insulating cathedral ceiling

[rob smith]

I am building a 16x20 small cabin w/ a cathedral ceiling.  I am putting on a metal roof w/ 2x6 rafters.  I wanted to insulate the roof w/ fiberglass batting and I am concerned about condensation on the metal roof.  Will the fiberglass insulation eliminate this or should I put some sort of barrier down before I put the metal roof on?  Thanks in advance for any advice.

[glenn kangiser]

I have been building steel buildings with vinyl covered insulation sandwiched between the steel sheeting and the steel purlins for about 30 years.  Vinyl to the inside.  I have never had a problem and this is an industry standard.  Pinching at the purlin does pretty well eliminate the R value at the pinch point.

Maybe the others have more comments on this.

Note that this is as applies to steel buildings - not houses.  Just an example of what works and is commonly done with them.

[PEG688]

 If your going to sheet the roof with plywood / osb then put your metal on you should I thnk , put in a insultray ,/ product that will provide a air flow space / force the insulation down / not touching the underside of that sheeting .  With a ridge vent and a soffit vent to move air thru that space .  
  If your could use 2x8 or 2x10 rafters you could get more R value , but should still use the air space maker to flow air above the insulation and below the sheeting .IMHO.



 Heres' a cut and paste http://www.energy.ca.gov/efficiency/qualityhomes/insulation.html


 //%20BATT%20CEILING%20INSULATIONUnfaced%20batt%20installation:batts%20shall%20be%20correctly%20sized%20to%20fit%20snugly%20at%20the%20sides%20and%20ends%20batts%20should%20fill%20the%20cavity%20where%20necessary,%20batts%20shall%20be%20cut%20to%20fit%20properly%20--%20there%20should%20be%20no%20gaps,%20nor%20should%20the%20insulation%20be%20doubled-over%20or%20compressed.%20When%20batts%20are%20cut%20to%20fit%20a%20non-standard%20cavity,%20they%20should%20be%20cut%20to%20be%20one%20inch%20(1")%20wider%20than%20the%20cavity.%20batts%20should%20be%20cut%20to%20butt-fit%20around%20wiring%20and%20plumbing,%20or%20be%20split%20(delaminated)%20so%20that%20one%20layer%20can%20fit%20behind%20the%20wiring%20or%20plumbing%20and%20one%20layer%20fit%20in%20front%20for%20batts%20that%20are%20taller%20than%20the%20trusses,%20full-width%20batts%20should%20be%20used%20so%20that%20they%20expand%20to%20touch%20each%20other%20over%20the%20trusses%20the%20insulation%20must%20cover%20the%20wall%20top%20plates%20hard%20covers%20or%20draft%20stops%20should%20be%20placed%20over%20all%20deep%20drops%20and%20interior%20wall%20cavities%20to%20keep%20insulation%20in%20place%20and%20stop%20air%20movement.%20If%20hard%20covers%20or%20draft%20stops%20are%20missing%20or%20incomplete,%20they%20should%20be%20completed%20before%20insulation%20is%20completed.%20[glb]required%20ventilation%20must%20be%20maintained:%20for%20eaves%20or%20soffit%20vents,%20one-inch%20(1")%20of%20unblocked%20free%20air%20space%20between%20the%20roof%20sheathing%20and%20the%20insulation%20is%20required.%20[/glb]where%20necessary,%20use%20baffles%20to%20keep%20the%20insulation%20from%20blocking%20the%20passage%20of%20air%20insulation%20shall%20cover%20all%20IC%20rated%20lighting%20fixtures%20fixtures%20that%20are%20not%20IC%20rated%20(e.g.,%20halogen%20lamps,%20heat%20lamps)%20need%20to%20be%20enclosed%20in%20an%20airtight%20box%20that%20meets%20fire%20codes,%20and%20the%20box%20covered%20with%20insulation.%20If%20fixtures%20are%20not%20IC%20rated%20and%20not%20enclosed%20in%20such%20a%20box,%20they%20should%20be%20replaced%20or%20boxed%20before%20insulation%20is%20completed.  

  Hope it helps .

[marty]

In our area R 38 is minimum in the ceiling.  You would never fit that into your 2X6's.  

I also believe you need an air space above the insulation.  It is important to have a free flow of air from the eave to the ridge vent to allow any moisture to escape.  Otherwise, you will have condensation at the point the insulation touches the roof above it.

I don't know how things work in dryer, warmer climes but you would have a problem here.  Likely you would eventually see your purlins rot and or get condensed water running down your ceiling and then into your outside walls.

[John Raabe]

If you are building a heated residence and have the rafters up you might consider a spray foam insulation such as Icynene. Since it is such a good airseal you may be able to do without the venting above the insulation (check with both the installer and the building inspector).

It will give you the closest thing possible to good insulation in such a small cavity.

http://www.icynene.com

[PEG688]

  We've used the spray  foam , work good cost alot more but when ya got to and it's not a huge area , it is a great option.