DIY Home Improvement, Remodeling & Repair Forum > DIY Home Improvement > Insulation and Radiant Barriers > Insulating a bonus room cathedral ceiling




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Old 06-02-2013, 09:32 PM  
nealtw
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Venting of the roof assembly has nothing to do with snow and everything to do with humidity.
If you're in a colder climate the venting has everything to do with snow and ice dams as well as humidity.


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Old 06-02-2013, 09:35 PM  
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GBR: This house has closed cavities where the jack rafters meet the hip, so unless the ceiling is completely rebuilt there would be no way to ventulate the roof properly.



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Old 06-03-2013, 03:36 AM  
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If you're in a colder climate the venting has everything to do with snow and ice dams as well as humidity.
True, but it does still get cold in FL (certain parts).

While the necessity drops considerably, it does have its place in even hot climate applications.
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Old 06-03-2013, 08:20 AM  
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True, but it does still get cold in FL (certain parts).

While the necessity drops considerably, it does have its place in even hot climate applications.
Venting of the roof assembly has nothing to do with snow and everything to do with humidity.

You want to be carefull when making statements like this. People living in different climates read this and may get the wrong answer to their question.
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Old 06-03-2013, 10:01 AM  
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Venting of the roof assembly has nothing to do with snow and everything to do with humidity.

You want to be carefull when making statements like this. People living in different climates read this and may get the wrong answer to their question.
Agreed.

I still stand by the statement, as does building science, that venting is for humidity.

While venting is often cited for the prevention of ice damns, I would argue that ice damns are the result of a poorly sealed envelope and/or inadequate insulation layer.

If ice damn prevention was solely accomplished by venting, those persons that have an insulated roof deck would be in serious trouble while they are usually just the opposite. Insulated roof decks normally perform better as a result of the materials used (i.e. SPF vs. loose fill insulation).
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Old 06-03-2013, 01:18 PM  
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Venting of the roof assembly has nothing to do with snow and everything to do with humidity

If this statement included closed cell foam, I might have agreed, but the if is always can you insulate enough over the outside wall to prevent heat from that wall getting to the roof sheeting when the snow is on it. The answer is most of the time, it often requires a close look at the size of the rafter and the birdsmouth cut in it.

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Old 06-03-2013, 07:14 PM  
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"Figure 2: Classic Control Approach—Keep heat from the interior from getting to the roof deck and then remove any heat that gets there. First, construct an airtight “lid” or ceiling plane. Second, insulate the top of the lid with lots of insulation. And then third, flush away any heat that gets to the roof deck by ventilating the underside of the roof deck with exterior air. Note the air seals at the top of the exterior wall. Exterior walls are like chimneys—you don’t want to vent the tops of them into your roof. Also, it is a good idea that the minimum thermal resistance directly over the top of the top plate be greater than the thermal resistance of the wall. So, if the wall is R-30, you want at least R-30 directly over the top of the top plate. Also, note that a 2 in. (51 mm) minimum airspace is recommended under the sheathing. Most codes call for only 1 in. (25 mm). The codes are wrong. In ice dam regions, where ground snow loads are greater than 30 lb/ft2 (146 kg/m2) you need 2 in. (51 mm). This is because ventilation only works to remove small amounts of heat. Therefore, you need a lot of it." From; http://www.buildingscience.com/documents/insights/bsi-046-dam-ice-dam

I don't see the part where jack rafters meet a hip..... in this thread, perhaps another? I read "gable roof" and 28" knee walls with 8' to finish flat ceiling on a 12/12 pitch, with 2x6 rafters he will furr.... and "I don't want to have to deal with trying to put up some sort of additional vent on the 12-12 pitch pyramid roof and spray foam insulation is too $$$." so I showed him that minimal venting is fine as you introduce hot exterior air with moisture. Add foamboard on knee wall and on sloped ceiling on room side. Cover for ignition barrier if required per local AHJ. ADA the drywall to keep warm exterior air from air conditioned interior.http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach

Wargle, any HVAC ducts in knee wall or top attic? Is there any radiant barrier?

Gary

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Old 06-03-2013, 10:05 PM  
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Sorry Gary, looks like I got this thread mixed up with another.

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Old 06-04-2013, 03:43 PM  
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Been there, done that...lol. I wasn't trying to be funny by thanking you,(just acknowledging it), couldn't remove it afterward, no harm-no foul. Gary

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Old 06-05-2013, 03:31 PM  
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No radiant barrier and we plan to install ducts in the knee wall.



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