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Old 12-09-2019, 12:28 AM   #1
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foam board, interior ceiling


Dealing with a fairly straight forward problem here. My house was built in 56, and thus has a poorly insulated ceiling.

Let me expand with more detail.
My house was built in 56, it has a flat roof,with a slight pitch with a cold ventilated space. Its 2x8 construction with a torch on top. Looks to be a single batt of insulation 3.5" thick maybe. My old timer neighbour seems to think it's only R-8 or R-12.
I live in British Columbia and am in climate zone 6
My house is a rancher style duplex, crawlpace below. My ceiling( where I lose alot of heat) are old staple on ceiling tiles. They are stapled to 1x6 furring strips that span my entire ceiling butted up to each other with maybe a 3/16 gap between them. I do not have access to the roof joist bays containing the 3.5" batt exept in one spot.The batt has a paper parchment backing.
When I demo the tiles I want to then screw 3" of foam board to the existing 1 x 6 furring strips. Then strap my ceiling again on 12" centers with new 1 x6 furring strips and install a 5/8 type x drywall ceiling.

I am having trouble finding examples of ppl doing this and info on it. Let me clarify why I want to do this.
The torch on roof is only 5 years old so I din't want to mess with it. Also as I stated before I have no access to the joist bays in my roof due to my ceiling being completly sheated in 1 x 6 furring strips.

I checked with my local building inspector and they said it would be fine. I also do not need a permit because I am only adding.

The lowes in my town is closing and today I picked up a bunch of silverboard xs exterior wall sheathing. I did some research before I bought and found that silverboard could be used as a vapour barrier. Which was my plan, I would use the silverboard as my vapour barrier then drywall as normal. But now that I am doing more research I am seeing 2 things on silverboards website

1.
The reflective laminated surfaces are micro perforated giving the panel high water vapor permeance greatly reducing the likelihood of trapped moisture through the increased drying potential.

2.
The reflective lamination can serve as a vapor barrier when taped and sealed.

Basically my question is,
1.am I doing something stupid insulating like this? Has anyone done a project like this insulating their ceiling from the interior with foam board

2. Should I have a vapour barrier? If so can anyone clarify if silverboard will act as one or I should add a poly vapour barrier?

Any help or comments would be greatly appreciated. I'm stepping out of my lane a bit here and it's hard to find resources because of how I am trying to do this project.
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Old 12-09-2019, 01:17 AM   #2
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Re: foam board, interior ceiling


You didn't say whether there was venting above the insulation now. What you are after is stopping house air from getting into the structure of the roof. I don't see a problem with taping the foam and sealing the perimeter with caulk

You can get drywall screws up to 3" long so you may not need to strap it.
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Old 12-09-2019, 04:22 AM   #3
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Quote:
Originally Posted by Nealtw View Post
You didn't say whether there was venting above the insulation now. What you are after is stopping house air from getting into the structure of the roof. I don't see a problem with taping the foam and sealing the perimeter with caulk

You can get drywall screws up to 3" long so you may not need to strap it.
Yes, there is venting above the insulation now.
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Old 12-09-2019, 04:29 AM   #4
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Re: foam board, interior ceiling


Quote:
Originally Posted by spudman17 View Post
Yes, there is venting above the insulation now.
I don't see a problem with your plan.
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Old 12-09-2019, 04:31 AM   #5
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Re: foam board, interior ceiling


Quote:
Originally Posted by spudman17 View Post
Yes, there is venting above the insulation now.
Zone 6 is big, where are you at or close to.
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Old 12-09-2019, 08:54 AM   #6
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Quote:
Originally Posted by Nealtw View Post
Quote:
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Yes, there is venting above the insulation now.
Zone 6 is big, where are you at or close to.
I'm in Prince George
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Old 12-09-2019, 10:33 AM   #7
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Re: foam board, interior ceiling


Quote:
Originally Posted by spudman17 View Post
1.
The reflective laminated surfaces are micro perforated giving the panel high water vapor permeance.....

2.
The reflective lamination can serve as a vapor barrier when taped and sealed.
These two statements are contradictory.

If I looked up the correct Silverboard product, it shows it is EPS with R5 per inch and permeability of .075 Perm, which makes it a vapor barrier. So you are putting a vapor barrier on the "warm in winter" side of the existing insulation, which is probably what is recommended for your area, but best to confirm with somebody more familiar with your local conditions.
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Old 12-09-2019, 10:45 AM   #8
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Re: foam board, interior ceiling


Getting ventilation to work on a flattish ceiling will be difficult.

Sounds like the joist bays are isolated from each other. Venting requires a high exit and a low intake AND some difference in elevation between them, like several feet.

Code in the north may require a vapor barrier but you said no permit was required. The need for a vapor barrier is debatable and they have now applied different levels of vapor retarders to this application. A vapor diffusion retarder would probably be sufficient and the foam board you describe is one.

The real concern from my perspective would be air flow for ventilation. Can you better describe what is there?

Bud
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Old 12-09-2019, 11:47 AM   #9
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Re: foam board, interior ceiling


If you are ok with losing 4" of ceiling height and adjusting all the ceiling light boxes and whatever else, would it ultimately be better to just remove it all (the 1x6s and R11 batts), address venting, air sealing, and insulation making it closer to the R26.5 required?
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Old 12-09-2019, 03:52 PM   #10
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Re: foam board, interior ceiling


Quote:
Originally Posted by Bud9051 View Post
Getting ventilation to work on a flattish ceiling will be difficult.

Sounds like the joist bays are isolated from each other. Venting requires a high exit and a low intake AND some difference in elevation between them, like several feet.

Code in the north may require a vapor barrier but you said no permit was required. The need for a vapor barrier is debatable and they have now applied different levels of vapor retarders to this application. A vapor diffusion retarder would probably be sufficient and the foam board you describe is one.

The real concern from my perspective would be air flow for ventilation. Can you better describe what is there?

Bud
Quote:
Originally Posted by Nealtw
Quote:
Originally Posted by spudman17
Yes, there is venting above the insulation now.

Zone 6 is big, where are you at or close to.


I'm in Prince George
You like this.

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Old 12-09-2019, 10:08 PM   #11
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[QUOTE=Bud9051;6002257]Getting ventilation to work on a flattish ceiling will be difficult.

Sounds like the joist bays are isolated from each other. Venting requires a high exit and a low intake AND some difference in elevation between them, like several feet.

Code in the north may require a vapor barrier but you said no permit was required. The need for a vapor barrier is debatable and they have now applied different levels of vapor retarders to this application. A vapor diffusion retarder would probably be sufficient and the foam board you describe is one.

The real concern from my perspective would be air flow for ventilation. Can you better describe what is there

Bud[/QUOTE

My house has some decent overhangs, 4' in front and back, and 2' each side. The underside of these overhangs is all soffit venting. Each joist bay is isolated. After further investigation it looks like there are 2 batts of insulation in my ceiling. Hard to tell what size they are exactly but maybe its 5.5" of insulation in total. This leaves about a 2" cold vented space above.

My roof is graded roughly 3/8" per foot. About a 8" drop from front to back
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Old 12-09-2019, 10:20 PM   #12
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Re: foam board, interior ceiling


Pitched roof with a peak in the middle or a single slope with one soffit higher than the other?

Very large soffits is a plus with minimal slope as wind can cause a positive pressure on one side and a negative on the other and it doesn't take a lot of wind to match the natural convection most homes rely on. An outside picture would help.

As in all construction, air sealing is a high priority, even higher than any vapor barrier or retarder. Far more moisture moves through leaks than moves through materials. Note, even a layer or two of paint scores as a vapor retarder, types of paint vary.

Bud
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Old 12-09-2019, 10:26 PM   #13
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Update, I took down the old ceiling tiles today. Turns out the 1x6 furring strips are not continuous across the ceiling. It was just double wide in the one place I looked originaly.
This means if I wanted I have access to the Roof joist bays.
I was considering just going on with my original plan of 3" of foam board, re strap then drywall. Using the silverboard as a vapour barrier or retarder.

I'm open to suggestions on how to get the best insulation value in my situation. I don't mind losing a few inches of ceiling height. There are also not too many lights to have to work around.

I will post some photos here. Open to any suggestions. Thanks
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Old 12-09-2019, 10:41 PM   #14
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The roof has one soffit higher than the other. The front of the house is higher than it is graded back at about 3/8 per foot
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Old 12-10-2019, 01:02 AM   #15
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Re: foam board, interior ceiling


What is or was the hole for and what is your plan for that?
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