Bonus Room Insulation Approach
Last Post 22 Aug 2014 12:07 PM by Dana1. 14 Replies.
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easyrider470User is Offline
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11 Aug 2014 02:09 PM
Need some guidance for my bonus room. Located above a 26x40 garage with radiant floor heat in the concrete floor in the 47460 area code.  Climate Zone 4 on the southern line of 5

Vented attic space, no HVAC in this room yet.  I plan to do a mini split in the future but not in the budget at this time.

Planning to insulate but not drywall at this time.

OPTION 1 :  Open cell against the roof deck from soffit to ridge (accuvent cathedral ceiling vents to give 1" spacing), open cell to seal garage ceiling drywall to trusses, blown cellulose 12" deep in attic floor area behind knee wall, dense packed cellulose under floor in bonus room with blocking at both ends sealed with foam

OR

OPTION 2 : Ruxol R-23 comfort batts in knee wall stud cavities, 1.5 " poly iso foam board sheathing on attic side of knee walls, Open cell foam to seal drywall ceiiling of garage to the trusses, accuvent cathedral ceiling vents and extensions to get air up above attic area above kneewall, blown cellulose in attic floor 12" deep, dense packed cellulose under floor in bonus room with blocking at both ends sealed with foam

OR

OPTION 3 : Dense packed cellulose (if possible) against roof decking with netting to hold it up, from soffit to ridge (accuvent cathedral ceiling vents to give 1" of spacing) open cell to seal drywall ceiling to trusses, R-23 comfort batts in knee wall stud cavities and 1.5 inch foam sheathing on attic side of knee walls, 12" blown cellulose


I need to know pros and cons but also would like to know if I am missing a part of the details in planning this out.  I think I have covered all the bases just not sure which method is safest for the roof deck for the long haul.  I have the 1.5 inch foam left over from the sheathing on the house and I have 3" XPS left over as well in 2x8 sheets...I could use the XPS where it would be beneficial
Dana1User is Offline
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11 Aug 2014 03:45 PM
It's damned near impossible to adequately air seal around kneewalls, making it better to just insulate the roof deck all the way down to the soffits, making that tiny attic space behind the kneewall conditioned space. By the time you've added up all the labor & can foam necessary to air-dam all the joists, you've blown any potential savings.

If you're going with a vented roof deck, do it all in cellulose, with the ~R10 polyiso on the interior side of the rafters.
easyrider470User is Offline
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12 Aug 2014 06:19 AM
@Dana
Thank you for the reply. When you say insulate from the soffit to the ridge vent I assume you mean block the soffit with accublock and create an unvented conditioned attic space. I assume this could be done with spray foam but I am concerned about open cell against my roof deck
Second, if I go the cellulose route in the rafters and leave the attic vented do I need to install the accuvent extensions in every single rafter and go all the way to the ridge vent? If that's the case then could I block off the soffits with the accublock and block the ridge vent somehow to create unvented attic and still use cellulose? Basically I am concerned about vented or unvented attic and how exactly to keep decking dry
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12 Aug 2014 02:46 PM
Dana?
jonrUser is Offline
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14 Aug 2014 09:50 AM
This should help. As Dana said, ventilation channels under the roof sheathing plus cellulose filled rafters bays plus interior side rigid foam. Use good tape for air sealing. Consider plywood (vs OSB).
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14 Aug 2014 10:16 AM
Thanks jonr. I have reviewed your link before. It helped me out a lot but I am very concerned about this roof deck. If I'm only doing cellulose do you guys think I could get away with just the 1" vent channel that the accuvent product provides. Not sure why you need more vent space for spray foam but I have heard 2" is necessary with that application.Here;s what I am thinking,

Airseal the garage from the bonus room as best I can
Netting to hold cellulose in floor system of bonus room where blocking is already installed
dense pack cellulose in remaining floor system after drywall is installed WITH blocking at attic ends
accuvent cathedral ceiling vents installed with extensions from soffit to ridge vent (DOES IT NEED TO BE EVERY SINGLE RAFTER?)
Open Cell Foam against attic side of knee wall drywall
Open cell foam against back side of drywall in functioing dormer walls
blown cellulose 12" deep on top of drywall in attic floor between soffit and knee wall
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18 Aug 2014 10:05 AM
Can I use the open cell foam directly against the roof decking in my climate or am I asking for trouble? They are doign TONS of houses like this.

I am worried about blocking off the soffits and running the foam all the way up to the ridge vent, although that does make for a fully conditioned bonus room including the tough to insulate and air seal kneewalls.
The other option is to use foam air tunnels all the way up in every single rafter and foaming from the knee wall up and the back side of the knee wall. That would make the attic space unconditioned.

What is the bets choice here?????
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18 Aug 2014 11:55 AM
I have a drawing that I made that depicts an idea I have for the insulation in my bonus room. I have read enough just today that makes me rule out open cell against the
decking. That's out. So what about this?  My question is what to do about the area between the two attics.  Of course I will have the baffles to allow the soffit to vent all the way to the ridge vent, and i'm pretty sure I will need some closed cell foam to detail the areas where the polyiso hits the roof deck/baffle.  I have some 2x8 sheets of 3" XPS left over as well, i could possibly install that against the roof decking in that open area between attics over the baffles....please let me know what you think.  Need to decide today or tomorrow so I can change insulation plan and prerock for the drywall man.




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19 Aug 2014 02:04 PM
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19 Aug 2014 02:08 PM
Posted By easyrider470 on 18 Aug 2014 10:05 AM
Can I use the open cell foam directly against the roof decking in my climate or am I asking for trouble? They are doign TONS of houses like this.

I am worried about blocking off the soffits and running the foam all the way up to the ridge vent, although that does make for a fully conditioned bonus room including the tough to insulate and air seal kneewalls.
The other option is to use foam air tunnels all the way up in every single rafter and foaming from the knee wall up and the back side of the knee wall. That would make the attic space unconditioned.

What is the bets choice here?????

There is some risk, it's fairly low in zones 4 & 5, but real, but it can be mitigated.

It's worth spraying the foam with "vapor barrier" latex paint, which will not perform to spec (which ~0.5 perms when applied to paper-faced wallboard), but will still reduce the vapor permeance to about 5 perms.  Avoiding the use of light colors on the roofing raises the average temperature of the roof deck, which lowers it's moisture content too. 

Ventilating the house at a rate sufficient to keep the interior relative humidity below 35% RH during the winter would be of benefit too.  Uncontrolled room humidity and localized moisture transfer via air leaks are the biggest threats to an unvented roof deck, but open cell foam is pretty air-tight when applied correctly, and with mechanical ventilation you have the power to monitor & limit wintertime humidity levels. (Some people control their HRVs via dehumidistat control in winter to deal with it.) 

Humans are healthy & comfortable anywhere between 30-50% RH, but maintaining 50% RH during the coldest weather would move more moisture into the roof deck than is healthy for the wood.

If you are putting foil faced polyiso on the interior side you MUST ventilate the roof deck, but it's a simpler assembly to air seal if the insulation follows the roof deck rather than the knee wall.




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20 Aug 2014 02:10 PM
So based off the drawing I attached would I be better off treating the ceiling and the knee walls as my thermal barrier and just allowing the attic space to remain unconditioned attic space? It seems like i eliminate all risk by simply letting the roof deck and the attic stay cold and ventilated but keep my walls into the room nice and insulated.
I wonder if the polyiso on the ceiling above the bottom cord of the truss would keep to much humidity in the room....I do not currently have ANY HVAC in this bonus room so it will probably have some higher and lower temeratures throughout the seasons until I get the mini split installed, Will the layer of Polyiso pose a problem in that location on the ceiling?
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20 Aug 2014 03:35 PM
The major cause of partition moisture problems is air leakage - in the wrong direction. You can try to address that with good (but never perfect) air sealing, a partition design that will dry when it does get wet and Winter humidity control. But controlling room air pressure can completely eliminate wrong direction air leakage. And uncontrolled air pressure differentials can make even well designed partitions fail.
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21 Aug 2014 02:23 PM
Posted By easyrider470 on 20 Aug 2014 02:10 PM
So based off the drawing I attached would I be better off treating the ceiling and the knee walls as my thermal barrier and just allowing the attic space to remain unconditioned attic space? It seems like i eliminate all risk by simply letting the roof deck and the attic stay cold and ventilated but keep my walls into the room nice and insulated.
I wonder if the polyiso on the ceiling above the bottom cord of the truss would keep to much humidity in the room....I do not currently have ANY HVAC in this bonus room so it will probably have some higher and lower temeratures throughout the seasons until I get the mini split installed, Will the layer of Polyiso pose a problem in that location on the ceiling?

The amount of humidity that escapes via vapor-diffusion is miniscule, irrelevant from a HVAC point of view. During the cooling season the vapor pressure is usually in the other direction- water vapor from the exterior would be coming INTO the house through the ceiling, not leaving.  In the winter the humidity would be escaping via vapor diffusion, but that's when you're more likely to want to retain some moisture. In a tight house wintertime moisture can be regulated via ventilation rates though, which is a much larger moisture transport mechanism than vapor diffusion through walls & ceilings.

But in unvented roof assemblies controlling the amount of moisture that goes through the interior surface is critical- if it's too vapor open the roof deck will accumulate moisture from the interior in winter,  but if it's too vapor tight it will prevent any mositure that finds it's way into the roof assembly from drying at all.  In your climate ~1-2 perms on the interior is about right, in cold climates it would need to be somewhat tighter, but never so vapor-tight as the <0.05 perm foil facers on rigid foam.  But as long as the roof deck is vented, a super-low permeance layer on the interior side is just fine. (Higher vapor permeance would also be fine- say. latex ceiling paint, which comes in at about 3-5 perms.)
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22 Aug 2014 10:29 AM
Also be sure that the interior side of an unvented roof is well air sealed. You don't want air coming in from the interior, depositing moisture and then exiting to the interior.
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22 Aug 2014 12:07 PM
That's right- there are orders of magnitude more moisture problems in buildings created by air-transported moisture than from vapor-diffusion alone. The focus on vapor retardency is much overplayed relative to the magnitude of the problem.

And there are an order of magnitude more moisture problems in building created by bulk moisture leaks than from air-leaks. In an unvented roof assembly some of the common bulk-moisture errors are sections improperly lapped roofing felt, improperly lapped skylight flashing or plumbing stack vent boots, etc.
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