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Old 08-26-2019, 09:51 AM   #1
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Insulating between the ceiling and a low slope roof


I’m renovating the top floor of a three floor building. The roof is a low slope for water drainage. The gap between the ceiling and the bottom roof joists varies from 9” to 24”.

The space measures 19.5' x 57'.

The ceiling is made from Armstrong short span - similar to a metal stud ceiling.

I want to insulate the ceiling before I start hanging sheetrock.

Is fiberglass roll out insulation the way to go? Should i get rolls or batts?
Any tips would be great!!!

I’m in NYC with gas steam radiator heat. According to the chart below I can use insulation from R38-R60 for my “attic”.

Thanks
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Last edited by jaketrades; 08-26-2019 at 10:09 AM.
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Old 08-26-2019, 10:10 AM   #2
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Re: Insulating between the ceiling and a low slope roof


Do you have high and low venting?
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Old 08-26-2019, 11:07 AM   #3
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Re: Insulating between the ceiling and a low slope roof


I have two vents on the roof.

One is a ridge vent on a skylight.

The other is a louvered box.


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Old 08-26-2019, 11:18 AM   #4
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Re: Insulating between the ceiling and a low slope roof


Essentially you have no venting. To achieve air flow you need both high and low vents, high for air to exit and low for it to enter. Your description doesn't sound like it will move much air.

With that limited space you cannot achieve the insulation levels you mentioned while maintaining an air gap above. Fiber insulation should not be in contact with the bottom of the roof.

No easy solution. In some cases they add a very thick layer of rigid insulation on top of the roof and then new decking and new roofing. Whether you could use spray foam on the bottom of that roof or not would need some onsite guidance.

If NYC is like many other locations they will want any modifications to be brought up to current code levels. What they are requiring is a question you should ask them.

Bud
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Old 08-26-2019, 11:26 AM   #5
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Re: Insulating between the ceiling and a low slope roof


Have you seen any videos on how to insulate that ceiling from below? bats and rolls won't be wide enough, getting a tight fit will not be easy.
Do the walls extend above the ceiling, will they be fire stopped?
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Old 08-26-2019, 04:00 PM   #6
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Re: Insulating between the ceiling and a low slope roof


The roof pitch is 3/8 : 12. Going to measure it more accurately later.

The ceiling height is set at two different heights through out the apartment.

To the left of the red line it is 12í. To the right it is 10í.

Between the blue line and the red line, it will be difficult to get insulation at proper thickness.

Everywhere else insulation can be properly fit. As @Nealtw pointed out there could be some difficulty getting that insulation installed.

Venting : The only proper vent in the unconditioned space is in the middle of the building (see green arrow). The ridge vent is in conditioned space.


@Bud9051 : You said that "Fiber insulation should not be in contact with the bottom of the roof.Ē Can the fiber insulation touch the bottom of the roof joist? The joists are 8Ē tall. Also can I use thinner insulation in the tight areas?

Good idea on spray foam on the bottom of the roof. Iíll get some quotes.



@Nealtw : The walls extend anywhere from 9"-24Ē above the ceiling. The walls are built with brick (brick noggin I think itís called.) Thatís the fire stop.
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Old 08-26-2019, 04:09 PM   #7
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Re: Insulating between the ceiling and a low slope roof


The walls the you built?.
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Old 08-26-2019, 04:18 PM   #8
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Re: Insulating between the ceiling and a low slope roof


Quote:
Originally Posted by Nealtw View Post
The walls the you built?.
Sorry i wasn't very clear.

The **building perimeter** walls are built with brick (brick noggin I think itís called.) Thatís the fire stop.

The building is attached on both the left and right side.
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Old 08-26-2019, 04:37 PM   #9
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Re: Insulating between the ceiling and a low slope roof


Quote:
Originally Posted by jaketrades View Post
Sorry i wasn't very clear.

The **building perimeter** walls are built with brick (brick noggin I think itís called.) Thatís the fire stop.

The building is attached on both the left and right side.
every wall has to be sheeted above the ceiling or have solid blocking below the ceiling.



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Old 08-27-2019, 07:29 AM   #10
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Re: Insulating between the ceiling and a low slope roof


Quote:
Originally Posted by Bud9051;
Whether you could use spray foam on the bottom of that roof or not would need some onsite guidance.

Bud
Getting some quotes for spray foam insulation. Closed cell and open cell. Does having spray foam insulation eliminate the need for ventilation? I've read that closed cell spray foam insulation keeps both moisture and temperature differences out of the unconditioned space.
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Old 08-27-2019, 07:51 AM   #11
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Re: Insulating between the ceiling and a low slope roof


Here are a couple of links that will provide more reading than you asked for, but they are my source for many answers like this as I have never built a flat roof building.
May need to use their one month free GBA Prime for this one. They are a trusted company.
https://www.greenbuildingadvisor.com...idential-roofs

That link came from this article:
https://www.greenbuildingadvisor.com...te-a-flat-roof

Bud
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Old 08-28-2019, 10:45 AM   #12
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Re: Insulating between the ceiling and a low slope roof


Did some reading and listed some of my options below. I want to preface this by discussing the condition of my roof. The roof is old and is composed of multiple layers of asphalt bitumen. Iíve managed to seal up the current leaks (weíve had quite a bit of rain in NYC this summer to expose the leaks), but who knows what the winter will hold for us. Replacing the roof is not in the cards right now.

I have 8Ē deep roof joists and then quite a bit of space b/w the bottom of the roof joists and the ceiling joists. Most areas are 2+ feet. There is one area that is very tight. 3-4Ē gap.


1. Closed cell spray foam insulation sprayed on the under side of the roof. This seems like a great choice - the roof becomes close to air tight and there is no need to vent the attic space. The concerns are that if my roof does develop a leak, the moisture will become trapped b/w the roof sheathing (white pine boards maybe??) and the close cell spray foam insulation. This could create rot on my roof sheathing. I cannot think of a way the moisture could escape. Sounds scary. Are my fears unfounded?

2. Open cell spray foam insulation the under side of the roof. Seems like a good choice. The open cell is permeable so water leaks in theory should be detectable. If O/C spray foam insulation is installed to 8Ē deep with a 3.5 R-value per inch = R-28. This leaves room for ventilation. I will need mushroom vents in the front and back of the building - in addition to the cupola in the center of the building.

3. Blown in cellulose. Install 5/8" drywall ceiling and IC rated recessed lighting. Then cut 3Ē holes in every ceiling bay and blow insulation until it reaches a certain depth. Issues with this are : how do I know Iíve reached the desired depth? As @Bud9051 pointed out the insulation cannot touch the roof sheathing. Seems like a tricky job with probability for under-filled and overfilled spots. IS there a smarter way of blowing in the insulation and getting adequate coverage? This method will also require mushroom vents front and back. The one upside of this job is itís DIY. $600-$700 for insulation and HD will waive the blower costs. So just my time.


Any thoughts or advice?
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Old 08-28-2019, 11:00 AM   #13
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Re: Insulating between the ceiling and a low slope roof


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Old 08-28-2019, 11:08 AM   #14
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Re: Insulating between the ceiling and a low slope roof


Quote:
Originally Posted by Nealtw;

Yup watched that last night. Seems like the open cell will eventually expose the leaks.



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Old 08-28-2019, 11:18 AM   #15
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Re: Insulating between the ceiling and a low slope roof


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