Question for the builders on insulating a log cabin ceiling

[moparguy]

I've started building a small log cabin on some remote property and have a question on which side of the ceiling insulation to place the vapor barrier.



I've always put the plastic on the conditioned space side of the insulation. However, in the cabin, the ceiling will be rough cut lumber nailed to the top of the exposed beams, with 2x6's on edge running the length of the cabin that will serve as lathing for the bright metal roof. Fiberglass batt insulation will be placed between the 2x6's.



My thinking is that I should in this application put the vapor barrier (thinking 6mill black plastic) on top of the insulation in case condensation forms on the bottom of the metal, don't want the insulation getting wet.



So, anyone have any suggestions?



Thanks, Ronnie

 

[Whitmore]

Condensation will form on the bottom of the metal just as you thought, I have in the past used housewrap plastic under the metal as a protectant and have never had any condensate problems inside the house.



Cheers, Kevin

 

[Doc Tinker]

Don't they tend to use spray foam insulation on metal buildings? Maybe that's the way to go.



Doc

 

[moparguy]

Doc, I'd briefly considered trying to use a two part foam system, but will likely go with conventional fiberglass batts.



I've about decided to place the vapor barrier on top of the insulation and not put any plastic on the bottom, I suspect that if moisture did migrate to the area between two sheets of plastic, it could cause the 2x6 lathing to rot in our often damp southern climate.

 

[fkovalski]

Vapour barriers are normally placed on the warm (in winter) side of insulated construction. In warm, humid climates, the barrier may have to be placed on the outer face of construction.



I am trying to envision the order of the construction that you described--

The exposed beams will form the stucture running to the peak in a gabled fashion.

Your rough cut ceiling should run at 90 degrees to this with the 2x6's (on edge) running parralel to the stuctural beams (not the length of the building as you stated?) You will want to have more space between the insulation and metal roof. Might consider using 2x8's and use ridge and soffit vents.



I honestly believe that you will want to use TWO barriers. One should definetly be on the warm side of the insulation. Any moisture generated from the inside will condense against a cold surface thus the need for it placed inside. One placed one the otherside of the insulation will be dictated by how humid your area is and how well you can vent the cavity above the insulation. If it is very humid and you get a cold snap to condense moisture, it could settle on the upper side of the insulation between it and the underside of the metal roof. Thus you would want a barrier located here too.



One other comment- Do you get hail where you live? It could dent the metal roof. Might consider adding OSB sheathing with felts then applying the metal roof. This allows for no voids under the roof should an oblect hit it. And, you might not need a second moisture barrier and it would be closer in set up as a conventional roof system with the metal taking the place of ashalt shingles. Just my opinion.



Where I live, it is so dry that we only spec vapour barriers on the warm side if the insulation.

 

[moparguy]

fkovolski said:



I am trying to envision the order of the construction that you described--



The exposed beams will form the structure running to the peak in a gabled fashion.



That's correct.



Your rough cut ceiling should run at 90 degrees to this with the 2x6's (on edge) running parallel to the structural beams (not the length of the building as you stated?)



No, actually the current plan is to run them parallel to the building or 90 degrees to the beams on top of the lumber that will form the ceiling. I'll add ties to prevent the 2x6 from rolling or trying to lay down.



The reason for running 90 degrees with the 2x6s is it eliminates the need for additional lathing normally used in this type simple construction.



And I had considered 2X8s but since I'm only planning on using 4" fiberglass, I thought 2" (1 1/2")of dead air space above the insulation would be sufficient.



I will use ridge vents and a soffit type arrangement.



According to my local metal supplier, the 26ga. metal I'll use can span up to 6' in our area with acceptable snow load capability. I am planning on a 4' span between the 2x6's.



The roof will be on a 12/12 pitch so I can have a small sleeping loft on one end of the cabin.



And we do have the occasional hail event but I'm counting on the 26ga. metal to be able to handle it.



This cabin will only be used as a weekend getaway. I'm not going to improve the fair-weather road that ends at the site, it'll be a ride the horses in and listen to the owls and whip-or-wills on the front porch kind of place.



And I've given the two barrier possibility a lot of thought, my concern is mosture getting between the two and causing a problem.



Prehaps a barrier such as Traveck (?) that can breath on the warm or bottom side.



I appricate all the input, thanks and keep it coming.



Ronnie

.

 

[fkovalski]

No, actually the current plan is to run them parallel to the building or 90 degrees to the beams on top of the lumber that will form the ceiling. I'll add ties to prevent the 2x6 from rolling or trying to lay down.

By doing this method, will you be able to vent properly (ie- convection from soffit to ridge?)



Venting in the direction of the 2x6's that you describe would mean soffit vents at the gable ends which might not vent properly (or as well as you might want. ) Unless there are enough seams (are you using standing seam metal roofing?) to allow upward convection. If you are relying on the gaps in the metal roof to allow for (warm)air passage, it could still potentially condense on your batts.

And I've given the two barrier possibility a lot of thought, my concern is mosture getting between the two and causing a problem.

If it properly vented, moisture should not be a problem between the two.

Prehaps a barrier such as Traveck (?) that can breath on the warm or bottom side.

I really do not think that you want a breathable barrier on the warm side. Moisture (from cooking, tracked in snow & rain on clothes, shower if any etc... ) will evaporate only to condense with cold as in the manner you propose. And... as we know, batts are not woth a damn when soaked.

 

[moparguy]

fkovalski, you're right, I do need to run the 2x6's (or 8's) parallel to the roof beams to insure proper venting.



The only additional material this will require is a few 1x4's or even 2x4's for lathing. Not a major change but I now see a necessary one.



I was going to rely on the ribs in the metal roofing for venting, but on reflection, that would be marginal at best.



Thanks for the thought provoking comments.

 

[daveshoe]

The best way I have seen to do this is to build a ''Cold roof system'' on top of the exposed ceiling. First, frame and insulate the roof and add a deck on the top -- just as if you were going to nail on an asphalt shingle roof. You can insulate this thing tight with no air space--just like a wall. Place heavy tar paper on top of the deck to catch any drips from the metal roof. Add 2x2's from eave to ridge [bottom to top ] of the roof. Then add purlins [ nailers ] at 90 degrees to the 2x2s to give you the surface to nail the roofing to.

Get ventilating eave starter or devise an intake vent for the area between the 2x2's. Ridge vent the top [ridge] and a constant airflow on the underside of the roof will not allow condensation to occur. Ice dams also will not be a problem. This system also works for asphalt shingles.

The main idea of this is that since there is no attic space to buffer against radiant heat, you add the air space at the roof level. The bad news is that plywood decking is very expensive right now. Hope this helps.