Looking for advice. Vapor barrier or not?

[speedwaytaco160]

I am having a new metal roof installed on my 36x40 detached garage. I am having a radiant barrier installed under the new metal roof. I live in west TN on the KY border. I have a product called "air mate" currently under my existing metal roof. Under that I have batt insulation that is stapled to my 2x6's.

My old metal roof roof was installed incorrectly so it was leaking in a few places. I am going to replace the insulation that was water damaged and then I'm wanting to install something to keep the insulation from falling (its stapled), and to cover it all up (look better)

I am confused on which product I would use because I don't want to cause my batt insulation to mold. My garage is heated with propane but it's not cooled during the summer time. Would you recommend:

1.Non perforated radiant barrier
2.Perforated radiant barrier
3. 6 mil plastic vapor barrier. I would choose a solid color to cover up the insulation.


Thank you for any info.

 

[K'ledgeBldr]

If you're doing a hidden fastener metal roof, I'd just spray foam it. Metal roof doesn't have to be vented, so no reason to re-invent the wheel.

 

[btdobie]

A properly sealed vapor barrier would be a good bet to keep the batts dry IMO. If you have a little budget to spare, consider using mineral wool insulation. Mineral wool has the advantage of not being damaged by water. If it gets wet, all you have to do is dry it out and it regains it's original R-value without the need to replace. If you have a lot of budget to spare go with closed cell spray foam as previously suggested.

 

[speedwaytaco160]

I currently have 1500 sq ft of batt insulation already installed. spray foam is out of my budget and tearing it all out is also out of the budget. Should have been more specific.

 

[btdobie]

Ok I didn't realize you already had the batts up. I would still go with a vapor barrier. Condensation can soak that insulation just as quick as a leaky roof if there is a lot of humidity.

 

[speedwaytaco160]

So would you recommend a 6mil plastic ? What is the difference between the non perforated radiant barrier and 6mil plastic vapor barrier? BTW thanks for replying Sir.

 

[K'ledgeBldr]

Putting a radiant barrier over the batt isn't going to do anything.
And since the batts already have a vapor barrier- no need to add another.

That's why I suggested the foam- not only do you get an R-value of your choice, but you also get a vapor barrier, and air infiltration barrier. Air infiltration is actually more of a problem than radiant heat.

Suffice it to say that you really can't improve on what you already have in-place.

 

[speedwaytaco160]

My main purpose is to hide the insulation batts. I would hate to use a radiant barrier and cause the paper faced batt insulation to mold or get wet for some reason.

 

[K'ledgeBldr]

My main purpose is to hide the insulation batts. I would hate to use a radiant barrier and cause the paper faced batt insulation to mold or get wet for some reason.

Change your fuel source for heat.
NG, propane, used motor oil, all produce quite a bit of moisture/vapor. Wood on the other hand, practically no water.

Just a thought!

 

[travisn1]

Just do it right and install liner panels. It's the same as exterior sheet metal except without any corrosion resistance so a little cheaper. About $0.60-$0.80/ sq foot plus J trim ($0.80/ft) and screws ($35/250 ct).

OSB is $0.30 per sq foot but then you should paint it. Plus it's heavy and is not forgiving on alignment, gaps, length tolerances.

The paper backing on the insulation is a form of vapor barrier and I've read having a double vapor barrier is detrimental.

Do you have soffet vents and a ridge vent? Did you install these? https://www.homedepot.com/p/ADO-Pro...h-Built-In-Baffle-10-Ctn-UDVB234610/202389935

 

[kbs2244]

Propane or NG will be fine for heat is it is vented.
If not vented you need a vapor barrier.

 

[speedwaytaco160]

I appreciate all the other suggestions fellas, I am just going to go with either perforated radiant barrier or sold radiant barrier.

The guy at atticfoil.com just told me to use either one. Now I am more confused than before.

 

[DC73]

I appreciate all the other suggestions fellas, I am just going to go with either perforated radiant barrier or sold radiant barrier.

The guy at atticfoil.com just told me to use either one. Now I am more confused than before.

Radiant barriers are seldom cost effective - and by seldom, I mean almost never. Once you air seal and get the structure properly insulated, any additional heat gain/loss that can be stopped by the radiant barrier doesn't make financial sense. You'd likely be better off putting the money for radiant barriers into additional insulation.

Go to GreenBuildingAdvisor.com and post your question on their Q&A forum. There are building science gurus who hang out there and can help you get this right. Get the details of this installation wrong and you are setting yourself up for mold.

DC

 

[speedwaytaco160]

Radiant barriers are seldom cost effective - and by seldom, I mean almost never. Once you air seal and get the structure properly insulated, any additional heat gain/loss that can be stopped by the radiant barrier doesn't make financial sense. You'd likely be better off putting the money for radiant barriers into additional insulation.

Go to GreenBuildingAdvisor.com and post your question on their Q&A forum. There are building science gurus who hang out there and can help you get this right. Get the details of this installation wrong and you are setting yourself up for mold.

DC

I appreciate the reply Sir.