the least cost-effective — then I would address the measures in the order they were listed,” Holladay writes.
“I would vote for insulating the above-grade walls or sealing and insulating the crawl space,” he continues. “Window replacement rarely makes sense, although it would probably be a good idea to install storm windows to protect any windows with single glazing.”
Rodriguez can always install a heat pump at a later date, he says. In the meantime, he could use his wood stove to lower his heating bills.
Go with exterior storm windows
Rodriquez has been using heat-shrink film to tighten up his windows in the winter, but a better option, says Dana Dorsett, would be weatherstrip the windows and add a low-e storm window on the outside. “A good low-e storm over a wood-sashed reasonably tight single-pane performs at about U-0.30 to U-0.33,” he writes, “as good as or better than cheap replacement windows.”
Two brands he mentions specifically are Harvey, a Massachusetts manufacturer that “makes the highest [quality] storm windows in the industry,” and Larson, whose storm windows are widely available from big-box retailers.
In any case, he adds, those old windows are worth hanging onto: “The quality of the clear grain old growth wood in 1880 vintage sashes and frames is unmatched by modern windows,” Dorsett says. “If they’re salvageable, it’s worth tightening them up. Putting the storm window on the exterior keeps them warmer and drier, and extends the life of the original window.”
Tighter house, smaller heating equipment
One reason to tighten up the house first, says Charlie Sullivan, is that the size of the heating system can then be smaller. “Investing in a better heating system first often means you paid for a bigger system than needed,” he writes.
Rodriquez, whose oil-burning furnace is a “guzzler,” has already had three heat pump contractors look over the house and provide him with estimates. Only one of them bothered to ask questions and take measurements while the other two “simply walked through the house and immediately recommended their top of the line offerings.”
Dorsett believes that Rodriguez would get much lower heating bills with an air-source heat pump. “The cost of operation of an 87% efficiency oil furnace at the recent 5-year average fuel and electricity costs is more than 50% higher than that of a pretty-good high-efficiency ducted heat pump and more than [twice] the cost of running ductless heat pumps,” Dorsett says.
Heating oil is headed lower this season, and electricity prices are on the rise, Dorsett adds, but “the fundamentals in both markets still favor heat pumps on a life-cycle operation cost. An open floor plan would allow a ductless version, which Dorsett thinks would pay for itself in reduced oil bills, in less than five years.
Try doing your own energy calculations
The solution to HVAC contractors who offer only seat-of-the-pants estimates is for Rodriguez to do his own energy calculations with the help of a website called LoadCalc, says Keith H.
“This is a very simple Manual J calculator,” he writes. “You can play with it quickly and see what effect on the whole house load some different options have. It’s just a model, but a model is better than what the window or HVAC sales guy has to say.”
The software would help Rodriguez see what window or insulation upgrades would do.
His own experience with replacement windows is that they didn’t make much of a dent in his heating bill, but they did make the house quieter and less drafty.
“These creature comforts were worth the money but I don’t believe they save much money,” Keith H said. “I put our old residence through this software and concluded that converting all the windows and two sliders would lower the load a 3/4 ton or about 16%. We didn’t see that on the bill, however.”
Our expert’s opinion
Here’s how GBA technical director Peter Yost sees it:
It could be that you have already considered moisture management in your renovation work, but I don’t see that addressed. As you work to better manage heat flow through your building enclosure, you will need to better manage moisture as well. Check out this building assessment form on GBA.
Given your current mechanical systems status (the 10-year old, working-well furnace and supplemental wood stove), I would continue to focus on your building enclosure, particularly the crawl space. You don’t say whether you would insulate the crawl space perimeter (unvented) or the first floor framing (vented), but I have found that getting a good air seal at the first floor framing, even with spray foam, is difficult. And I have also found that crawl space venting rarely if ever actually results in significant air changes in this space, so making your crawl space a basement for short people is the best approach.
I also think that low-e, airtight storms are a good investment (see Efficient Window Coverings), particularly if they are exterior and weather-protect your primary windows (see this BuildingGreen blog). Two considerations in this case, however, can be wintertime condensation on the interior of the storms and heat-trapping that damages the primary window seals.
For the former, the general rule is that to manage wintertime condensation, the most airtight plane of the dual window system must be to the interior. In my own home, the double-hung, dual-pane, sash replacements are pretty leaky (even after some seal retrofitting) and the storms are more airtight so we get quite a bit of deep-winter condensation and frost on our storms.
For the latter, our storms are high-SHGC, pyrolytic, low-e while our dual-pane primary windows are low-SHGC, sputter-coat, low-e. That combination, on the western side of our house in particular, has led to some pretty significant primary window degradation, including the insulated glass seals, as intense solar heat gets trapped in the window system. (I have to add here that while the seals look horrible, we have yet to have a seal failure, but keep reading…)
I have measured surface temperatures on the primary windows above 165 degrees F, and air temperatures between the primary and storm window above 140 degrees F. The main way we are managing this is with exterior awnings in the summer — comfort and thermal problem solved.