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Choosing what type of attic insukation

Bsw9090 | Posted in General Questions on

I have read a ton of articles on the differences for the ways to insulate the attic. 

My home has stucco exterior live in NC, with high humidity and heat. There are 4 different access holes to my attic, 2 are for the same attic, the other 2 are entirely separated, and in one part, it has a roof that runs up to a sidewall with no vent at the top. Currently there are ridge vents, although with my roof style, I question the efficiency of them, because it is beyond unbearable in the summer. I am a contractor and spend my summers in attics, but can’t stand my own. 

I am interested in encapsulating it, but is there a way to have a semi conditioned attic with insulation on the ceiling and spray foam on the underside of the roof? Roof is 1 year old, so no for re roofing. Our AC unit is in the attic, and we are on a slab, so can’t relocate to the crawlspace. Can someone help me out with this, or point me in the right direction?

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  1. walta100 | | #1

    The point of venting an attic is not to keep it cool in the summer but to allow any moisture that might get in the attic in the winter to escape so nothing rots and keep the roof about the same temp as the outside air to avoid ice dams and the leaks from them.

    The summer temperature differentials are really pretty small compared to the winter. Let’s say it might get up 120° in the attic and it is 78° = 42° in the winter it could be -10° and 68° = 78°

    The way I see it a semi conditioned attic is very risky in that the dew point of air in the attic will often be above the surface temp of the enclosure. When that happened things get wet and mold and rot could happen.

    The fully conditioned attic is a bad idea because putting the insulation at the roof line requires you to cover 40-120% more area. That larger area will always loose that % more energy if you can afford to get the same R value. Lot of people cover their attic floor with R60 of cheap fluffy insulation. When you move the insulation to the roof line the only real option is Closed Cell Spray Foam. CCSF is the most expensive way to buy an R of insulation! CCSF is the riskiest insulation you can buy! CCSF is the least green insulation you can buy! Almost no one buys code minimum R value of CCSF simply because it is just so darn costly it would neve ROI. Most people end up with R28 roof that is laughably poor. From time to time the installers get the CCSF mix wrong or apply it outside of the required temps and it fails the cure leaving a stick smelly mess.

    The only compelling reason to condition an attic is if you are going to use it as living space. If you are stuck with HVAC ducts and an equipment in your vented attic. A truly stupid idea only then is the bad idea of a conditioned attic marginally better. If you have a ton of money to spend on a conditioned attic, I think the money would be better spent relocating the HVAC equipment but neither will ROI in your lifetime.


    1. Bsw9090 | | #2

      There is no possible way to relocate the HVAC equipment out of the attic. There is concrete slab for foundation, no crawlspace. I understand what you mean as far as venting is the ideal situation. Also, as far as ice dams, honestly we get minimal snow here, maybe 2-4" on average a year.. We got 10" about 7 years ago I think, and this year weve had none so far.. if that matters?

      The roof is not an ideal roof, it has multiple hips and valleys, the lower roof had 3 different ridge lines, with 3 different valleys, that make it very difficult to have the proper attic ventilation from the venting. It also is recorded 150+F in the attic in the summer. The semi conditioned attic was just a curiosity question of mine.. the roof line also meets multiple living spaces, that have significant heating and cooling hot spots that are under insulated within the attic on the sidewalls..

      If you'd like for me to get multiple pictures, let me know what you would like to see, and I'll get the pictures as soon as possible 😀 I truly appreciate your input! I am only truly to find the solution to insane power bills from heating and cooling...

  2. freyr_design | | #3

    You could spray foam your roof deck and keep your venting without causing any issues. I imagine this would cool your attic a bit, but doesn’t give you most of the energy benefits of encapsulating your attic. But you can’t spray foam, get rid of venting, and keep ceiling insulation. You would almost be better just increasing your ventilation within the attic, possible with powered ventilation or increased ridge vent area. You could build a site build ridge vent sim to coupula to get better air flow. This doesn’t really solve your mech equip though… for that you would need to fully encapulate, which would entail either all spray foam, or flash and batt along roofline. Or exterior insulation, but you said that is no.

  3. Expert Member
    Akos | | #4

    For any attic mounted HVAC, your biggest bang for dollars and effort is duct blaster directed duct sealing. This will greatly reduce your operating cost and usually a big improvement in comfort.

    You can also look at encapsulating just the ducting and boots with spray foam, much less money than a full attic encapsulation.

    Radiant barriers tend not to do much in insulated attics but with an attic that hot it will help. You can even try say 1" metal foil faced insulation as the radiant barrier to take even more of the edge off. The rigid won't help your ceiling insulation R value but if drops the attic even 10F it will help. You still need to vent the attic space though.

    As for venting, there are many ways of venting roofs but be careful with power ventilators and over-venting. With equipment in the attic, putting the attic under negative pressure will simply pull conditioned air from the house through all that ducting and typical house ceiling air leaks. All that conditioned air leaking into the attic is great for cooling the attic but terrible for efficiency.

    Stick to the code required net free area with about 60% of it down low, 40% up high. Many ways of venting complicated roofs: ridge vents, hip vents, gable vents and box vents all work great.

    1. Bsw9090 | | #5

      Could I do something like adding 2" foil faced foam board to the bottom of the joists up to say 2ft from the ridge vents and seal the gaps with fsk or mastic and go all the way down to the bottom by the soffit vent?

      Even something like that while increasing the blown in insulation or something like this. There are severe air leaks in the home, and im working on correcting them over time.. im open to all options, just trying to make sure it's the most efficient. I have no problem with a full encapsulation either, with removing the insulation on the floor of the attic either.

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