Home>Discussions>INSULATION & HVAC>Is A Vapor Barrier needed over my Exposed Joist Ceiling?
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Is A Vapor Barrier needed over my Exposed Joist Ceiling?

>Do I need to install a vapor barrier over an exposed joist ceiling before spraying open cell foam insulation?

> I have constructed a small 16' x 21' living quarters in an open bay, in one corner of my metal (shop) building. The metal building has double bubble double faced radiant barrier in the roof (and walls). The living quarters has 2" x 4" stud wall construction, sheathed w/ 1/2" OSB, wrapped w/ 30# felt and sided w/ 3/4" western red cedar. The ceiling has exposed 3" x 8" reclaimed Douglas Fir beams with 3/8"- 4' x 8' sheet T1-11 rough siding, installed as the ceiling material. I have installed a 2" x 6" joist sub-floor over the ceiling, as I intend to use the overhead space for light/medium wt. storage. I intend to fill the sub-floor joist system w/ 0.5 lb. open cell urethane foam and then deck over with 1/2" plywood.

>The attic of the living quarters is closed on one end and the two sides. The remaining end has been left open for access to the overhead storage. The open end is still protected from the elements because the common roof line extends out an additional 21 ft. past the open end.

> QUESTION - Do I need to install a vapor barrier on the ceiling before spraying in the open cell urethane foam. I asked my local insulation contractor about this an he seemed to be unsure about this. He seemed to think that a vapor barrier wouldn't be needed in this situation. I want to make double sure whether a vapor barrier is needed in this construction. I am located in East, TX zip 75766, DOE climate zone 3. We have high humidity levels year round in my area. The humidity averages in the 80 - 90% range for the mornings and in the 60% for the afternoons. I do not want to create any moisture problems with the possibility of mildew/black mold in this ceiling/sub-floor system.

>I will heating/cooling this living space with a 12,000 Btu Mini-Split heat pump and an Aux. 10,000 Btu LP radiant space heater.

Any experienced advice about my project would be greatly appreciated. -:confused: Thanks, Perplexed in Texas

Re: Is A Vapor Barrier needed over my Exposed Joist Ceiling?

A vapor barrier should not be applied over the joists before spraying on foam. Typically a vapor barrier is placed on the warm side, which is generally considered the interior. The idea is to prevent moisture from migrating to the cold side then condensing. A VB on the outside would trap moisture that would condense and have no place to go.
That said I'm not sure a VB is required with open cell foam, but if I did one I would place it on the inside.
Besides, if the VB is over the joists and subfloor the foam would be supported by the vapor barrier and not the wood in the space where it was sprayed. I'm not sure the foam would want to stay in place before it was sheathed over.

Re: Is A Vapor Barrier needed over my Exposed Joist Ceiling?

Your walls will need a vapor barrier inside of the insulation, but the ceiling will not. The walls need to have more ventilation on the outside of the insulation in order to keep it dry. The siding really restricts the ventilation so you need even more restriction on the inside, hence the vapor barrier.

The attic has a lot of ventilation because it is open. The ceiling will restrict ventilation from below the insulation, therefore even with out a vapor barrier, you have more ventilation on the outside of the insulation than on the inside. This will keep it dry.

The short answer is no.

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