Thread: Garage Rafter Insulation

Hi

I am new to Inspection News. I am not a home inspector, I am a DIY. I hope Inspection News will answer a DIY question.

My current project is to insulate my garage ceiling.

Garage Information:
Location: St Louis County, Missouri
Size: 21'x21'
Structure: 2x6 roof rafters 16" OC only three collar ties hold the walls togeather.
House Age: 40 yrs

I want to attach with screws 3" or 2" rigid foam insulation to the roof rafters.

MY QUESTION IS?
Does code require me to put something over the rigid foam insulation (drywall....plywood....) or can I just leave the ridgid foam insultaion exposed on the garages cathedral ceiling.

Thanks in advance
Frank

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can I just leave the ridgid foam insultaion exposed on the garages cathedral ceiling.

FA: Your garage has a cathedral ceiling?

My Garage has no Ceiling. The roof rafters are all you see if you look up. I priced installing Ceiling joists but kind of expensive. That is why I would like to put up 4'x8' rigid foam on th rafter to insulate.

Frank

Is the garage heated?

Is it attached to the house?

Generally, foam insulation should be covered by a fire rated material. If the garage is heated or you plan to heat it you'll need to create an attic space with the proper vapor barrier between the garage and insulation and you'll need to have proper ventilation in that attic space.

If you put the insulation in like you propose and have a heat source in the garage you're going to create a lot of condensation on the roof sheeting.

Originally Posted by Frank Albanello

Hi

I am new to Inspection News. I am not a home inspector, I am a DIY. I hope Inspection News will answer a DIY question.
My current project is to insulate my garage ceiling.
Garage Information:
Location: St Louis County, Missouri
Size: 21'x21'
Structure: 2x6 roof rafters 16" OC only three collar ties hold the walls togeather.
House Age: 40 yrs
I want to attach with screws 3" or 2" rigid foam insulation to the roof rafters.
MY QUESTION IS?
Does code require me to put something over the rigid foam insulation (drywall....plywood....) or can I just leave the ridgid foam insultaion exposed on the garages cathedral ceiling.
Thanks in advance
Frank

Frank,

I realize that this does not necessarily answer your question, but you need to provide ventilation for the rafter bays if you are insulating the underside of the rafters. In this case, I believe that the best bet would be to drill holes (generally 3 - 2 inch holes in each) in the frieze blocks at the eaves and install a screen to keep out bugs. Then, remove the ridge cap and cut a slot down the ridge so you can install a ridge vent. This will allow free air flow in the rafter bays that will be covered with the insulation. If you fail to provide adequate ventilation, you could end up with moisture condensation on the hidden framing, which could rot out your roof. In addition, the ventilation will cool the rafter bays which will help to prevent prematurely killing your roof shingles (assuming that it is a shingle roof).

Gunnar and Ken

Thank You for your response

Ken
- The garage is NOT heated Normally. I would like to warm it up occasionally so I can work in there from time to time.

- The garage IS attached to the house

Ken and Gunnar
- Sofit vents AND a Ridge vent are already installed. So if I fasten the Rigid Foam to the bottom of the Rafter with screws and seal the seams with foil tape and spray foam insulation. There should be 6" of free air space for ventilation between the roof sheeting and insulation. So Ken's "Attic space" would be the 6" of space between the roof sheeting and rigid foam insulation, right?

Thanks again
Frank

Oh one other thing!

This is a single story home. No living space above the garage roof line.
Frank

Originally Posted by Frank Albanello

Oh one other thing!

This is a single story home. No living space above the garage roof line.
Frank

FA: Insure that you have a minimum of 1/2" drywall separating the garage and the rest of the house, and the space above the garage from the space above the remainder of the house.

Install the panels of rigid foam perpendicular to the rafters. Install a soffit vent in the soffit between each set of rafters and on each side of the garage for ventilation. If you have a ridge board, drill a 1" hole in the center of the ridge between each set of rafters.

Be happy.

Originally Posted by A.D. Miller

FA: Insure that you have a minimum of 1/2" drywall separating the garage and the rest of the house, and the space above the garage from the space above the remainder of the house.

Install the panels of rigid foam perpendicular to the rafters. Install a soffit vent in the soffit between each set of rafters and on each side of the garage for ventilation. If you have a ridge board, drill a 1" hole in the center of the ridge between each set of rafters.

Be happy.

ADM: There is already drywall on ALL the WALLS of the garage. There is NO living area above the roof rafters of the garage.

Since there is no living area above the garage are you saying rigid foam insulation on the roof rafter is acceptable and will pass a code inspection

Frank

Foam insulation is relatively hard to ignite but when ignited, it burns readily and emits a dense, black, smoke containing many toxic gases. The combustion characteristics of foam insulation products vary with the combustion temperatures, chemical formulation, and available air.
Because of the dangers described above, foams used for construction require a covering as a fire barrier. One half-inch thick gypsum wallboard is one of the most common fire barriers. Some building codes, however, do not require an additional fire barrier for certain metal-faced laminated foam products. Check with your local building code/fire officials, and insurers for specific information on what is permitted in your area.

Originally Posted by Frank Albanello

ADM: There is already drywall on ALL the WALLS of the garage. There is NO living area above the roof rafters of the garage.

Since there is no living area above the garage are you saying rigid foam insulation on the roof rafter is acceptable and will pass a code inspection

Frank

FA: The only person who can "pass" your installation is your building official. Most of them will be happy to help you if you just give them a call. Even if you are in an unincorporated area you can call the nearest town's building inspection department and they will set you on the right path.

Frank, can you put some photos up? How high is the ceiling at the highest point? Adding some ceiling joists onto the rafters would make insulating easier, and greatly reduce the amount of heat needed to make the garage more comfortable. If you moved them up to where a 16' span would make it across, it's a pretty easy project.

AD and KD
Thank you for your response. It does not sound like my idea will work base on what I hear you saying and I don't thing the 2x6 Rafers are enough to support drywall. I did not think the county building inspector would be available for questions to help come up with a solution. But I will have to give them a call I guess.

KR
Your suggested alternative (16' span) is a interesting idea. I will have to see what the pitch of the roof is to determine how high I would have to go and I will get some pictures. Currently there are (3 collar ties/Joists ?) that hold the walls togeather. They are about 10' off the floor.

Thank
Frank

Originally Posted by Frank Albanello

MY QUESTION IS?
Does code require me to put something over the rigid foam insulation (drywall....plywood....) or can I just leave the ridgid foam insultaion exposed on the garages cathedral ceiling.

The foam needs to be protected if installed as you are describing it.

From the IRC. (underlining and bold are mine)
- R314.4 Thermal barrier. Unless otherwise allowed in Section R314.5 or Section R314.6, foam plastic shall be separated from the interior of a building by an approved thermal barrier of minimum 0.5 inch (12.7 mm) gypsum wallboard or an approved finish material equivalent to a thermal barrier material that will limit the average temperature rise of the unexposed surface to no more than 250°F (139°C) after 15 minutes of fire exposure complying with the ASTM E 119 standard time temperature curve. The thermal barrier shall be installed in such a manner that it will remain in place for 15 minutes based on NFPA 286 with the acceptance criteria of Section R315.4, FM 4880, UL 1040 or UL 1715.

The foam would not need to be protected if it met the following:

From the IRC. (R314.5 is a long section this is the part which applies to your question)
- R314.5.3 Attics. The thermal barrier specified in Section 314.4 is not required where attic access is required by Section R807.1 and where the space is entered only for service of utilities and when the foam plastic insulation is protected against ignition using one of the following ignition barrier materials:
- - 1. 1.5-inch-thick (38 mm) mineral fiber insulation;

- - 2. 0.25-inch-thick (6.4 mm) wood structural panels;
- - 3. 0.375-inch (9.5 mm) particleboard;
- - 4. 0.25-inch (6.4 mm) hardboard;
- - 5. 0.375-inch (9.5 mm) gypsum board; or
- - 6. Corrosion-resistant steel having a base metal thickness of 0.016 inch (0.406 mm).
- - The above ignition barrier is not required where the foam plastic insulation has been tested in accordance with Section R314.6.

Basically, the exception is not applicable ... unless the foam has been tested in accordance with R314.6:
- R314.6 Specific approval. Foam plastic not meeting the requirements of Sections R314.3 through R314.5 shall be specifically approved on the basis of one of the following approved tests: NFPA 286 with the acceptance criteria of Section R315.4, FM4880, UL1040 or UL1715, or fire tests related to actual end-use configurations. The specific approval shall be based on the actual end use configuration and shall be performed on the finished foam plastic assembly in the maximum thickness intended for use. Assemblies tested shall include seams, joints and other typical details used in the installation of the assembly and shall be tested in the manner intended for use.

Which means you would need to check and see if the foam insulation has been tested to one of the above stated criteria.

Adding ceiling joist would not only make it easier to insulate, but will help keep the walls from spreading when you add the additional weight of the drywall/osb/plywood......

Frank,

If you are just looking to add some insulation to your garage, you could just install unfaced fiberglass batts against the rafters supported by wire. This would give you the desired insulation, and fiberglass does not burn. However, if the fibers become airborne it would cause itching.

Just emphasizing the key word:

Originally Posted by Gunnar Alquist

install unfaced fiberglass batts

Originally Posted by Gunnar Alquist

Frank,

If you are just looking to add some insulation to your garage, you could just install unfaced fiberglass batts against the rafters supported by wire. This would give you the desired insulation, and fiberglass does not burn. However, if the fibers become airborne it would cause itching.

Don't forget the baffles.

For further nitpicking confusion:

http://www.icc-es.org/criteria/pdf_files/ac12.pdf

Thank to all for your response

JP
Thanks for the cut outs from the actual regulation. Does not appear my idea will pass code inspection.

CM
If I add the ceiling joist then I don't need to install rigid foam on the rafters, I can just insulate the ceiling.

GA
This would be a work area and unfaced insulation would allow fibers to get in the air so I don't think that would pass either.

ADM
Thanks for the actual regulation and thanks for marking the sections in red for me. But reading it gave me a big headack. Based on what I read the rigid foam insulation has to be covered with one of the surfaces listed. SO MY IDEA WILL NOT PASS A CODE INSPECTION.

Based on everyones comments I am going to have to go back to a add ceiling joist solution. That is actually the one I prefer but I was just trying to save some money with a simpler (less expence) solution. Something I could do myself.

Thanks to everyone again.
Frank

Frank, I like your original idea (with caveats). A vaulted ceiling in the garage makes a lot of sense for storing things high and out of the way. If you work on your car, motorcycle, etc., it gives you room to install a hoist.

It would seem that you shouldn't be so hesitant in calling whatever municipality governs building questions. They are the only folks who can tell you specifically what they would like to see done.

I suspect they will tell you that the foam should be covered with DW and be fire-taped. You have already thought about the venting, etc.. As to any additional support, it's the local guys who will tell you this. Remember however, none of these things are free, cheap or without a fair amount of labor involved. Before you commit to the project, be prepared for the expense and the work.

This forum is a great place for ideas, but remember, we are "red flaggers" and natural born skeptics. Shoot, we could probably find dozens of reasons a house should not have a stove/range in a house, for safety reasons. Good building!!!

Hi TR

I kind of like the idea also, for all the same reasons. I am no stranger to work. I enjoy working on my own house but the biggest reason is sweat equity labor is less expensive and the end result is generally better if you do it yourself.

It is a small garage the ridge is only 18 feet over the floor. So it would be a sweat equity job to just put up 4 by 8 sheet of ridgid foam insulation. The bids I have been getting to put up ceiling joist and drywall have been around $2500. If I have to put up Drywall on top of the ridgid foam, I don't think the 2x6 roof rafters are enough to support that weight. If that is the case I may as well just go the add ceiling joists, insulate and drywall. Putting up the ceiling joists is I job I don't think I can do.

I have attached a picture of what I have to work with. There are 3 (collar ties/ceiling joists)???? only two show in this picture.


Your comments are appreciated

Frank

Frank, those would be rafter ties, not collar ties.
Have you considered having spray foam installed rather than foam boards?
I think some of it passes the smoke/flame spread test but you would have to have the AHJ sign off on the specific product.
The foam is no DIY job but might not be any more expensive than some of your other options.

Hi JL

Thanks for your correction. I was not sure what to call them. How can I tell them apart? Is there a simple definition for Collar tie, Ceiling Joist and Rafter tie?

Your suggestion on the spray foam insullation is a interesting idea, I will look into it (cost, drawbacks....AHJ sign off).

Thanks for your comments

Frank

Originally Posted by Frank Albanello

Hi JL

Thanks for your correction. I was not sure what to call them. How can I tell them apart? Is there a simple definition for Collar tie, Ceiling Joist and Rafter tie?

Your suggestion on the spray foam insullation is a interesting idea, I will look into it (cost, drawbacks....AHJ sign off).

Thanks for your comments

Frank

Here are some diagrams that might be helpful.

OK JL thanks for the pictures

What I see is Ceiling Joist and Rafter tie are basicly the same if they are at the bottom of the rafter. If they move up the rafter they become Rafter ties and if they are in the upper 2/3 of the rafter they are called collar ties. Thats probable putting it to simply but I think what I am saying is correct.

So I would need to have more Ceiling joists installed if I go that route. The spray foam insulation route I will look into its cost and pitfalls. The only pitfall to spray foam I can think of is if there was a roof leak you may not find out about it till it was to late and if you had to replace sheathing it would be more difficult to remove the sheathing and then it would be a problem to reinsulate. I will check it out.

Thanks
Frank

Originally Posted by Frank Albanello

.............and the end result is generally better if you do it yourself.

Frank,
As a general rule I would have to disagree with this statement, I have seen the end results of a lot of DIY projects that ranged from from ugly to dangerous.

I am not necessarily including you in this category because you are doing your homework and say you know your limitations. The AHJ (at least in my area) will not make recommendations, but will only let you know if what you are doing is acceptable or not.

I see that you have garage door opener(s) to work around, you also have load issues weather you are adding the weight to the rafters or the ceiling joist, which appear to be 2x8's and would (most likely) not be sufficient for a 21' span.

I realize that budget is an issue, but paying someone to show you your options and what you need to do would be money well spent in the long run.