R12 insulation for ceiling??

[green manalishi]

Finishing up a garage reno/extension on a pre existing structure, and roof insulation is proving to be a bit of a dilemna. The garage is less than 400sq/ft (32x12) and will be heated by the wood stove pictured, as well as a 220volt electric construction heater as needed. Because it is an existing and older structure, the walls and ceiling are all 2x4 construction. I have already installed r12 batts with vapor barrier in the walls, as well as insulated the garage door (it will be getting a layer of reflectix over top the foamboard as well). I had originally planned to create a ceiling, but the garage is small and I have decided I would like to keep the ceilings as they are for max room for potential storage and feeling more open. The plan is to run rafter vents 1/2" from the roof, insulate with r12 batts to the peak, vapor barier, and then cover that completely with reflectix. The 1/2" gap should be sufficient as the soffits are fully vented, but my concern is keeping things warm. I am in Canada, and plan to spend a good amount of time out here in the winter evenings and weekends- half of me says some insulation in the roof is better than none and a wood stove with insulation will keep me toasty in 400sq/ft, and the other half of me says it won't be enough.

Anybody heating and older 2x4 framed garage with r12 on the walls and ceiling? How efficient is it? I feel like im over thinking this, but also dont want to regret my decision. This whole project has been diy and also on a budget. The wood heating is going to be free, but that said, I dont want to burn through wood at a much faster rate because of my ceiling insulation choice. Any experience or input is greatly appreciated!

 

[slimpickins]

I'm in Saskatchewan so I'm familiar with the cold. Here's my comments....
* R12 is very thin for the ceiling insulation. I'd go at least R20 or higher. At R12 you WILL have ice dams on the roof. R20 may be minimum spec for code.
* Reflectix is not code for insulation and AFAIK it is not fireproof either. Bad idea for a garage, and particularly with a wood burning stove.
* A 1/2" for attic venting is probably not enough. Particularly if you plan to park a vehicle in there during the winter. Snow melting off a vehicle creates a lot of humidity. That humidity WILL get into the vent space. How are you venting this at the ridge??? Without at attic you need a continuous ridge vent or you vent every rafter space on both sides of the ridge.
* Wood burning stove- better check your insurance rates. With any "solid fuel" heating system, your insurance may cost more than what you think you'll save. Running a gas line might be less expensive.
* Also, a wood burning stove needs lots of fresh air supply. Plan for a special combustion air vent to avoid creating a vacuum in your garage and sucking smoke and co2 down the chimney.
* IMHO any diy insulated overhead door is only about 50% as good as a factory insulated door. This is another big heat loss for a garage

 

[green manalishi]

I'm in Saskatchewan so I'm familiar with the cold. Here's my comments....
* R12 is very thin for the ceiling insulation. I'd go at least R20 or higher. At R12 you WILL have ice dams on the roof. R20 may be minimum spec for code.
* Reflectix is not code for insulation and AFAIK it is not fireproof either. Bad idea for a garage, and particularly with a wood burning stove.
* A 1/2" for attic venting is probably not enough. Particularly if you plan to park a vehicle in there during the winter. Snow melting off a vehicle creates a lot of humidity. That humidity WILL get into the vent space. How are you venting this at the ridge??? Without at attic you need a continuous ridge vent or you vent every rafter space on both sides of the ridge.
* Wood burning stove- better check your insurance rates. With any "solid fuel" heating system, your insurance may cost more than what you think you'll save. Running a gas line might be less expensive.
* Also, a wood burning stove needs lots of fresh air supply. Plan for a special combustion air vent to avoid creating a vacuum in your garage and sucking smoke and co2 down the chimney.
* IMHO any diy insulated overhead door is only about 50% as good as a factory insulated door. This is another big heat loss for a garage

I appreciatethe insight, especially from another individual north of the border in the cold. Because this is an old construction garage, its built on 24" centers with 2x4 construction, even the roof. I have thought about sistering another 2x4 to each rafter so that I can run thicker insulation, but I will lose a bit of room and it involves more work and money....although I did not consider ice damning with only r12.

As for the reflectix, it was going to be on the door and on the ceiling just to help reflect the heat back, not so much as actual insulation as I know it really has no r value. Around the stove in the ceiling i was not going to run it, and stove and garage door are complete opposites to eachother. The stove will be getting a steel barrier around it to decrease distance to combustibles, with the plan being to keep the fire in the stove so that insurance doesnt have to know.

As for the roof venting, other then my fully vented soffit, there is nothing. I guess i thought the air gap on both sides of the roof meeting at the peak would just allow the air to flow back down the other side per say. I guess i will have to add some roof vents then..

 

[slimpickins]

Your other alternative to venting is closed cell spray foam which will eliminate the need for a vapour barier and venting, but definitely requires a fire barrier, such as gyproc. This is sprayed directly on the underside of the roof sheathing.
Also, FYI, for 24" centers the minimum thickness of gyproc is 5/8" otherwise it sags too much.
I also note that the tie-joists of the roof/ceiling structure (the lowest cross member) appear to be 2x4s. Just in case you are thinking about changing your mind on where to put the ceiling, those tie-joists do NOT look strong enough to support a ceiling - certainly not the minimum 5/8 gyproc required for the 24" spacing of the rafters. The rafters themselves would probably sag under the weight of insulation and gyproc as well. You could try to jury rig/make up a truss arrangement to better support the roof and/or ceiling, but then you're into some really sketchy non-approved, non-code construction that will void your insurance ...

 

[green manalishi]

Your other alternative to venting is closed cell spray foam which will eliminate the need for a vapour barier and venting, but definitely requires a fire barrier, such as gyproc. This is sprayed directly on the underside of the roof sheathing.
Also, FYI, for 24" centers the minimum thickness of gyproc is 5/8" otherwise it sags too much.
I also note that the tie-joists of the roof/ceiling structure (the lowest cross member) appear to be 2x4s. Just in case you are thinking about changing your mind on where to put the ceiling, those tie-joists do NOT look strong enough to support a ceiling - certainly not the minimum 5/8 gyproc required for the 24" spacing of the rafters. The rafters themselves would probably sag under the weight of insulation and gyproc as well. You could try to jury rig/make up a truss arrangement to better support the roof and/or ceiling, but then you're into some really sketchy non-approved, non-code construction that will void your insurance ...

Thanks for the tips and insight. Those cross joists at the lowest point are sistered 2x4s. Im no engineer, so we just copied the section of the garage that was original before the addition. Also, the new plan is now sistered 2x4s under each rafter so that I can run r22 batts with a 1.5" air gap, as well as a flat spot near the top spaced properly and supported via 2x4 to allow venting from one side of the garage to the other via the soffits. I know the way I am explaining it may not make the most sense, but we have some warm weather this weekend and I am hoping to crank it out then and I will post some pics. Should follow code, give proper roof ventilation, and allow me to install some covering over the vapor barrier at some point if not before this winter.

 

[racecougar]

Also, the new plan is now sistered 2x4s under each rafter so that I can run r22 batts with a 1.5" air gap, as well as a flat spot near the top spaced properly and supported via 2x4 to allow venting from one side of the garage to the other via the soffits.

Venting doesn't work that way. You'll need to add a ridge vent.

 

[rancherbill]

I appreciatethe insight, especially from another individual north of the border in the cold. Because this is an old construction garage, its built on 24" centers with 2x4 construction, even the roof. I have thought about sistering another 2x4 to each rafter so that I can run thicker insulation,.....

You do not have to sister. Rip a 2x? into 2" strips, nail then on and you have a 2x6 or bla bla and get a 2x8. We did an old 2x4 house and made it 2x"6" and then insulated, it worked well and was easy to do.

 

[slimpickins]

Thanks for the tips and insight. Those cross joists at the lowest point are sistered 2x4s. Im no engineer, so we just copied the section of the garage that was original before the addition. Also, the new plan is now sistered 2x4s under each rafter so that I can run r22 batts with a 1.5" air gap, as well as a flat spot near the top spaced properly and supported via 2x4 to allow venting from one side of the garage to the other via the soffits. I know the way I am explaining it may not make the most sense, but we have some warm weather this weekend and I am hoping to crank it out then and I will post some pics. Should follow code, give proper roof ventilation, and allow me to install some covering over the vapor barrier at some point if not before this winter.

It won't vent from one soffit to the other. It just won't, regardless of the air gap you leave above the insulation. Hot air rises. The sun heats the roof. The air in the gap you leave will be full of hot air and will be trying to rise. Both sides will be venting toward the peak. You need to vent the peak or at least both gable ends if this "flat spot" you describe will form a tiny attic. I would put a small fan at both gable ends to exhaust the hot air, and control them with a temperature switch that switches the fans on if there is more than a 10 or 15 degree difference between the attic and outside temps.

The purpose of venting twofold: First is to keep the roof surface as cold as the outdoor air. This is what prevents ice dams. Second is to vent any moisture that gets into the attic so it will dry out. Both of these require good air flow! Spray foam may be your best option if you can't or won't provide adequate venting. It seals to the underside of the roof sheathing so moisture cannot condense on the underside of the roof. And with the insulation directly on the underside of the roof, (assuming has adequate R-value) the roof will stay very close to the outside temperature, so no ice dams.

Even with spray foam there can still be a sufficient temperature differential between the insulated roof and uninsulated portion over the eaves that can cause ice dams in the right conditions. Depending where you are, even with spray foam you could possibly still need some sort of heating cable to prevent ice-dams.

 

[Sumboodie]

I'm in Saskatchewan so I'm familiar with the cold. Here's my comments....
* R12 is very thin for the ceiling insulation. I'd go at least R20 or higher. At R12 you WILL have ice dams on the roof. R20 may be minimum spec for code.
* Reflectix is not code for insulation and AFAIK it is not fireproof either. Bad idea for a garage, and particularly with a wood burning stove.
* A 1/2" for attic venting is probably not enough. Particularly if you plan to park a vehicle in there during the winter. Snow melting off a vehicle creates a lot of humidity. That humidity WILL get into the vent space. How are you venting this at the ridge??? Without at attic you need a continuous ridge vent or you vent every rafter space on both sides of the ridge.
* Wood burning stove- better check your insurance rates. With any "solid fuel" heating system, your insurance may cost more than what you think you'll save. Running a gas line might be less expensive.
* Also, a wood burning stove needs lots of fresh air supply. Plan for a special combustion air vent to avoid creating a vacuum in your garage and sucking smoke and co2 down the chimney.
* IMHO any diy insulated overhead door is only about 50% as good as a factory insulated door. This is another big heat loss for a garage

Agreed, aside from CO the stove makes is the concern.

 

[Sumboodie]

You do not have to sister. Rip a 2x? into 2" strips, nail then on and you have a 2x6 or bla bla and get a 2x8. We did an old 2x4 house and made it 2x"6" and then insulated, it worked well and was easy to do.

Wouldn't be a bad idea to sister 2x6. 2x4 24" OC is fairly lightweight for a roof, even when it's in a truss.

 

[rancherbill]

Wouldn't be a bad idea to sister 2x6. 2x4 24" OC is fairly lightweight for a roof, even when it's in a truss.

There are a ton of houses here with 2x4 rafter that are 100+ years old with 24 inch centers. My Garage has engineered trusses 30' long out of 2x4, it's all in the design.

In Canada 24" exterior walls were common now they must be 16"

 

[nickz]

You are going to have a ton of very warm air up there , you will be amazed and annoyed at the temp difference . I would suggest a ceiling, it will be way more efficient.