foam board question

[ratrodder123]

Can I put up plastic sheeting on my garage ceiling joists to keep out any moisture and cold air from the roof vents and then install foil faced foam board to help hold heat in when running a heater out there. The actual roof deck is 12" or more from the ceiling joists.

 

[walrus]

Can I put up plastic sheeting on my garage ceiling joists to keep out any moisture and cold air from the roof vents and then install foil faced foam board to help hold heat in when running a heater out there. The actual roof deck is 12" or more from the ceiling joists.

Aren't the roof vents supposed to vent moisture out of the building, why would you want to stop that?
Foil faced foam is a vapor barrier you don't need the plastic, foam or caulk the cracks between the boards so you have a tight barrier

 

[ptschram]

My experience has shown that plastic stapled to the joists will lead to the roof rotting as there isn't enough air movement to drive the moisture out.

 

[Vvmvbb]

With soffit vents and a ridge vent a believe you can cover the rafters with silver board and air will flow beautifully behind it. This is what a contractor told me and what I am planning to do - hope it's true

 

[ratrodder123]

My experience has shown that plastic stapled to the joists will lead to the roof rotting as there isn't enough air movement to drive the moisture out.
Today 02:26 P[/QUOTE

Why would putting plastic up be any different than putting rolled insulation up and installing plastic. Does putting the foam board up change it to a "double vapor barrier" trapping moisture? If so, I'm doing a good job building moisture traps in my garage. I'm still learning about this whole insulation buisness.

 

[ptschram]

Beats me. I rotted the roof on a hella nice garage years ago by stapling plastic to the rafter.

 

[walrus]

Why would putting plastic up be any different than putting rolled insulation up and installing plastic. Does putting the foam board up change it to a "double vapor barrier" trapping moisture? If so, I'm doing a good job building moisture traps in my garage. I'm still learning about this whole insulation buisness.

As long as you are still venting above the insulation you should be ok. The plastic isn't needed with tight foam board install but if you want it should be toward the heated side of the room, so insulation and then plastic and then sheet rock or whatever with venting above your insulation

 

[trbomax]

Aren't the roof vents supposed to vent moisture out of the building, why would you want to stop that?
Foil faced foam is a vapor barrier you don't need the plastic, foam or caulk the cracks between the boards so you have a tight barrier

My thoughts exactly. I'm a big user and believer in foam board anyway. The foil faced urethane board carries an R of about 9/inch. I wouldnt use less than 1" board in a ceiling application. Attach it with # 9 deck screws and i/4" x 2" fender washers. If you are going to caulk the joints,leave a bit less than 1/8" between all the sheets.Use a flexible caulk (I like Big Stretch) because the building will move with temp/humidity changes and "normal" caulk will crack when it does. Installing 1 x 3 battens over the foam board will support the joints and look good. Paint the battens before putting them up and it will look good too.

edit) you can get the "heavy foil faced" foam board in thicknesses from 1" to 2" and in lengths of 10,12 and 16'. My supplier has 1 x 4 x 16 for $22.40 and 2 x 4 x 16 for $35.20.

 

[brownbagg]

no, moisture be trap by the vapor, its never a good idea to place moisture barrier on ceiling

 

[pattenp]

That sure is a one size fits all answer. May be where you live, but vapor barriers are required here to be on the heated side of exterior walls and ceilings.

no, moisture be trap by the vapor, its never a good idea to place moisture barrier on ceiling

 

[trbomax]

I'm not a believer in vapor barriers for most applictions,I prefer a retarder instead so that the structure can breathe both ways.This is a different application with the foam in close proxsimity to the roof deck and limited attic volume for venting.Full length ridge and soffit vents would be required to do what the op is considering , and ,in this case I would want vb on the warm side.If it were me,I would run purlins on the bottom of the rafters and then attach the foam board to them,thus allowing the rafter cavities to vent to each other as well.

 

[rburke65]

The OP said he wanted to put up a VB on the ceiling joists. He made no mention of rafters. But, I could be confused.

 

[lt1nut]

I was a contractor before I trashed my back, listen to what "trbomax" posted. The vapor retarder/barrier ALWAYS goes to the warmer/hotter side, which is facing towards the inside of your shop. You ABSOLUTELY do not want insulation trapped between two vapor barriers - at the least it will freeze in the winter, in essence turning your building into an igloo. The foil-backed insulation you mentioned has a vapor-barrier built in --- the foil. Tape the joints with foil tape or caulk them when done. Also, PLEASE check the fire rating on whatever insulation you are going to leave exposed, not covered with drywall.

The underside of the roof deck can rot when there is not enough ventilation, preferably through convection. There is a code requirement for how many free-air inches of venting area per "attic" volume, probably best to ask your building department what it is or search on the net, I'm not sure if it changes due to location.

Hope that helps!

 

[Daniel Dudley]

So, no vapor barrier with out insulation on the cold side. Check.

Go ahead and put up silver faced foam board. Tape the joints with clear tape and you are good to go. Thicker is always better, but if you had an inch of foam, you could always top it with blown in cellulose later. It will make a huge difference in your ability to heat the place.

 

[trbomax]

My thread on building walls also has a section on ceilings.