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Old 01-31-2019, 09:09 PM   #1
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Insulating Attic: Floor or Ceiling?


We bought our first home back in October and have come to realize there is hardly any insulation in the attic. Its a single story home built in 1938 with part of the roof slanted and part is flat. The insulation is so thin that in some places you can see the sheet rock of the room below. We have a heat pump and most of the ductwork is in the attic along with the heater itself. Should we insulate the attic ceiling to keep the heater and ductwork at milder temps or just do the floor to keep the main living space insulated. Weve read articles saying both is correct so were just not sure.

Thanks!
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Old 01-31-2019, 09:25 PM   #2
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Re: Insulating Attic: Floor or Ceiling?


There is no one answer, so a lot more information will be needed.
Climate zone, city or state.
venting you have now, soffit, peak, box, gable.
holes between house and attic, pipes chimney wiring light boxes, many houses are like Swiss cheese.
Duct work, joints sealed and insulated.
Picture where you can.
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Old 01-31-2019, 09:34 PM   #3
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Re: Insulating Attic: Floor or Ceiling?


Thanks I’ll try to get up there and take pictures tomorrow
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Old 02-23-2019, 01:00 PM   #4
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Re: Insulating Attic: Floor or Ceiling?


Always insulate the attic floor in an unheated attic space. Never the ceiling.
The attic should have proper ventilation to the outside air. In new construction, this is done with soffit and ridge vents. In a 1938 home, you may need to add vents or an attic fan.

Last edited by Let it Snow; 02-23-2019 at 01:03 PM.
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Old 02-23-2019, 06:09 PM   #5
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Re: Insulating Attic: Floor or Ceiling?


With ducts in the attic, you should do the ceiling if you can.

Duct losses are very significant.

You also have all the penetrations for hvac vents/ducts, electrical, plumbing stacks and it's better to bring all that into the conditioned space than try to air seal. Houses shift, things expand and contract and seals around pipes in particular fail over time.


You can install baffles and have a chase from the soffit vents to the roof vents so any moisture that gets past the vapour barrier can be removed.

Or use impermeable insulation -> rigid or spray foam.


Conventional wisdom with respect to attic ventilation is wrong:

They don't always need ventilation.

Attics were mold free and didn't have issues until people started insulating them without air sealing.

Attics are ventilated when the floor is insulated because it's impossible to make them air tight even when vapour barriers are used. The floor insulation makes the surfaces in the attic cold; the moisture that leaks out from the house needs to be purged to stop condensation, mold growth and rot.



The ventilation also helps control heat in the summer, but the impact it over-blown.

When you insulate the roof deck, the exposed attic surfaces get very close to room temperature so you don't have to worry about condensation any more.

If you keep the heat out to begin with, the attic doesn't get hot.

If the insulation is impermeable to air and moisture, it can't hit the cold side and condense. So you can use spray or rigid foam (using some great stuff foam as adhesive) on the roof deck and if it's done properly, it will never rot out (unless the roof leaks) despite being covered by impermeable shingles.

When the insulation on roof deck is permeable and a imperfect vapour barrier is installed, the ventilation is required but only under the insulation, the entire attic doesn't need to be ventilated in this case.

The baffles installed form ventilation channels, insulation batts are installed under the baffles, a vapor barrier is used on the warm side.



Most of this post is relevant in climates that get cold weather.
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Last edited by user_12345a; 02-23-2019 at 06:13 PM.
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Old 02-23-2019, 10:22 PM   #6
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Re: Insulating Attic: Floor or Ceiling?


Hi K, as you expected guidance varies and as Neal requested the additional information will help. Having your mechanical equipment in the attic complicates the traditional attic floor approach but once we know your climate we will do our best.

Bud
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