Spray foam insulation contractors and homeowners alike have debated this questions over many years and since spray foam first gained popularity in late the 2000’s, we’ve had many case studies to learn from. What is the best practice for homeowners when they are considering spray foaming their attic? In a hot dry climate like Phoenix, spray foam is worth the extra cost and is the ideal insulation method because all of our ductwork and HVAC systems are inside the attic space. Spray foam contractors in Arizona have differed though in their reasoning over if the existing insulation should be removed or kept along the attic floor.
The case for leaving the loose fill insulation along the attic floor as is the attic floor insulation will keep doing its job and slow the heat coming into the home. If all the insulation on the attic floor is gone and the attic will still get warm, isn’t it a good idea to leave the insulation up in the attic floor to stop that heat from coming inside the house? The insulation is still good right??
laying devil’s advocate, the case for removing the insulation on the attic floor (aka extraction) is that leaving the insulation on the attic floor will work against the spray foam insulation on the roof deck. It will trap the heat up in the attic and the attic will get even hotter than it would without insulation on the attic floor. With zero insulation on the attic floor the temperatures start to equalize more with the temperatures inside the house. This keeps the attic cooler and with the spray foam along the roof slope.
What’s the true answer then, when you get spray foam in your attic, should you remove the insulation on the attic floor or leave it in? The best answer is to remove the insulation. That is, to get the most benefit in comfort, cost savings and reduction in heat gain you need to extract the insulation out from the attic floor. OK but why? Let’s go through an exercise of how heat flows based on the Second Law of Thermodynamics. The Second Law says that heat flows from high potential to a lower potential or from hot to cold. This means that heat is not flowing from inside your house up into the attic in the summer time. Yes, warm air rises, but if the net temperature is greater in the attic than in the house, heat will flow from the attic into your home. So when hot outside air starts its penetration into your home from the roof, it hits your great spray foam insulation barrier and slows way down. The heat still gets into the attic space mind you but it’s greatly reduced by 20% on average.
So let’s say instead of a 140 degree attic in the middle of a Phoenix summer without spray foam, with spray foam you get a 112 degree attic. That’s still hot but we aren’t done yet. If you remove the insulation on the attic floor, then the temperature equalizes between the inside and the attic space. At first, yes the air conditioner will have to overcome a larger amount of heat gain with no insulation. Once that initial heat load is taken care of, you’re free and clear so to speak... the hard part is behind you. Now if you keep the temperature inside at 78 degrees, your attic space will also get cooled to 82 and mix with the 112 degree air to come to an average of 93 degrees (average of 112 and 82 degrees).
If you leave the insulation on the attic floor, the heat will not easily transfer from the inside of the house to the attic space because the insulation on the attic floor is doing its job and slowing the heat transfer from the attic to the house. Now your attic is at 112 degrees after it gets through the spray foam layer, but it has nowhere to go… it gets trapped in the attic space and heat starts to build up in the attic. From our Second Law, the heat will go from hot to colder areas. It goes from the outside to the inside of the attic, and then it will go from the attic into the house. There will be a greater delta T or temperature difference driving that heat transfer into the house.
If you really want to get into the formula driving heat transfer of our homes (I love this formula because it’s what all our load calculations are based from) we can apply the Second Law to find the heat gain from U x A x delta T = heat flow.
U = U-factor or heat transfer across a surface such as a window or from the attic into the house. U is the inverse of the R-value. You may be familiar with R-values when we talk about attic insulation, the higher the R-value, the more it resists heat transfer. An attic with 12” of cellulose insulation has an R-value of R-38, an attic with only 6” of cellulose has an R-value of R-19. In the case of our example, our U-values will vary depending on if we leave the insulation on the attic floor or remove it.
A = area, in the case of finding our heat gain from the attic into the home, our area will be the area of the attic space or in a single story home, it will equal the sq ft of the house. If you want to find the heat gain from a wall, your area will change to that wall area as will your other data points like temperature differences and U values.
Delta T = temperature difference between the attic and the house. In our example case we will say our inside the house temperature is 78 degrees. What will vary is the attic temperature depending on if we leave the insulation on the attic floor or remove it.
Heat flow = how much heat is coming into the home in one hour (BTU/hr). This is how much heat the air conditioner will have to remove and how we size air conditioner systems, based on how much heat gets into the house. The larger the number, the larger the AC system will be.
Now let’s play with the formula with our two scenarios of removing the existing insulation on the attic floor verses leaving it in the attic. Let’s say we have a single story home with 2000 sq ft of roof decking, 6” of existing cellulose insulation on the attic floor (R-19) and 6” of open cell spray foam along the roof deck (R-22).
Option 1: remove the insulation on the attic floor with spray foam on the roof deck
U x A x delta T = heat gain
U = 1/R = 1/22 = 0.045. Our R-value is 22 with the open cell spray foam on the roof deck and our attic is defined as conditioned space when we have spray foam on the roof deck (as long as it’s done right).
A = area of roof decking or 2600 sq ft.
Delta T = 93 degrees – 78 degrees = 15 degrees
So plugging them into our formula we get: 0.045 x 2600 x 15 = 1,755 BTU/hr
1,755 is our number to compare to Option 2. If we get a higher number, it is better to remove the insulation on the attic floor because the air conditioner will have to work more to offset the additional heat gain in the house. If we get a smaller number, it is better to leave the insulation on the attic floor. Let’s see what we get.
Option 2: leave the insulation on the attic floor with spray foam on the roof deck
U x A x delta T = heat gain
U = 1/R = 1/22 = 0.045. Same as Option 1.
A = area of the house or 2000 sq ft.
Delta T = 112 degrees – 78 degrees = 34 degrees. The temperature is hotter inside the attic because we have 6” of existing insulation on the attic floor that’s trapping the heat.
Plugging them into our formula we get: 0.045 x 2000 x 34 = 3,063 BTU/hr
The results mean that our there will be more heat to remove in the summer and our A/C's will work almost twice as hard if the insulation is left on the attic floor. So from a cost savings view, it is better to remove the attic floor insulation when installing spray foam on the attic slope.
Now are there other reasons to extract the existing insulation from the attic floor? Yes, in fact there are health benefits from removing that old, dusty insulation you won’t be breathing it in your home and the air quality improves. Although, hot dry climates like Phoenix don’t get a lot of moisture, removing the insulation from the attic floor reduces moisture build up in the attic as well.
11/27/2018 05:41:40 pm
If the insulation you remove is fiberglass you might want to dispose of it properly. It might be easiest to use a removal company to remain safe. There could be asbestos if it is an older home.
I like it when you said that attic floor insulation is vital as it slows down the heat coming into the home and protects the equipment stored in your attic or even inside your house. My father has been planning to use spray foam insulation into our attic floor where the HVAC equipment is stored. I think it's a great idea given the benefits you mentioned in your article. I will help him find a high-quality spray foam equipment soon.
7/16/2019 05:31:55 pm
My sister wants to make sure that her apartment is properly insulated in order to save on electricity bills. It was mentioned here that insulating the attic and floors will help slow down the heat from coming into the house. Moreover, it's recommended to hire professional spray foam insulation contractors for quality services.
7/31/2019 11:20:31 am
It's interesting that even though hot air rises, heat will flood from the attic into your house. My attic is very very hot in the summer and now that you mention it, I notice that the rooms next to it are definitely warmer. I'll have to think about getting better insulation.
8/6/2019 09:24:04 am
It's interesting to know that leaving the insulation on the attic floor will trap the heat in there and make it hotter. My husband got a recommendation about installing spray foam in our roof, and we are looking for advice. I will call a professional to help us determine if we need spray foam roofing.
8/13/2019 10:59:05 am
I appreciate you helping me learn more about insulating the attic. I didn't know that if you leave the floor insulation down when attic ceiling is sprayed that it can, in turn, make the attic hotter. In general, I think most of us want our attics to be well insulated so it helps the rest of the house stay at the temperature it should.
11/27/2019 10:27:27 am
I'm glad you explained that the best answer is to remove the insulation from your attic before you replace it. My house isn't staying heated very well, so I've decided to fix the insulation in my attic. It'll be a lot warmer once I have it replaced.
5/28/2020 02:29:02 pm
It's great to know that spray foam is worth the extra cost in hotter climates. I would love to do this to my attic. I'll look into finding someone to help with this.
11/19/2020 07:54:03 am
Is there a way to do the work without removing the entire attic? It would definitely cost a lot by manpower itself. Please let me know if there any information about this.
1/16/2021 10:38:22 am
What if we replace the blown in on the attic floor with foam? Won't this let the attic "breathe" to remove heat.... and at the same time seal the lower floor, stop airflow between it and your attic, and insulate it from the high attic temps? Seems the transfer of heat is the same from the roof(with foam) and the floor of the attic, but you will have to "cool" less cubic space if the attic isn't part of the cooled space. Also allows the duct work to be insulated so your cold air will arrive at the vent cooler than without the blown in around it?
1/19/2021 01:59:28 pm
Thanks for the advice to remove the insulation on your attic floor when you go to install spray foam. I don't want my home to overheat in the summer. The heat is already unbearable that time of year so I need good insulation.
Thanks for helping me understand that getting spray foam insulation will make the temperature drop from 140 degrees to 112 degrees as an example. I guess I would be looking for professionals that can install those in our home. It would be perfect for my house since I am located in an area where it gets the hottest during summer.
8/5/2021 01:12:36 pm
Yes, the attic floor insulation should be removed when doing spray foam to prevent moisture problems and to get a better air seal.
8/12/2021 08:52:19 am
Typically we recommend removing that old insulation but you can still get results without it
10/7/2021 08:27:50 am
I want to make sure that my attic is properly insulated. It makes sense that I would want to get a spray foam specialist to help me out with this! They would know what to remove in order to have the spray installed correctly.
11/11/2021 03:11:45 pm
It's good to know that spray foam is worth it in hot dry climates. I live in a dry part of the country. So I'll have some spray foam installed.
11/30/2021 03:00:13 pm
I like how you mentioned that spray foam insulation is an ideal way because all of our HVAC systems are inside the attic area. My uncle mentioned to me last night that he is hoping to find a reliable service for his home to protect from mold and mildew growth and asked me if I have any idea what is the best option to do. Thanks to this informative article and I'll be sure to tell him that he can consult a residential spray foam insulation service as they can answer all his inquiries.
1/6/2022 05:48:46 am
I want to make sure that my roof is fully insulated. It makes sense that I would want to consider looking into spray foam insulation! I'll work with a professional to ensure that it gets installed correctly.
2/28/2022 07:44:43 pm
My favorite part of this article is spray foam insulation contractors and homeowners have discussed this subject for years, and we've had a lot of case studies to learn from since spray foam first became popular in the late 2000s. Someone recently told me about this and it is different from what I understand. Thanks for helping me understand spray foam roofing.
7/1/2022 07:23:23 am
I want to make sure that my home is properly insulated, but I'm not sure how to go about it. It makes sense that I would want to get spray foam insulation put in! I'll be sure to work with a professional to help out with this.
7/7/2022 01:08:17 pm
Thank you for sharing this information.
11/1/2022 06:43:23 am
I'm planning on getting my attic reinsulated. It's good to know that I shouldn't use the spray foam on the attic floor. I can see how that could end up trapping heat in the attic.
12/1/2022 02:14:38 pm
Thanks so much for talking about why using spray foam insulation rather than loose floor insulation is better for the home. My parents have been doing a lot of renovation on their home this year and they want to try to get into the attic easier. To help with that, we've been looking into getting a contractor down to help install new insulation to make sure they're house stays the temp they want it.
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