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Is my uninsulated garage making my house hot?

RoastingInAustin | Posted in Energy Efficiency and Durability on

I have an attached uninsulated garage with its own attic, is that why it is so hot in my Austin, TX house?
My house is insulated with R19 walls, R30 attic and R13 floor, the windows are low-e vinyl. It has Hardi lap all around, and Hardi continuously vented soffits; the house faces SW and the shade ends before it reaches the house during the hottest part of the day. Right now there is some shade on the porch. The attached garage is not insulated at all; the white door and concrete driveway are in full blazing sun every afternoon. At 6:59 PM it is 82.1 degrees in my house, it is 102.3 in the garage, and the air temperature where the shade just reached the garage door is 112 down from 118.9 only 30 minutes ago. (It has gotten as high as 130+ on my porch when it is in the direct sun.)

The garage was built before the modular was assembled. For some strange reason I am able to see shingles on the attic roof when I look at it from inside the attic of the house. From inside the garage attic I see plywood for the same span. I have lost the soffitt vents for that length of the garage. Both attics have passive box vents, two over the garage and seven over the house.
The central AC and heat ducting goes under the house then up chases into the attic with ceiling registers. I have 3.5 ton for 1335 sq. ft. of living area (exterior measure, not including the covered porch). I am ready to bite the bullet and spend money to make this house comfortable. Ten years in Hades are enough.

What will get me the most bangs for my bucks? Re-roof, remove the passive vents, make sure the soffitt vents are adequate and have a ridge vent. Remove the cellulose—some leaks evident on the ceiling of the house—seal everything before I replace the insulation? Would batt insulation be better than cellulose? I am pondering rock or mineral wool to deaden sounds. What is a good re-roofing material? I currently have architectural singles as does the rest of the neighborhood, but if a metal roof will make me comfortable I will spring for it. I am also considering rock wool in the garage walls in case there is ever a fire. I want to avoid spray foam insulation based on internet searches concerning off gassing and stability over time.

Is the convective heat from the garage and the radiant heat from the garage door and driveway why this house is so hot? Do I need to have the garage attic opened up and remove that portion of its roof that is visible from the house attic to allow for more venting? (There were permits pulled and various trades’ work was inspected by the city and I hired a private inspector before I purchased BUT there was no attic access cut until right before the state inspected so I am clueless if that garage attic configuration is code. Plus the state inspector was only concerned about the installation and construction of the house) .

I am waiting for a shade cloth to arrive so I can shade the garage and the driveway, will that help?

Or is this house hot because our AC systems which are located in our houses have ducts that first run under our houses before running in a chase to the attic then across the attic floor to ceiling registers? (A giant waste of energy?) Many neighbors have ditched the AC configuration for attic installed units with no additional attic insulation, house installed with floor registers because the ducts run in the cool crawl space, or they have added window units to boost the central AC and some roast like I do. The owners of the reconfigured ACs haven’t complained about them and some of them may have insulated their garages.

I am considering a ductless mini-split because I rarely go into two bedrooms. If I have the funds I want to close off my open floor-plan kitchen to shield the heat coming from the refrigerator, the gas burners and the gas oven (which is pretty much only used in the winter because of the heat.) I will likely be purchasing a radiant stove top/convection oven soon and that may negate the need to partition off the kitchen/dining area which also face SW.

The house is on a crawl space; the garage is on a slab. The yard has a good slope so drainage is not a problem, the soil is caliche.

Also how do I find a reputable company to do the work? A big box retailer has info about their services at their stores, I paid to be on that list of service providers, but previous internet searches and queries leave people stymied about the garage attic and garage insulation and how to best address it. Plus people seem to want to sell me radiant barrier which research tells me isn’t a good deal for the cost.

While a few people wouldn’t spend money on a manufactured modular, the homes in this part of Austin have doubled in price over ten years so I would likely get my money back if I ever sold. My main concern now is my comfort level which right now is nil.

My parents 1950s house has gable and turbine vents, insulation added in the 1960s and central air and heat added in the 1970s and their house is never as hot as this. They have a ranch style with the longest wall facing west and trees that took years to grow tall enough to shade. So why is it so hot in here in my house? I appreciate any suggestions that are cost effective for the energy and cooling I desire.

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  1. GBA Editor
    Martin Holladay | | #1

    If you house had adequate insulation -- it probably doesn't -- then the temperature in your garage would be irrelevant.

    Attic venting has nothing to do with your problem.

    Your biggest problem is the fact that your ducts are located in the attic. (If I were you, I would beef up the R-30 insulation to R-49, but that is a secondary issue compared to the attic duct problem.)

    At this point, there is no cheap fix to your problem. The best solution is probably to transform you vented unconditioned attic into an unvented conditioned attic. Here is a link to an article that explains your options: Creating a Conditioned Attic.

    You probably also want to read this article: Keeping Ducts Indoors.

  2. iLikeDirt | | #2

    Your problem is caused overwhelmingly by the sun heating up your house, in a nutshell. There are not enough barriers and impediments between the sunlight and the things you don't want to get hot (yourself, the interior walls, the ceiling, the ducts, etc.). There are a bunch of ways to fix this.

    One is keep the sun off your house. That could be done with shade trees. It's never too late to plant some nice shade trees. Mesquites grow well in Texas and can provide great shade. Awnings over the windows or exterior solar shades could help too. Sun that doesn't hit your house can't heat it up. This stuff makes a big difference.

    Another way is to keep the solar heat that strikes your house from making it to the rest of the building. This is done with light-colored claddings and radiant barriers, which make a big difference in places where the heat is primarily caused by the sun. A light-colored metal roof will reject a lot of solar heat, and if the bottom is bare metal, then installing it over ridge-to-eave ventilation channels will act as a makeshift radiant barrier and the channels will carry away heat. A lot of Texan homes have this kind of roofing arrangement and it works well. Radiant barrier roof sheathing works well too, but it sounds like it's too late for that. If you have enough space to work in your attic, stapling up a radiant barrier yourself can be cheap and effective.

    Finally, you need more insulation. Once the sun is striking your house, a certain amount of heat is going to enter it, irrespective of the color of the materials or the presence of radiant barriers. To slow down this heat transfer, you need more insulation. R-30 in the attic is not enough; as Martin said, shoot for R-49 or better. If access is not a problem, you can easily do this work yourself with a rented blower and some bags of cellulose or fiberglass. Go higher if you can. This is actually really important for your attic, because insulation doesn't stop heat transfer, it just slows it down. The more insulation, the greater the time it takes for heat in the attic to enter the house. Ideally you want so much that by the time the heat is entering the house, the sun has already gone down, so the attic itself is cooling off, which will reverse the direction of heat flow. More wall insulation is important too but unlikely to be easy or cost-effective unless you're already planning to replace your siding soon.

    Finally, you need to make sure that the things you want to stay cool are protected within the envelope of all these barriers. Your attic ducts are a major violator of that right now; they're more or less exposed to the exterior, which is destroying your AC's ability to function properly. To fix this, you can bring the attic into the conditioned space like Martin mentions, or you can abandon or remove those ducts and build new ducts within the conditioned space of your house, say, above a false ceiling in a central hallway.

  3. RoastingInAustin | | #3

    Martin and Nate, thanks for the info.
    I purchased an infrared thermometer and at the hottest and sunniest part of the day my walls are hotter than my ceiling by 3 to 4 degrees and run 81+. My windows, even with 90% sunblock screens and /or a variety of window coverings are hotter than my walls by 5 or more degrees and run 86+. The temperature at my ceiling installed AC registers averages a cool 71 degrees that is not reaching the rest of my house or me.

    Additional questions: Since the AC registers are cool, could there be a benefit in installing a duct fan to push the air faster and hopefully make it fall further?

    Should the attic be sealed before additional insulation is added?

    Nate, what did you mean " bottom is bare metal" if I have a metal roof installed? Around here I see metal roofs added on top of the existing shingles, so should those be removed? Do they add an additional layer of insulation?

    I've read pros and cons of radiant barrier. as far as efficiency and payback.....working around roof trusses would add to the cost, correct?

    When I replace my HVAC would a heat pump be more efficient than whatever the standard is? I tend to run my AC 9+ months out of the year. (Still researching duct placement options).

  4. GBA Editor
    Martin Holladay | | #4

    I strongly advise you to transform your vented unconditioned attic into an unvented conditioned attic. This will bring your ducts inside of your home's conditioned space.

    It's quite possible that your duct system is undersized, disconnected, or leaky. There's no way to evaluate your duct system without a site visit. A knowledgeable HVAC contractor could inspect your duct system.

    Insulation is almost always a better investment than a radiant barrier.

  5. RoastingInAustin | | #5

    Correction, my AC is 2.5 ton.
    My attic/roof has trusses. Would it be less expensive to condition the attic by using foam on top of the exterior decking than have someone work from inside the attic? If I go the exterior route, do the gable ends need insulation inside the attic? And would a metal roof be more efficient than architectural shingles on the roof? Lastly, I should deal with the roof before considering a new HVAC system, correct? I've been researching variable speed units and may replace my current system with one of those, is there a cost benefit to a variable or dual speed over single speed systems?

  6. GBA Editor
    Martin Holladay | | #6

    K C,
    Q. "Would it be less expensive to condition the attic by using foam on top of the exterior decking than have someone work from inside the attic?"

    A. The only way to compare costs is to get some bids from contractors -- or, if you are doing the work yourself, to prepare some cost estimates. Remember, if you install rigid foam above your roof sheathing, you have to install new roofing.

    Q. "If I go the exterior route, do the gable ends need insulation inside the attic?"

    A. The gables will need to be insulated. In most cases, it is easier to insulate the gables on the interior (between the studs) than on the exterior.

    Q. "And would a metal roof be more efficient than architectural shingles on the roof?"

    A. I'm not sure what you mean by "efficient." If you plan to install an adequate thickness of insulation along the sloped plane of your roof assembly -- and by adequate, I mean enough insulation to meet minimum code requirements for roofs or ceilings -- then there will be no difference in thermal performance between a roof assembly with asphalt shingles and a roof assembly with metal roofing.

    Q. "I should deal with the roof before considering a new HVAC system, correct?"

    A. That would make sense.

    Q. "I've been researching variable speed units and may replace my current system with one of those. Is there a cost benefit to a variable or dual speed over single speed systems?"

    A. I'm not sure what you mean by "cost benefit." Variable-speed or two-stage systems may provide better comfort and in some cases may require less energy to operate than a single-speed system.

  7. slopecarver | | #7

    A variable speed unit would be of no benefit if the system already runs at 100% most of the time, I assume it cools down enough overnight to shut off momentarily.

    Pictures would help us point you in the right direction and I would be cautious of any HVAC contractor that wants to install a bigger system instead of fixing your existing ducting problem. Don't forget an average HVAC contractor installed your system incorrectly in the first place, probably under the guidance of an oblivious GC or archetect. An energy auditor may be able to help you even more and be worth their expense in the long run, especially when power companies sometimes reimburse you for energy audits.

    Don't forget to address the ducts in the crawl space too, while the crawl space isn't as hot as the attic it is probably warmer than your house and much warmer than your ducts transporting air chilled down into the 60's or lower.

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