# Insulating Ceiling Under Flat Roof - South Florida



## Yamilo (Jul 4, 2017)

Hello Everyone,

My home has a primary roof(gable) and a secondary roof(flat). I would like to insulate the ceiling under the flat roof section but I am scared to cause a mold/condensation problem by doing it incorrectly. I am attaching some pictures which I hope will help paint a better picture for you guys. 

The first picture shows the old insulation which I've torn off and looked quite raggedy. I know the previous owners had some leaks but the roof has been re-surfaced with new mineral cap sheets. There is close to 9-1/4" of space from the bottom of the joist to the roof sheeting. I want to re-insulate, and drywall under the joists so I can also add some recessed lighting. 

In the 2nd picture, you will see the current ceiling and how the joists are mostly exposed thanks to the original owner of the house wanting to add to the look of the ceiling (and probably just wanting to melt some excess weight off, PERMANENTLY, or maybe they just loved high energy bills). 

In the 3rd picture, you will see that there is a slight space above the drywall which connects to the gabled roof and I think this is where the venting is happening. I guesstimate there is probably about 2 to 2.5" of space above the current drywall before you get to the roof sheeting. 

In the 4th picture, you will see that the flat roof is vented. I know that over the current drywall there is insulation as you were able to see in the first pic but I don't know how much ventilation there.

My objective is to make the rooms cooler by re-insulating, to add recessed lighting, and not cause a mold problem while I'm doing it. 

Which product do I use? Should I be concerned with moisture/mold? Budget is an issue, I don't think I will be able to afford spray foam insulation as this will be done in two rooms, one room is 16x20(family room) and the other is 16x14(bedroom). Thank you all in advance and have a great day!

-Yamilo


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## Windows on Wash (Aug 30, 2011)

How old and what is the flat roof? If it is near the lifespan, you will find it easier to insulate the exterior surface in this case vs. all the interior demo.


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## Windows on Wash (Aug 30, 2011)

Condensation issues are less problematic in your climate than say, upstate NY where roomside moisture condenses on cold exterior connected surfaces. 

Biggest areas of condensation are going to be at that cold supply side HVAC and mixing with hot and humid exterior air. 

While insulating the rafter bays and converting it to a conditioned (i.e. hot) roof are probably more efficient, you are not going to address the thermal bridging of the exterior framing with cavity insulation. 

See option 3: http://www.finehomebuilding.com/pdf/021221068.pdf

https://buildingscience.com/documents/reports/rr-0108-unvented-roof-systems/view


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## Yamilo (Jul 4, 2017)

The flat roof still has a 10-15 year life span, as I just redid the roofing paper with asphalt and mineral cap sheet.


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## SeniorSitizen (Sep 10, 2012)

I can't help you on this with the exception of advising that in south Florida the odds are about ten thousand to one, I would estimate, of dew point temperature ever being reached to cause condensation and potential mold in the area in question.


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## Windows on Wash (Aug 30, 2011)

Maybe it is just me, but I would prefer to do roofing than drywall and all the mud and tape work. 

The links have some decent reading in them about approaches to sealed roofing approaches.


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## Yamilo (Jul 4, 2017)

Windows on Wash said:


> Maybe it is just me, but I would prefer to do roofing than drywall and all the mud and tape work.
> 
> The links have some decent reading in them about approaches to sealed roofing approaches.


Thanks for the links. I went through them but insulating above the sheeting is not an option right now as I just re-did the top surface a few months ago. I really want to insulate between the joists and add some recessed lighting which currently I can't because there isn't enough space above the drywall. I have another place to stay while the drywall work is being done but I want to make sure I use the right insulation for the job. 

-Yamilo


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## Bud9051 (Nov 11, 2015)

I would have replied sooner if I had a good answer for you but your objective is difficult with a limited budget. But, here are some comments.

1. Natural ventilation required a vertical distance between intake and exhaust. You have inches and need feet. One option is to add a vertical chimney of sorts. Google Venmar roof vents.
2. Recessed lights create a thin spot in your insulation so choose carefully and get IC and AT rated (insulation contact and air tight).
3. Here is another link which may provide some more information on your insulation issues. http://www.greenbuildingadvisor.com...ented-roof-assemblies-be-insulated-fiberglass

Bud


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