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Community and Q&A

Insulation and mechanical upgrades

Jeffrey S | Posted in Mechanicals on

We recently purchased a 1500 square foot colonial outside of Philadelphia. I’d like to triage energy upgrades for this house and establish a long-term, holistic plan. We’re in climate zone 4, close to the border with zone 5. It was built in 1984, has a finished basement with 1 inch eps foam on the walls a floating slab. Radon mitigation is underway and they will be sealing the slab-wall gap with polyurethane foam. 1300 square feet are split between two floors, and the remaining living space is behind the attached garage, on a slab foundation. This 200 square foot room also contains a fireplace that we plan to convert to a gas insert. Above grade walls are 2×4 with fiberglass batts, and 8 ft ceilings. House faces south, and there are 45 sqft of double pane glass on the south wall and 70 sqft on the north wall. The attic for all living space is accessible and the roof is vented soffit to ridge. It’s forced air heat with central air conditioning. The hot water heater and furnace pull combustion air from the living space and the are ~10 years old. Both run on natural gas.

I suspect a low-hanging fruit is air sealing and adding insulation to the existing R-19 attic, but I think I would feel more comfortable doing this with sealed-combustion heating and hot water. Is it worth upgrading both at the same time? I did a preliminary Manual J calculation and came up with heating load of ~25000 Btu/hr and cooling load of 13000 Btu/hr. I’m not sure I filled out the spreadsheet entirely correctly, but those numbers surprised me given the 70,000 Btu furnace that’s there. We usually keep the heat at 66F, but I used 70F in the calculations.

If the heat load is closer to 25,000 Btu/hr, would it be feasible to heat the house with the gas fireplace and a small wall furnace upstairs after air sealing and insulating the attic? I’m not crazy about the ducted system (hard to access ducts to air seal and poorly balanced), and it doesn’t work very well as air conditioning because the second floor vents are on the floor. The cooling load seems within the realm of one or two minisplits. The heating and cooling equipment will need to be replaced in the future, and I wonder if a wall furnance/mini split strategy is a better route. We will be installing engineered floors and have briefly considered radiant, but I suspect that’s even less cost effective.

What should I plan for short and long term? Is it better to just insulated and then upgrade the existing, ducted system even though duct-sealing and performance aren’t ideal?

Thanks.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jeffrey,
    If you plan to tighten up the house and add more insulation, it makes sense to rethink your heating and cooling system. If you are thinking of getting a couple of ductless minisplits for cooling, why not use them for heating, too?

    That would make more sense than trying to heat a house with a gas fireplace -- which would be a decidedly inefficient approach.

  2. Charlie Sullivan | | #2

    I agree with Martin.

    You might have a bit of a chicken and egg problem--which do you do first, air sealing and attic insulation, or heating system upgrades? If you can do both in the same year, that's ideal.

    If you are worried about CO, etc. in the house without sealed combustion and with air sealing, you can, for one thing, get a good quality CO monitor. But also, if you focus on sealing leaks at the top of the house (attic) and save the basement leaks for later, you will make the average pressure in the house higher, and reduce the potential for backdrafting. So you can do attic airsealing and insulation right away.

    Oversizing by more than a factor of 2 is not surprising. The incentives for contractors are such they have no financial incentive not to oversize, and oversizing is a lot easier than doing sizing right. And given that absurd oversizing is standard, it's has been hard to find furnaces with ratings smaller than yours--probably particularly around the time your house was built.

  3. Jeffrey S | | #3

    Martin,

    Our home has natural gas, which is considerably cheaper in our area. Electricity costs 8.6 cents/kWh, while gas costs the equivalent of 1.2 cents/kWh. While the minisplit is more energy efficient, it would still cost more to run than a standard-efficiency gas burning appliance.

    I suspect I misunderstand gas fireplaces. It appears to me that some of the gas fireplace inserts have efficiencies comparable to those of direct-vent wall heaters (~80-90%). The latter have been mentioned on this site as a means of heating well-insulated houses. While I don't think we'll be able to reach superinsulated levels, we do have a potentially small heating load. What in particular makes the gas fireplace approach inefficient?

    The ductwork is within the building envelope, though most supply vents are located near the rim joist. (I plan to insulate and seal the rim joist when replacing the floors.) Is it perhaps better to stick with the ducted system and attempt to remedy the uneven cooling and heating in another way?

    Charlie,

    Thanks for your comments and suggestions. As you mentioned, it would be ideal to time the heating and insulation upgrades as closely as possible. I'm trying to understand what would be the ideal heating/cooling solution so I can estimate the cost of doing both simultaneously.

    Another issue will be deciding on ventilation. I can use the bathroom exhaust for the short term, but would like to install a balanced system eventually.

  4. GBA Editor
    Martin Holladay | | #4

    Jeffrey,
    While there are a lot of inefficient gas fireplaces out there, some are more efficient.

    Here is a list of those with efficiencies over 75%: Gas Fireplace Models.

  5. D Dorsett | | #5

    Gas a 75-90% vs a mini-split at 350% efficiency- do the math on the real numbers.

    There is no efficiency loss to heating with an oversized hot air furnace, unless the ducts are horribly designed & leaky or routed outside the pressure boundary of the house. There isn't any advantage to buying wall furnaces or gas fireplaces to heat the place, and they don't air-condition. If you're going to air condition the place there's probably a rationale for going with heat pumps rather than modestly cheaper AC only mini-splits, at which point you'll have options as fuel prices change, or if you only needed to heat half the house some of the time.

    A 13K cooling load isn't likely to change much with insulation, some, but not much. It would take a 1.25 ton mini-split to manage the load with any margin.

    A 1.25 ton mini-split can deliver about 18,000 BTU/hr @ +15F (the approximate 99% outside design temp in Philly.), and would probably cover most of the house if it's an open floor plan.

    A 3/4 ton Fujitsu AOU-9RLS3 or 3H can deliver over half your calculated heat load, and pair of those might make more sense, depending on the floor plan.

    An -18RLFCD mini-duct cassette will deliver about 20,000 BTU/hr, but that's probably not where you're going with this. It's also somewhat less efficient than pair of Mitsubishi -FH09NA or Fujitsu -9RLS3, but allows you to duct the heat where you need/want it.

    The heat load is going to shrink a bit as you insulate and air seal. It may be worth improving the insulation on the foundation at some point too, since that's probably disproportionately high loss for what it is, but you have the numbers to compare.

  6. GBA Editor
    Martin Holladay | | #6

    Jeffrey,
    Whatever you decide to do about your heating and cooling equipment, I urge you to do a good job of improving your building envelope. Important steps usually include basement air sealing (paying attention to the rim joist area) and attic air sealing. This air sealing work is often performed with the help of a blower door.

    Once the air sealing work is complete, you can add a deep layer of cellulose insulation on top of the existing attic insulation.

  7. Jeffrey S | | #7

    D Dorsett,

    Thanks for adding some information about practical mini-split sizing to the conversation. Adjusting for efficiency, energy prices approach 1.3-1.6 cents/kWh for gas and 2.5 cents/kWh for electricity. But as you mention, a leaky, ducted system fully contained within the envelope doesn't lose efficiency. I suppose this presumes a fairly tight pressure boundary. Which ties into the next point...

    Martin,

    This is priority number one. I plan to air seal all accessible attic areas and basement rim joists, and take Charlie's suggestion of carefully monitoring CO in the basement. I hope a blower door will confirm that this is a safe situation, or dictate an upgrade.

    When replacing the upstairs flooring I'm considering removing a 1-2 ft strip of subflooring to air seal and insulate the rim joists, and then replace it. Is there a better way to accomplish this, and is it worthwhile?

  8. GBA Editor
    Martin Holladay | | #8

    Jeffrey,
    There are three ways to insulate rim joists located between the first and second floor:
    1. The way you describe -- by pulling up the subfloor above.
    2. By opening up access holes in the ceiling below.
    3. From the exterior, using cellulose.

    The last approach is described in this article: How to Install Cellulose Insulation.

    That article quotes Bill Hulstrunk, who says:

    "You climb up the ladder and remove the siding at the rim joist. You drill a 3-inch hole and look in the hole. If you are looking down a joist bay, you take a feed bag — we used to use one of the 100-pound grain bags from Blue Seal Feeds, the plastic ones, but now that most grain is sold in 50-pound bags, they’re harder to get. But we’ve found a source for the big bags — we can still get them.

    "You pull the bag over the end of your tube, so the tube is ready to fill the inside of the bag, and you insert the tube with the bag over the end into the 3-inch hole. You want to keep holding on to the bag so the bag doesn’t go flying when you start to fill it. The bag is mostly in the joist bay when you do this. You fill the bag with cellulose — you literally fill the bag.

    "Now the key to this technique is getting around the edges of the filled bag. The bag doesn’t always completely fill the joist bay that it is in. So after you have filled the bag, you pull the hose out. You stuff the end of the bag into the hole. You push it back 6 inches or a foot, and then you dense-pack the gap between the bag and the hole. You aim the hose to seal off any gaps."

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