We added about 480 square feet on to our home. The addition is added where the back patio doors were originally installed in our kitchen on the back of the house. So the addition is made up of three rooms, a dining room, small office and then a bedroom on the end with a hallway that connects them all.

We've noticed a significant temperature difference between the original part of the house and the new part. All the rooms in the addition are about 3-5 degrees colder than the rest of the house. Even the kitchen area that is closest to the entrance of the addition is significantly warmer than the addition.

So we consulted a HVAC company to help, thinking that was the issue. They upgraded us to a larger unit and reworked the ductwork to be more efficient. However, we have not improved the situation at all with the new system. The addition is still 3-5 degrees colder.

We've uncovered a new issue that we think would possibly be the reason for the temperature difference. We are in zone 4 (TN) and the builders put in rolled R30 insulation in the ceiling. However, they installed the insulation perpendicular to the joists and therefore, the insulation is sitting on top of the joists instead of being in contact with the sheetrock. There's probably about 7 inches of space between the sheet rock of the ceiling and the insulation.

So, would we be able to improve the temperature issue if we reposition the insulation to be parallel with the joists and sitting directly on the ceiling and also blowing in an additional 5 inches of insulation? The blown-in insulation will be R-19 at 5 inches.

Obviously I don't want to go through all the trouble of changing the orientation and adding additional insulation if it's not going to help. I'm looking to see if our thought process is correct and if there are any additional steps we need to look into. Thank you!

  • I am sure that the insulation has some effect. But it's more likely to be the HVAC, especially if there is no sensor for the thermostat in the addition. That being the case, you will never get the mix of flows to different areas right. Jan 21, 2023 at 5:10
  • : If you just hang a pice of insulation in the middle of the room, it will do nothing ,same applies to you 7 inch gap, allowing the clod to go under....so go ahead insulate, the more the better. –
    – Traveler
    Jan 21, 2023 at 6:04
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    I'd be interested in how the ducting was done. The furnace needs to push A LOT of air into those new rooms... is it sufficiently sized for both the existing house and the addition? Did they run new ducts all the way from the furnace or just extend duct sections that were never intended to heat that much space? Are the ducts above the insulation (if so, when the attic is cold, they are losing heat through the walls of the ducts). Maybe adding a mini-split heat pump would be a plan. More efficient than gas. Jan 21, 2023 at 8:09
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    "we added..." Who did the work? What insulation was put below the floors? In the walls? in the roof? I there vapor barrier? Where is the dew point in the wall? Our house design had 12" or 30cm of insulation and the heating needs are very low even when winter temperatures drop to -15 deg C.
    – Solar Mike
    Jan 21, 2023 at 9:51
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    Yes, fixing the insulation you already have paid for is a great way to make it better; no downside. It might not make it 100% better, but it will make it better.
    – dandavis
    Jan 21, 2023 at 10:42

3 Answers 3


It's an incorrect assumption that insulation above the joists is inherently wrong. Many commercial buildings, for example, have large gaps between the finished ceiling and the insulation envelope. In your case it was probably done that way to take the lower R-value wood out of the system. An insulation envelope amounts to the average R-value over all the area. Wall studs and ceiling joists are detrimental.

If it was done well (no open cavities at the outside of the area) it's just fine. If there are openings where the insulation ends, that's a problem that should be addressed. R-13 (for 2x4 joists) or R-19 (for 2x6 joists) fit between joists at the perimeter would do nicely.

This situation is similar to many others. It's most likely a matter of balancing airflow by adjusting output vents. I would start there. Partially close those closest to the furnace and in rooms that are warmest. Verify that new supply vents aren't closed or blocked with furniture. See if you can get things to balance.

Then, compare energy consumption from before the addition. Is the increase reasonable for periods of similar weather? You could also have someone do an infrared assessment of the attic to check for heat loss. Only then do you really know whether you have an insulation problem.

  • As much as I am here to share the knowledge I possess I am here to learn from others. I just can't seem to find any substantial bases for your beliefs. Would you please direct me to valid sources of information substantiating such because energy.gov/energysaver/insulation, owenscorning.com/en-us/insulation/pink-next-gen-fiberglas, knaufnorthamerica.com/en-us/attics, ieccode.com/2019/08/22/what-is-a-buildings-thermal-envelope, insulationinstitute.org, NAIMA, and countless others all disagree with you. Please and thank you!
    – SOHR
    Jan 22, 2023 at 6:28
  • You'll need to be more specific with your links if you expect me to look at something in particular there. I don't have beliefs. I have knowledge based on science. It's a simple fact that there's nothing magic about insulation touching drywall that makes it somehow better. Insulation is insulation no matter where it is, assuming it's installed correctly.
    – isherwood
    Jan 22, 2023 at 20:48
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    With the insulation above the joists, and assuming (1) adjacent bats are tight to each other, and (2) the air gap under the bats is fully enclosed by insulation, then above the joists is better. (Note that both of these can be assessed by visual inspection.) Why is this better? The R-value of the insulation itself is unchanged, but the system R-value is better by eliminating the thermal bridging of the joists. The air gaps, when bounded by insulation, are insulators. Obviously filling the air gap with insulation is better, but that's a different comparison. That is your scientific explanation.
    – blarg
    Jan 23, 2023 at 11:37
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    And the reason manufacturers recommend the simple bay insertion is because it has fewer ways if going wrong (installer error is still very possible), and it's the way things have been done for a long time, especially in walls, where dealing with thermal bridging is a pretty new activity.
    – blarg
    Jan 23, 2023 at 12:24
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    The point is having an air cavity that is done correctly is not an inherent problem. More insulation is also better. These are not opposing statements. More insulation has diminishing returns and may not be your primary or only issue. Nothing unreasonable here.
    – blarg
    Jan 24, 2023 at 4:20

Sorry to hear this has happened to you! I would be calling the owner of the company immediately to come and look at it and correct it! Look on your invoice to see how much attic insulation you paid for.

Maybe they thought you would never get up there and look??

Maybe they miss understood the true concept of the first layer goes in between the joists and the second layer is installed perpendicular to the first and figured they would just do the second step and it would be just fine???? Have them watch the Lowes installation video @: https://www.lowes.com/n/how-to/install-insulation

Attic insulation installation

So there are a couple of things at play here and the more you understand them the more this will all make sense.

"Newton's Law of Cooling describes how the temperature of an object changes. It states that the rate of change of its temperature depends on how much hotter it is than its surroundings." So the rate of change from the inside of your room through your ceiling and into your attic would be slower if the insulation was installed properly and therefore you ceiling was warmer.

"Heat is the flow of energy from a high temperature to a low temperature. When these temperatures balance out, heat stops flowing, then the system (or set of systems) is said to be in thermal equilibrium." https://energyeducation.ca/encyclopedia/Thermal_equilibrium

"Thermal bridging is the movement of heat across an object that is more conductive than the materials around it. The conductive material creates a path of least resistance for heat. Thermal bridging can be a major source of energy loss in homes and buildings, leading to higher utility bills." This refers to materials such as your wood 2X4's https://www.progressivefoam.com/thermal-bridging-and-how-to-stop-it/#:~:text=Thermal%20bridging%20is%20the%20movement,leading%20to%20higher%20utility%20bills.

Energystar.gov states that for Zone 4 the attic should have R38 to R60. It would have made sense for them to use 2 layers installed in a perpendicular layout! Also note that it is recommended: "Zones 3–4: Add R5 insulative wall sheathing beneath the new siding". I wonder if they did that? https://www.energystar.gov/campaign/seal_insulate/identify_problems_you_want_fix/diy_checks_inspections/insulation_r_values

My opinion: No, it was not installed properly. You are not over reacting and it should be corrected!

Fun little experiment you could do to see the difference it would make before going through the trouble of rearranging it:

  1. Get 3 thermometers and dress properly to handle insulation.

  2. Place one thermometer sitting on top of the drywall in the 7" space under the insulation as it is now.

  3. A little distance away, reposition an area of the insulation to fit properly down into the cavity (after all this is why the manufacture made it the width that it is!) and place a thermometer under it sitting on top of the drywall.

  4. A little distance away from that, reposition an area of the insulation with 2 layers, the top layer perpendicular to the lower as seen in the photo, and once again place a thermometer under this section in the same manner.

  5. Wait a few hours and check the thermometers to see if there is a difference in the temperatures.

In theory: The highest temperature wins the race! The area with the highest temperature is causing the least amount of heat transfer out of your living space. Note: If you do see a difference you have your proof and encouragement to change it but if you don't see a difference, just know, there could be variables at play causing that and it still should be corrected.

If you do this experiment please post the results here. I would love to know if you found a difference!

And you should ask the builder what all was done to properly insulate your space.

  • In addition to Newton's Law (conduction/radiation), in HVAC convection (air movement carrying heat) is often a more important practical issue. Much depends on the air flow above the OP's ceiling, which we haven't been told or shown much about.
    – Armand
    Jan 21, 2023 at 7:00
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    It's an incorrect knee-jerk assumption that insulation above the joists is inherently wrong. It was done that way to take the lower R-value wood out of the system. If it was done well (no open cavities at the outside of the area) it's just fine.
    – isherwood
    Jan 21, 2023 at 18:17
  • Insulation above the joists is an excellent installation practice! As long as it is installed perpendicular and over the first layer located in the cavity.
    – SOHR
    Jan 22, 2023 at 6:08

Sounds to me like they messed up your insulation. I would definitely call the company that did it and ask them to come put it in correctly.

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