insulation humid climate


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Old 07-15-13, 02:45 AM
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insulation humid climate

hi. my home is a 1950s very low pitch asphalt roof in new orleans. need to insulate but having tough time making decision. there is no attic space, only rafters that are unvented.

2x6 rafters with only 5.5 inches of insul. space.
no ventillation
i wont make the r30 but want to do best job possible
trying to avoid spray foam because of high cost
ac ducts and air handler will be in conditioned space

should i put a rad. barrier with styro spacers 1in from ceiling btw rafters and then press r13 3.5 in fb batts under the rb to fill in the rest of rafter... or

r22 5.5 in high density pressed up to ceiling. or???

please any advice appreciated
 
  #2  
Old 07-15-13, 02:59 AM
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Welcome to the forums! Are the rafters exposed to the living area? The picture you draw indicates since you don't have an attic, there are no ceiling joists between the living space and the roof rafters. If so, fiberglas is not the way to go, especially since you don't have any ventilation. Pressing fiberglas against itself only reduces its insulative qualities, so that's out. R19 is the best you can hope for.

You may look into Roxul, which is far superior to fiberglas insulation, but at a cost. I do believe, however, spray foam by professionals is going to be your best bet, since all the odds are stacked against you....including the Hot Louisiana sun.
 
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Old 07-15-13, 07:00 AM
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yes. rafters are exposed to living space. is a perforated radiant barrier worth applying spaced an inch space from roof deck with insulation then applied under the rb. since there is no ventilation in the rafters, will a rb be a waste of effort.

also ive been told that sprayfoam will hide any type of roof leak and may allow water damage to roof above the foam. not to mention the difficulty in tracing a leak. we get loads of rain, so an eventual roof leak is always possible.

i will look into the insul. u mentioned. thanks. i know ime asking for alot in a small space.
 
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Old 07-15-13, 09:03 AM
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Do you currently or in the future plan on running air conditioning? With it cool inside and hot and humid outside you should determine the merits and proper location of a vapor retarder.

With those rafters extending from roof to inside they are acting as a thermal highway to conduct the heat right past any insulation you install. Do you plan on covering the inside and do you have enough height to increase the 5.5" currently there.

Although RBs can be used in some situations, eliminating an inch of insulation to make room for the RB decreases any benefit. Plus those rafters will be conducting right across that gap. The purpose of a RB would be to reduce the temperature that the insulation has to deal with. With no ventilation and your configuration I doubt the air in that gap will be any cooler.

I like the Roxul Chandler suggested. Fill the cavity and add 2" of rigid foam and cover with sheetrock, being very careful to eliminate (or seal) any air leakage paths, like recessed lights or electrical boxes.

Bud
 
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Old 07-15-13, 10:00 AM
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yes. ac handler and ducts will be in living space. vapor barrier? where would best place to install this in my situation? would a vp make a roof leak undetectable?

option 1:
roxol has r-23 5 1/2 in. batt that i could press in cavity and cover with 5/8 rock.

would the insulation touching the roof deck heat up more without a buffer space btwn the two. (lil confused btwn radiant vs. conductive heat in this small of a cavity as well as the rb unvented value)

option 2:

screwing 2 in x 1 3/4 in. foamboard furdowns cut to fit bottom of 2x6. this gives room for 7 1/4 in. batting at r-30. then rock with longscrews thru furdowns. this would be cheaper than having to buy 1400 sq ft of 2 in. foam.

opt. 3:

going with the full on 2 in. foamboard as bud suggested


am i over thinking this? is all the extra work worth an r-7 upgrade. or should i just get on with the r-23 and call it a day.

thanks for the pointers, they help
 
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Old 07-15-13, 10:45 AM
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styro -

You are splitting hairs a little too closely. This is especially true when you are tossing R values around when they do not represent the actual R-value of the roof assembly. The R-values (lab tests of the actual individual materials under ideal situations) materials are just for the insulation and there is a thermal short circuit caused by the joists than can be substantial in the real world. Also, it takes very little moisture (1% or so) to reduce the effective insulation by as much as over 33%. The R-value of a layer of XPS foam provides real insulation because it is a continuous layer with no short circuits.

The reason for the apparently high R-values required for wall and ceiling assemblies is because they do not actually perform as well as the advertised advertised claims when used in a total wall/ceiling.

Dick
 
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Old 07-15-13, 11:36 AM
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Vaulted ceilings present a similar problem with limited space for insulation. This is a topic which has had a lot of discussion. Here is a link to several related articles. I can't say I've read them all or that they will agree with me, but they are much higher on the experience scale than I am.
Search | GreenBuildingAdvisor.com

Now, about half way down in this next link he gets into why you shouldn't dense pack a flat roof. Dense pack is a term related to blown in cellulose, but Roxul is similar in being very dense. Code requirements as well as best practices will be working against you.

However, you are in a hot humid climate so the risks associated with cold climates do not apply.

If you are required to bring this up to code, then another approach will be needed and perhaps one that includes venting. I hate to leave you without a specific answer, but hot humid conditions are not common way up north. I'll let you read.
Joe Lstiburek says, Don't dense-pack unvented cathedral ceilings | GreenBuildingAdvisor.com

Bud
 
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Old 07-16-13, 05:06 AM
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thanks for links. i have read most everything. unfortunately i am still a bit puzzled by many conflicting studies.

i suppose i could add ventilation by drilling 2 in holes in each soffit with caps. but i also read studies summarizing that in my climate zone, ventilation does more bad than good because of the humidity.

from what ime understanding, seems like a rad. barrier is pretty useless in my situation also.

looks like my only realistic approach is as suggested, going with r13 fiberglass batting with a 2 in foamboard over rafters bringing it to about r-28.

seems that my vapor issue would be the hot attic air hitting the cooler drywall being that it only dips under 32 once or twice a year.
 
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Old 07-16-13, 10:18 AM
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my only concern with 2 in foamboard. when the roof springs a leak, it will be impossible to detect. water will make it thru batts and run down foam board to walls.. potential nightmare where rain is an everyday summer occurrence. guess i have to choose a poison here.
 
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Old 07-16-13, 11:07 AM
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I'm still following and still don't have a concrete suggestion to offer. One of the reasons I joined here was to encounter problems outside of my travel range and it has worked well . But moisture is a challenge and still being heavily debated by the experts, so no wonder we struggle.

Whatever is on the roof will probably prevent any drying to the outside. Warm climate and air conditioning means the condensation risk would be allowing that outside air to reach the cool drywall. That would imply no venting and if no venting, do we need a gap?

Your suggestion of 2" of rigid to act as a thermal break and increase the depth to allow for more insulation may be the best. I still like the Roxul and omitting a vapor barrier on the inside would allow drying to the air conditioned side. Use a latex paint as oil based will act as a vapor retarder.

If you will be installing any electrical boxes or recessed lights, air tight and insulation contact for the recessed. For any electrical, just be sure they are air sealed. When it is all open be sure to seal any air leaks to the outside.

From 1,500 miles away that's about the best I can guess. Since you are there, checking with codes and standard practices will be up to you. I hope some of this helps.

Bud
 
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Old 07-16-13, 01:52 PM
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thanks bud. i appreciate your input. today i begin sealing all cracks in fascia plates.
 
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Old 07-22-13, 10:19 PM
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Hope I'm not too late... the foamboard below the joists will stop the thermal bridging and keep the side facing the warm attic --- warm, not cold so there is no condensation of the attic air, Fig 3-41; Moisture Control Handbook: Principles and Practices for Residential and ... - Joseph Lstiburek, John Carmody - Google Books

Use Roxul rather than R-19 fiberglass (low density= low results), IMO.

Gary
 
 

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