Insulation on attic ceiling or floor?

10 years ago

I am looking to add more insulation to the attic, currently there are two options from the contractors I've talked to, some recommend air seal the attic floor and thicken the existing insulation, others recommend spray foam the attic ceiling which would take care the air seal and insulation in one shot. Now, I don't plan on adding HVAC on the attic in the future, so what are the pros and cons to ventilated and unventilated attic? Or should I just go with whichever the cost is lower.

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Comments (4)

mag77
10 years ago
Short story. I'd go with air-sealing the attic floor and more blow-in insulation. Make sure the air-sealing is done well - it's important.

Long story. Both methods work, but whether to vent or not is a topic that causes arguments, even among experts (and I'm not an expert). I go with the conventional approach because you'll get payback within your lifetime and it's straight-forward work any insulation contractor can do. That said, there doesn't seem to be any doubt unvented attics are the Rolls Royce of attic insulation, but they're expensive, difficult to very difficult to implement if the roof isn't simple (dormers, hips, valleys, etc), and require knowledgeable, conscientious installers.
Bruce in Northern Virginia
10 years ago
Some of the answer depends on where you are locatedl=.
- If you air seal and add insulation to the attic floor, and then increase the attic ventilation as much as possible (for example, continuous soffit vents and a ridge vent) it will be the most effective for a hot climate where you are trying to reduce heat gain from the sun. You are effectively creating a vented umbrella over the house and keeping most of the heat away.
- If your primary need is to keep out the cold, I'm not sure which would work better. However, good ventilation is also needed in a cold climate to keep down condensation, so I think sealing and adding attic floor insulation would also be effective.

Bruce
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Sorry, but your old-timer's logic is totally wrong. Too little or no insulation directly above the living space ceiling in your attic will result in a dramatic increase in the amount of radiant heat entering your living space through the ceiling. Your attic can store an enormous amount of heat during the day, which will slowly dissipate after the sun goes down. Until it dissipates, and without sufficient insulation above your ceiling, heat will radiate into your living space during the day and continue to do so at night until the attic completely cools. Radiant heat from the attic will dramatically increase the load on your air conditioning system. Sufficient ceiling insulation will block radiant gain through your ceiling and lower your cooling costs (it will also decrease your heat loss in colder months). Perhaps you misunderstood your old-timer. It's a good thing to have insulation above your living space ceiling, but not good to have insulation in the attic on the underside of the roof. You also want good ventilation in the attic. This allows the attic to cool quickly after the sun goes down. Putting good insulation directly above your living space ceiling allows the attic to heat up during the summer without radiating heat into your living space, and also allows the attic to remain cool during the winter so that condensation doesn't form up under the cold roof.
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energy_rater_la
10 years ago
more info please.

what climate?

what type of insulation do you currently have?

any recessed lights?

where is heating equipment & ductwork currently
located?

what is roof pitch?
simple roofline or complicated?

cheapest is not always best.
nor is most expensive always best.

adding insulation without air sealing between
attic & will lower performance of insulation.
creating as perfect of an air barrier at the
attic floor/ceiling plane is a very cost effective
upgrade. then attic temps, insulation particles,
& humidity stay in attic.

we do a lot of unvented attics here in La. because
we do stupid things like putting ductwork &
equipment in the attic. then add dozens of recessed
lights that aren't air tight, cut oversized holes in
ceiling for the can lights, supply boxes for hvac,
bath fans, stove vents etc.

foaming the roofline takes these multitude of
building failures and makes them less of an issue because
the air barrier & thermal barrier of the foam insulation
has now been moved from the attic floor to the
roofline. when this is done the holes in the
attic floor no longer suck air from attic into the house.

take time to answer some questions &
educate yourself before just adding insulation.
read some of the threads about cellulose & 'dust'
issues.

foam air seals & insulates as one install, when properly
done. other insulations slow air movement, but do not
stop it.

best of luck.
aidan_m
10 years ago
Bottom line, you don't want to pay money to heat and cool the additoinal space of the attic. Insulate the attic floor and seal it off from the house.

If you insulate the attic ceiling, even with the sealing off the outside air, the result is more volume of space within your building envelope that must be heated and cooled.

Insulation on attic ceiling or floor?

Comments (4)

mag77

Bruce in Northern Virginia

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energy_rater_la

aidan_m

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