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??
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 2000 sq ft.
Delta T = 93 degrees – 78 degrees = 15 degrees
So plugging them into our formula we get: 0.045 x 2000 x 37 = 1,351 BTU/hr
1,351 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
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.