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Old 04-04-2019, 04:53 PM   #1
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Insulating with two ceilings


Hi,
I am in NJ. I would like to insulate the uninsulated ceiling in my kitchen. It is the only part of the house that extends out of the house.
The kitchen back in the day had plaster and lath walls and ceiling. Over the years, drywall was installed on the walls and the ceiling
So the kitchen has two ceilings. First ceiling is 3/8 inch drywall with 14 inches OC and then there is 1 foot space and then there’s the plaster ceiling. The kitchen roof like the rest of the house has flat roof.

I was thinking of having it insulated with fiberglass insulation by cutting open a 2 feet wide row of sheetrock perpendicular to the studs that are holding on the drywall. Then sliding in the fiberglass in each of the 14 inches of studs and putting the sheetrock back in place with screws and putting compound on it.

Any suggestions or feedback would be appreciated.
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Old 04-04-2019, 04:58 PM   #2
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Re: Insulating with two ceilings


Fibrous insulation in flat roofs doesn't usually work well. Vapor permeability and condensation issues.



Any chance you could buy a Froth Pak kit. How many square feet we talking about?
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Old 04-04-2019, 05:17 PM   #3
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Re: Insulating with two ceilings


One concern with "two ceilings" is that somewhere inbetween the cold outside and the warm inside there will be a dew point temperature. Guidance says to be sure any humid air does not reach a surface cold enough to cause condensation.

In your case it is difficult to define where that dew point will occur and that 14" of OC foam is moisture permeable. You need to follow the traditional approach of drywall, insulation, and then ventilation. If you are not going to have ventilation you are going to be at high risk of moisture problems.

More details needed.

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Old 04-05-2019, 09:45 AM   #4
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Re: Insulating with two ceilings


Isn't it that I would have condensation issues as it is now, whether I insulate or not? The drywall ceiling is completely sealed so there's no air leakage to the second ceiling.
Since the kitchen is connected to the part of the house, it does share and there's open space (2 ft wide and 1 foot height) in the ceiling to the adjoining living room.
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Old 04-05-2019, 10:40 AM   #5
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Re: Insulating with two ceilings


I don't really have an alternate answer for you because your roof assembly doesn't show up in my many technical links. Venting a flat roof is between difficult and bad but might be done with a chimney style vertical vent. Little guidance on that approach.

Filling with insulation requires attention to all air sealing, walls as well as ceiling and requires a perfect vapor barrier, which never happens.

The one common fact for all assemblies is they need to keep the moisture out and then they need a direction to dry if moisture gets in. Well, straight up I expect is well sealed, that means you need another way to dry out any moisture that gets in and I don't have an answer for that.

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Old 04-05-2019, 04:11 PM   #6
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Re: Insulating with two ceilings


Thanks Bud for your answer.
My main question is will adding fiberglass insulation make things worse compared to as it is now?
Only thing I'll be doing is sliding in the insulation and patching the drywall back up.
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Old 04-05-2019, 05:07 PM   #7
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Re: Insulating with two ceilings


I'll first turn the question around, will adding fiberglass in that one foot gap do any good? You say you already have 14" of open cell foam up there which would be over r-50. That's already well above code level requirements for NJ so basically it would not be cost effective even if it is just your labor.

Will it do any harm? You stated you aren't already having a problem so why risk changing anything. The bottom line is answering your question is a guess either way, so pick an answer you like and go for it but it didn't come from me.

There are solutions, I read one. Remove the ceiling up to the bottom of the roof deck and add 12" of rigid foam. then replace your ceiling. Or go up top and add similar amount of rigid above the existing roof and install a new rubber roof above that. I don't consider either of those methods to be a serious option.

Good luck,
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Old 04-05-2019, 05:44 PM   #8
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Hi Bud! Thanks for your informative reply.

I think you misunderstand me. By 14 inches OC, I meant 14 inches on center wood beams. Currently, I don't have any insulation in the ceiling. With the building materials, I have like max R-4 value. I was thinking of using the fiberglass insulation with the paper backing.

So back to my original question, will adding insulation this way cause more harm than the way it is now?

Last edited by michaeldrehl; 04-05-2019 at 05:47 PM.
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Old 04-05-2019, 09:06 PM   #9
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Re: Insulating with two ceilings


Good old abbreviations, I read OC as open cell, you apparently have 16" oc (on center) making the space between them 14.5" tyoically. Standard batt with will fill those cavities so do the best you can at getting it in there. Still no good way to ventilate a flat ceiling so long term may not do well.

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