# Garage ceiling insulation



## MNHouseRepair (Dec 1, 2011)

Hi all, first time poster! I did a few searches, but haven't found an answer to what I'm looking for, so hope y'all can be of assistance.

We have an older detached garage built in the 60's that we're wanting to convert into a gym/workshop space to use year-round. I've started off by insulating the walls and putting up drywall, but am at a bit of a loss as to what to do with the ceiling.

I want to keep the cathedral-style ceiling because I'm fairly tall and wouldn't be able to run on a treadmill or jump if we just closed off the rood - but the joists that run up the roof are 2x4's - meaning I can only fit R-13 faced insulation between them? I'm not 100% sure. I'd love to fit thicker stuff between them, but I know I'm not supposed to compress the insulation.

Anyone have any ideas on how to get the best bang for my buck insulation-wise up there? I was also planning on cutting some soffit vents into the side and running channels up behind the insulation to get some airflow in there - but with only 3 1/2"-4" to work with, that really limits the space I can use. Ideas?

Thanks all! Please direct me to any existing threads if there any on this, in case I missed them too!


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## Windows on Wash (Aug 30, 2011)

How are you going to be conditioning the space?

2x4 @ R-13 is about 1/3rd of what is required and even less of what is recommended.

http://www.ornl.gov/sci/roofs+walls/insulation/ins_16.html


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## MNHouseRepair (Dec 1, 2011)

Thanks for the reply! I realize that R-13 is too little, and I'm wondering what else I can do with the space I have? I really do want to keep it cathedral-style (I hope I'm using the right term there) because if we close off the ceiling, I'm pretty tall and won't be able to use the space as well as I'd like to.

If I use thicker insulation, and it sticks out from between the joists but is held in place by a plastic sheet that would be used as a vapor barrier - would that work? Does the insulation have to fit in between the joists?


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## Windows on Wash (Aug 30, 2011)

How are you heating the space and how do you want it finished?

You don't need to have a drywall ceiling but you will need a proper air barrier across that ceiling to prevent the warm air and diffusion of moisture to the underside of the roof deck.


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## joecaption (Nov 30, 2011)

The right way to do it would be add collar ties to act as a ceiling at a height you can live with, add 2 X 2's to the bottoms of the rafters, Add foam baffles from where the ridge vents will be to the point where the collar ties are.
Now you can add the R13 and not compress it and still have air flow. On the flat part of the ceiling you could go as thick as you want to.
You also going to need to add a ridge vent so the hot air can escape in the summer.
If you install insulation tight againt the sheathing it will super heat the shingles and shorten there life by about 5 years and the heat in the summer will transfure down into the room.
By having that flat raised ceiling you could add can lights or a ceiling fan.


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## Windows on Wash (Aug 30, 2011)

joecaption said:


> The right way to do it would be add collar ties to act as a ceiling at a height you can live with, add 2 X 2's to the bottoms of the rafters, Add foam baffles from where the ridge vents will be to the point where the collar ties are.
> Now you can add the R13 and not compress it and still have air flow. On the flat part of the ceiling you could go as thick as you want to.
> You also going to need to add a ridge vent so the hot air can escape in the summer.
> If you install insulation tight againt the sheathing it will super heat the shingles and shorten there life by about 5 years and the heat in the summer will transfure down into the room.
> By having that flat raised ceiling you could add can lights or a ceiling fan.


I would agree with about 95% of this with the exception that there are very slight differences in shingle temps and no evidence to indicate that unvented assemblies shorten shingle life. 

Unvented assemblies that leak moisture with shorten roof life in general but there is nothing to indicate that shingle life suffers.


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## joecaption (Nov 30, 2011)

90 % of my work is remodeling in older houses 100 years old or older. Dozens of times I've seen where over the year DIYS have gone in there and tryed packing in the insulation to tight with no soffit vents, no baffles and no ridge vent, The paper on the insulation got so dry out and had gotten so hot it was falling apart, the 10 year old architural shingles would crumble in you hand and the granuals just filled the gutters.
http://www.gaf.com/Roofing/Residential/Key-Danger-Signals/key-danger-signals.aspx
Notice how they show at least 4, differant pictures of what poor venting can do.


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## Windows on Wash (Aug 30, 2011)

Poor ventilation in a vented assembly and a hot/insulated roof deck are two totally different things.

Shingle manufacturers will forever attempt to blame the conditions and not the shingle.

There is no data to indicate that an insulated, sealed roof assembly shortens shingle life.

Most shingle cooling happens to the exterior and there are studies showing only a 5-7° difference in peak shingle temperature in Las Vegas.

Ventilation and moisture drive to the back of the shingle can kill it early.


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## MNHouseRepair (Dec 1, 2011)

Hi all, thanks for all your help with this. I'll most likely go the route of adding collar ties to set a higher ceiling height, and work from there. Thanks!


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## shazapple (Jun 30, 2011)

No shingle manufacturer, roof inspector, or qualified roofer would ever tell you that it is ok to insulate against the deck without ventilation. Heat causes breakdown of anything in any application, so why are shingles any different? 

This is a literature review by the Florida Solar Energy Centre. It does not cover shingle life in much detail, but it does compare traditional vented, unvented, and cathedral vented roofing systems. They all have their strong points but the cathedral vented is the recommended. 
http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf


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## Windows on Wash (Aug 30, 2011)

shazapple said:


> No shingle manufacturer, roof inspector, or qualified roofer would ever tell you that it is ok to insulate against the deck without ventilation. Heat causes breakdown of anything in any application, so why are shingles any different?
> 
> This is a literature review by the Florida Solar Energy Centre. It does not cover shingle life in much detail, but it does compare traditional vented, unvented, and cathedral vented roofing systems. They all have their strong points but the cathedral vented is the recommended.
> http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf


Shazapple,

All of the science and data that is being collected today is in complete contradiction to your assertions.

The idea of ventilated assemblies in Fl and other hot and humid climates was based on very antiquated recommendations and using the shingle manufacturers of build code inspectors as you source of information is flawed at its source.

95% of the shingle cooling happens to the exterior surface via convection.

I have never seen a report, although cited, showing a 32-40° difference between and unvented and vented assembly. As a matter of fact, orientation, color, and material have everything to do with surface temperatures. That being said, Las Vegas shingle roofs that were either well ventilated (1:150) vs. completely sealed only show a 9° difference in surface temperature (well withing guidelines and allowable maximums).

Lets be clear about one thing, attic ventilation is for moisture. It always has been and will continue to do so. Removing moisture to prevent rot, mildew and mold is all it is intended for.

Beside the energy efficiency reductions in a vented assembly (i.e. wind stripping of R-value, more building leakage, ice damns in low slope assemblies), vented assemblies can also entry pathway points for rain and bulk moisture and embers from brush fires.

Don't get me wrong, if you are going to run a sealed attic/hot roof the details need to be correct. The vapor diffusion concerns are more pressing in colder climates.

http://www.thermalsealexperts.com/air03.php

http://www.buildingscience.com/docu...as-nevada/view?topic=/doctypes/researchreport

If surface temperature really ruined asphalt...wouldn't the roads be crumbling much faster?


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## AndyGump (Sep 26, 2010)

I have to agree with Windows (wish you would put in a real name) on this.

After doing some research on this whole issue I have come to the conclusion that for most applications an un-vented assembly is usually the best assembly.

Especially here in dry fire prone conditions in So. Cal.

Andy.


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## Windows on Wash (Aug 30, 2011)

AndyGump said:


> I have to agree with Windows (wish you would put in a real name) on this.
> 
> After doing some research on this whole issue I have come to the conclusion that for most applications an un-vented assembly is usually the best assembly.
> 
> ...


Andy...my name is Eric.


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## shazapple (Jun 30, 2011)

Windows on Wash said:


> Shazapple,
> All of the science and data that is being collected today is in complete contradiction to your assertions.


My assertion was that manufacturers require ventilation. The Asphalt Roofing Manufacturers Association has a technical bulletin talking about roof ventilation and how it is required. http://www.asphaltroofing.org/

A couple examples of how improper ventilation will void warranties (with exception to CertainTeed)
http://www.iko.com/warranties/
12. Any damage or distortion caused by inadequate ventilation either at the eaves or on the rooftop of the building. This includes failure of ventilation caused by blocked, non operative or defective vents or any other condition that renders the ventilation system ineffective. Roof system ventilation should meet local building code standards for total vent area. Ventilation must also be distributed evenly between the rooftop and the eaves of the building;
http://www.bpcan.com/documentation-and-tutorials.aspx
(b) the roof and each part of it must be designed and built in accordance with the applicable local and National Building Codes. All roof structures must be provided with thorough ventilation and the deck over which the shingles are installed must meet minimum building code requirements. Where local building codes have specific requirements which differ from National Building Codes, the more stringent requirement must be followed.
http://www.certainteed.com/resource/roofing/warranties
Fiberglass shingles can be non ventilated, and asphalt will only have a 10 year warranty.
http://www.powerhrg.com/_files/files/Roof_Warranty.pdf GAF-ELK
Goes into less detail than the rest, but says warranty does not apply if there is "inadequate attic ventilation"

I do agree that shingle colour, orientation, etc have a lot to do with temperature, but I'm not sure what the maximum guidelines are that you refer to. 



Windows on Wash said:


> Beside the energy efficiency reductions in a vented assembly (i.e. wind stripping of R-value, more building leakage, ice damns in low slope assemblies), vented assemblies can also entry pathway points for rain and bulk moisture and embers from brush fires.


Please go back and read the article I posted. It clearly shows that a vented catheral ceiling provides better energy efficiency. I'm not sure why ice daming is lumped in your list? 



> http://www.thermalsealexperts.com/air03.php
> 
> http://www.buildingscience.com/docu...as-nevada/view?topic=/doctypes/researchreport


It should be noted that this study refers to tiled roofs, not asphalt, and only models the asphalt shingle temperature. I read the above links and they mostly refer to HVAC being present in the attic space. Yes, it is true if you insulate the deck you will have less HVAC losses because the system is now in the conditioned area. The link I posted agrees with this, but also states that attic air temperatures are lower if you add ventilation above your insulated deck. 



> If surface temperature really ruined asphalt...wouldn't the roads be crumbling much faster?


The asphalt used in shingles is a different type than that used in roads (and the asphalt used in modbit roofing is different still). I realize they recycle shingles for road construction, but they are either used as an aggregate or in a small percentage of the road asphalt mix (5%).


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## Windows on Wash (Aug 30, 2011)

shazapple said:


> My assertion was that manufacturers require ventilation. The Asphalt Roofing Manufacturers Association has a technical bulletin talking about roof ventilation and how it is required. http://www.asphaltroofing.org/
> 
> *Actually you said:* _No shingle manufacturer, roof inspector, or qualified roofer would ever tell you that it is ok to insulate against the deck without ventilation. Heat causes breakdown of anything in any application, so why are shingles any different?_
> 
> ...


The long and short is that you claimed heat kills the shingles. Maybe true but only by driving the volatilization of the components. With and average temperature rise (summertime) of only 1.3° in Florida, that is not of any significance and your study indicated that if you modeled it out, it might equate to 11%.

Page 10: _Accordingly, if the average shingle temperature was elevated 
by 2°F, the shingle life expectancy *might *be reduced by 11%_

Mind you, average temp differences were noted at 1.3°. Just using a simple linear relationship would indicate that at 1.3°, the reduction in shingle life *might* be 7%.

Page 9:_ One published reason indicates that attic ventilation keeps shingles from reaching excessively high temperatures and reduces the rate at which oxidation and hydrocarbon volatiles are driven off that make aged shingles become brittle (Terenzio, 1997). The role of temperature versus UV exposure is not well known, although temperature is commonly cited as having a critical role in shingle longevity (Cash and Lyon, 2002). However, experiments shows that ventilation is a lesser factor in resulting shingle temperature than is shingle color or geographic location. Rose (2001) showed that ventilation of a black shingle covered, truss framed roof only reduced temperatures by 2-3% whereas the impact of color was 20 - 30%. FSEC testing at its Flexible Roof Facility (Parker and Sherwin, 1998a) also shows that roof color and reflectivity is a very large effect– and larger than ventilation._

So if color is the larger impact on shingle life, it begs the question again why any roofing manufacturer would sell a dark shingle if they are concerned about shingle life and surface temperatures.

You can do any roof system improperly. I have seen spray foam done wrong just as I have seen vented roof done wrong. My big issue with a vented cathedral ceiling is the sacrificial R-value that is donated to a venting zone. If I were build a home today I would insulate with rigid foam to the top, vented overdeck, and use more traditional insulation materials to the interior with an airtight ceiling.


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## Windows on Wash (Aug 30, 2011)

By the way...if anyone can show me an 30-year shingle that has made it that long...you will be the first.

You will forgive me if I take every shingle manufacturer's warranty claims and exclusions with a grain of salt.

Interesting that the most significant piece of evidence for shingle degradation was...

_Perhaps the most rigorous analysis of the impact of attic ventilation on shingle life comes from work 
sponsored by the *Certainteed Corporation* (Shiao et al, 2003) in which the authors developed a *mathematical model* of cumulative shingle damage to evaluate the kinetics of roofing material degradation. Again, this work showed that fundamental increases to molecular activity were the fundamental driving force in reducing shingle life expectancy. In particularly, the high temperature history in a hot climate was showed accelerated aging of composition shingles. For instance, the higher temperatures in a hot climate such as Miami, were shown to accelerate aging during during winter months at a rate about 10 times faster than in Minneapolis. The effect of attic ventilation was found to reduce the uneven distribution of cumulative damage across the roof deck due to unbalanced thermal regimes in the unvented roof deck as well as to reduce the times at higher temperatures. The *work did not look at how no ventilation and an insulated roof deck would influence the time-temperature history*, but there is little doubt that such an evaluation would find even larger differences in the rate of cumulative shingle aging (Shiao, 2005)._

Again, the point being that region has much more impact that ventilated vs. sealed.


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## Windows on Wash (Aug 30, 2011)

Good read on cathedral ceilings and attic venting.

www.fpl.fs.fed.us/documnts/pdf1999/tenwo99a.pdf

Unvented-cathedralized attics: Where we’ve been and where we’re going

Roof Ventilation Update


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