# Insulate / vapor barrier drop ceiling question



## AGWhitehouse (Jul 1, 2011)

Insulate B, Vapor Barrier 2...

Insulate around and over beam.


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## Windows on Wash (Aug 30, 2011)

What climate zone are you in?

While insulation value can be cumulative if the spaces are connected succinctly, you should make the insulation in one plane.

Climate zone will have everything to do with vapor retarder recommendations.

What is the venting of the current assembly like?

I personally would furr out a vent space in between the joists with some strips of rigid foam, install rigid foam EPS or ISO (install it air tight between the joists) that would serve as both a vent channel with the strips and part of the insulation layer, install additional insulation in between the joists, install rigid foam over the joists to break up the thermal conductance.

Leave all the insulation off surface B. Nothing about drop ceilings are air tight so don't bother with IC cans for that matter.

http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting/view

Depending on the zone, you may not need to vent but venting is a proven application and if done properly, does not have any real drawbacks in most cases.


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## joeh (Dec 15, 2011)

> What climate zone are you in?


Sorry, I had read so many people asking where you are and had it in my mind but didn't put it in there. I'm in Canada (Hamilton area, ON), zone B by their standards, zone 5 based on the US map.



> What is the venting of the current assembly like?


The roof is low pitch (1:10 or so) and venting is through the soffits only along the entire north and south faces of the house. There are no roof vents located anywhere at a higher point to release the hot air. There is a clear run from side to side though - the center 16" beam that supports the roof has the 2x8's sitting on top so air flows over. I don't anticipate changing this anytime soon - flat roof = expensive roofing materials = <3 years old! BUT... recommendations for the future are always welcome!

When I mentioned those roofing channels, I was going to have the clear airflow, but you are recommending actual rigid foam insulation in place of these channels. Understand there. I planned on vapor barrier over the structure, as it's required by the Ontario building code - not sure how the rigid foam over the joists would impact that.

Thanks for the response.



Windows on Wash said:


> What climate zone are you in?
> 
> While insulation value can be cumulative if the spaces are connected succinctly, you should make the insulation in one plane.
> 
> ...


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## Windows on Wash (Aug 30, 2011)

Rigid foam, depending on the type, does qualify as a Class I vapor retarder (i.e. vapor barrier with less than 0.1 vapor permanence).

I think all the solutions for your roofing/insulation quandary are in that link I posted.

I would run the rigid foam, make your own vent chutes, and supply some ventilation exhaust on the top side of the home.


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## AGWhitehouse (Jul 1, 2011)

Windows on Wash said:


> Rigid foam, depending on the type, does qualify as a Class I vapor retarder (i.e. vapor barrier with less than 0.1 vapor permanence).


Polyisocyanurate foam board is what you will need to achieve the Class I vapor retarder rating. XPS foam is too permeable and would take a large thickness to reach the permeability rating of a Class I vapor retarder. EPS is not even an option when a Class I foam is needed.


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## Windows on Wash (Aug 30, 2011)

AGWhitehouse said:


> Polyisocyanurate foam board is what you will need to achieve the Class I vapor retarder rating. XPS foam is too permeable and would take a large thickness to reach the permeability rating of a Class I vapor retarder. EPS is not even an option when a Class I foam is needed.


Polyiso with a foil facer will give you the Class I vapor retarder as will XPS with a poly facer.

EPS can be had with a foil facer as well which makes it class I but you would never use it in this case because of the lower R-Value per inch. 

Might as well use ISO because it is the "greenest" of the foams and gives you the highest R-Value per inch.

If the roof is going to remain vented, the permeance of the foam is less of an issue as compared to the airtighness of the ceiling/foam assembly. Airtightness is far more critical in this case as any diffused moisture through a tight, painted drywall ceiling should be removed by the venting.


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## AGWhitehouse (Jul 1, 2011)

Windows on Wash said:


> as will XPS with a poly facer.


Haha, I agree...facings are definately a game changer on permeability. My post's information was for unfaced foam boards.


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## Windows on Wash (Aug 30, 2011)

AGWhitehouse said:


> Haha, I agree...facings are definately a game changer on permeability. My post's information was for unfaced foam boards.


:thumbup:

I know that you know this stuff backwards and forwards. Just clarifying for the customer's sake.


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## joeh (Dec 15, 2011)

Whew, you guys started talking over my head, but I think I'm finally caught up! And FINALLY onto the insulation process, as the process fell behind a bit.

I have listened to everything and read the sheet, and have come to a plan of attack, and wanted to bounce it off of you all.

First off, I picked up some cheap EPS insulation and cut furring strips to place between the ceiling joists. 1 1/2" to start, moved to 2" quickly - I wanted to make sure I could fit 2x6 insulation in at the lowest slope, and needed that 1 1/2" gap. I'm currently installing 1" EPS, ripped to pressure fit, into the joists to form my channel. Question 1:


When I read the instructions for the cheap stuff, they said to leave gaps between 4' pieces - is that still the case here? I have those X supports that form a perfect break, and would be hard to seal up if there is not supposed to be a gap.
I have 2" EPS that will be doubled or tripled up and placed into the cavity under the vent channel on top of the frame wall. So R-20-30 of insulation there.

Next, I'll be using fiberglass into the joist cavity. Easy. 

Now, you mention placing the thermal break of rigid insulation on the sloped ceiling to keep away from using IC boxes on my potlights, but this just won't work. My drawing wasn't very well to scale, and this is a BIT better, still not there though:












My actual dimensions are of a 10'6" 2x6 drop ceiling, so around 5'6" overlap, while only the last 5 feet have a gap between the drop joists and the sloped joists. I believe that you said I should place the thermal break to the top side of the letter "A". I just don't see how that can work.

So... my plan, unless you convince me otherwise, is to fill with fiberglass batts, insulate the walls, bringing vapor barrier around onto the ceiling, and then place 1/2" thick faced PolyISO IsoGard onto the flat ceiling, taping all seams to form a vapor barrier. Is this just too much work for 1/10 of the surface area of the roof?


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