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Air sealing, vented vs. non-vented attic, and a new HVAC system possibly in the attic

freddie03 | Posted in Energy Efficiency and Durability on

Hi All,

My husband and I were hoping to get some unbiased information related to our first major home renovation project.

Our Goals Are: Air sealing, updating our attic insulation, and installing a new 2-Zone HVAC system plus ductwork in a complementary and cost effective way.

Our Home: colonial style house built in 1980, 2 floors plus basement, 1800sq ft + finished basement, vented attic with attic fan, no ridge vent, soffits on front of house only, existing insulation is on attic floor that is the same age as house – rolled batt fiberglass, 4/12 Roof Pitch, no existing duct work.

Located in Maryland at the very top of zone 4A, bottom of zone 5A.

We had a blower door test done and found the house is very leaky, mostly from the attic into our 2nd floor. The contractor recommended that we air seal and add 15″ of blown in fiberglass insulation to the attic floor before doing central AC. The HVAC contractors recommended putting the air handler and duct work into the attic. To us it seems doing the insulation first will make the HVAC contractors work much harder.

We are also concerned about putting the air handler and ducting into an unconditioned space. We read about the loss in efficiency, sweating, and higher energy costs and started to research a vented vs. non-vented attic space. Doing a non-vented attic seemed like a good idea at first but then we became concerned with the health of our roof (ice dams, moisture issues) and the cost of the project.

We have discussed putting the air handler into a closet on the 2nd floor but there doesn’t seem to be a way of retrofitting the duct work without using the attic space.

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  1. GBA Editor
    Martin Holladay | | #1

    You don't want to put a new air handler and ductwork in a vented unconditioned attic. Here is a link to an article that explains why:
    Keeping Ducts Indoors

    The usual advice to someone like you would be to install the new insulation along the sloping roofline, thereby transforming your vented unconditioned attic into an unvented conditioned attic. If you want to do that, here is a link to an article that explains the necessary work:
    Creating a Conditioned Attic

    These days, however, building science experts are beginning to realize that a less expensive solution is to install one or more ductless minisplit units on your second floor to handle your air conditioning needs. If you do that, your attic can remain a vented unconditioned attic. (If you do this, you should perform air sealing work in your attic and add more insulation to the attic floor.)

    And no matter how you solve your other problems, make sure that you unplug your powered attic ventilator. Here is a link to an article that explains why you don't want a powered attic ventilator:
    Fans in the Attic: Do They Help or Do They Hurt?

  2. freddie03 | | #2

    Thank you.

    I looked at the mini splits as well for the upstairs. We have 4 bedrooms and 2 bathrooms. 2 of the bedrooms get beat on by the sun all morning and tend to be fairly warm. From the blog posting below I would think we would need 3 heads upstairs, one in each of the hot bed rooms, and one for the hallway. I will see if there is a HVAC contractor who can come over and talk to us more about it.

    Does anyone know how the minisplits impact resale-ability? Most people I know seem to have the impression that they are "junk"

    If we remove the powered attic ventilator (attic fan) do we risk damage to our roof?

  3. Expert Member
    Dana Dorsett | | #3

    HVAC contractors always want to put the ducts in the attic, since it's easy to install, but in your location that is likely to add at LEAST a ton of cooling load to the ducts, and increase the heating load as well.

    HVAC contractors are also notoriously lousy at getting the sizing right. Before specifying the equipment, get a qualified third party such as a certified professional engineer or RESNET rater do perform room-by-room Manual-J load calculation using the most AGGRESSIVE assumptions possible, taking advantage of every realistic factor that would reduce the load numbers. Only then is it possible to design/specify the right-sized system (including ducts, if ducted) that will achieve the highest comfort & efficiency. It's downright scary how often HVAC contractors install equipment 3x or more oversized for the load, resulting in a more expensive (both up front, and annual operating cost), noiser, less comfortable system!

    Modulating mini-ducted mini-split cassettes serving heat & cooling to 3-4 adjacent rooms can often be tucked into the tops or backs/sides of closets with very little impact on storage space. Even if it takes more than one mini-split to serve a bunch of doored-off rooms it's usually a better solution than an oversized air handler serving the entire floor as one zone.

    What is the existing heating system? Do you have heating energy use data over a full winter for this house? If yes, it's useful to do a fuel-use based heat load calculation on the "before improvements" picture as a sanity check on the "after improvements" Manual-J. (see: ) Sloppy calculations even from well intentioned people can sometimes come up with post-improvement load numbers higher than the pre-improvement reality, as measured by the energy use. A fuel use load calculation doesn't tell you the room-by-room loads (and they're not exactly proportional to the room size), but puts a firm upper bound on what the load can be, establishing appropriate size limits.

    Sure, removing the fan with a crowbar would risk damaging the roof, but it's possible to do it correctly. Removing all of the power wiring to it and leaving it there until the roof needs replacement is always an option.

  4. freddie03 | | #4

    Wow, that is a lot of math. It is going to take me a while to get through that last link :) Thank you for the information.

    We are not looking to replace our heating system at this time. We run a fuel oil powered furnace with hot water base boards.

    When I asked about roof damage I meant from the heat build up in the attic, not the actual removal of the vent. From the limited reading there seem to be two different schools of thought about shingle life/warranty and attic venting. I really don't want to buy a new roof any time soon.

  5. GBA Editor
    Martin Holladay | | #5

    Warm air in your attic will not significantly affect shingle life.

  6. Expert Member
    Dana Dorsett | | #6

    In my area (MA,with some of the highest electric rates in the lower 48- about 20 cents/kwh to MD's 14-15 cents) it's cheaper to heat with better-class mini-splits than with an oil boiler & baseboards. In MD it will be cheaper still, both due to the lower electric rates and warmer wintertime outdoor temperatures. (At higher outdoor temps heat pumps operate at higher efficiency, and modulating heat pumps such as mini-splits can magnify this effect if sized properly.)

    Run the heat load math on the oil-burner and you'll probably find that it is more than 2x oversized for your 99% outside design temperature, probably even 2x oversized for the absolute lowest temperature ever recorded in your location. It's also likely to be oversized for the amount of baseboard in your house (and certainly oversized for any single heating zone.) Being oversized by even 2x means it's as-used AFUE is going to be below it's tested efficiency, and at 3x oversizing it's WELL below it's rated efficiency.

    Spending $8-10K on a dumb oversized ducted split AC system, spending that or even a bit more on a modulating ductless (or mini-ducted) heat pump solution would be a better deal. Assuming you have about 3000' of above-grade house the odds are that an aggressively calculated cooling load will come in at about 2-2.5 tons- MAX! Two tons of ductless heat pump can deliver about 25-30,000 BTU/hr of heating capacity @ +17F (the 99% outside design temperature in Baltimore.) Even if a ductless heat pump sized for the cooling load doesn't quite cover the Polar Vortex low temperature heating loads, it should more than cover the average wintertime load, at an operaing cost well under that of the oil boiler.

    For rough budgeting purposes, in my neighborhood pretty-good name brand 3 zone 2-ton multi-split (wall coils, not mini-ducted) runs about $7-8K in competitive bidding. Add about $1000 per zone for zones with mini-ducts to cover the higher cost of a mini-duct cassette vs. wall coil, plus the duct costs.

    Going with a 1 or 2 stage split AC system with ducts in the attic they'd probably install 3-4 tons of compressor (sometimes more) to cover the air-handler driven infiltration and the duct gains in the attic, providing ZERO heating capacity, and oversized for the average cooling load. It's sad but true that many HVAC installers are still using a ridiculous "ton per 500' of conditioned space" or similar type of rule, reliably oversizing by more than 2x.

  7. Expert Member
    Dana Dorsett | | #7

    To get a sense of the comparative heating costs, assuming your oil boiler is averaging ~83% AFUE (not likely, if 2x oversized) you're getting about 115,000 BTU/gallon out of the oil. Normalizing to gallons per million BTU (MMBTU) that's 1,000,000/115,000 = 8.7 gallons/MMBTU. Assuming a cost of $2.50/gallon (it was a bit higher than that this year) that's 8.7 x $2.50 = $21.75/MMBTU.

    If the oil burner is 3x oversized for the 99% heat load, it's as-used AFUE is really going to be more like 75%, even if it has a steady state combustion efficiency or labeled AFUE of 85%. See Table 3, system #1 on page 9 (p14 in PDF pagination) of this document:

    That would make heating with $2.50/gallon oil more like $24/MMBTU.

    That doesn't include pumping power or electricity used for the burner & controls, just the cost of the oil.

    A right-sized 3-4 zone multi-split will have an HSPF of ~10-11, maybe only HSPF 9 if it's all done with mini-duct cassettes. An HSPF of 9 means it delivers 9000 BTU/kwh, so normalizing to MMBTU that's 1,000,000 / 9000= 111 kwh/MMBTU...

    ...which at 15 cents/kwh costs $16.65/MMBTU.

    So even with an all mini-ducted solution it's at least a 25% reduction in heating costs compared to the fossil-burner, even assuming oil pricing modestly lower than this past winter's pricing, and modestly higher electricity prices than the current state average.

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