Join the MRA | Member Login
Find a Professional
Sign Up Log In Add New Topic

TOPIC: Roofing over existing roof

Metal over T&G 2x6

Return to Roofing over existing roof

Return to Archive

Return to Forum

I have a cabin style home built in the mid 1930's that has T&G 2x6 over beams. Over that is a composition roof. There is no attic space. We are going to reroof and are looking into metal as an option. We would like to add some insulation but the house can't support much weight. A contractor suggested putting rigid foam on top of the 2x6's with sleepers between and putting metal on top of that. We live in a cool, damp climate (costal California). Will condensation on the inside of the metal roof be a problem?
Guest User

8/30/2001
My concern is that moisture from inside the house (generated by cooking, showers, laundry, plants, etc.) will raise up through the T & G and through the insulation. It will then condense on the coolest surface it hits. This would be the bottom of any roof decking that is installed or the bottom of the metal roofing panels themselves if you install a product that does not require decking. Therefore, I would strongly suggest creating a vented chamber of at least 2" depth that allows for clear venting from the soffit (eave overhang) and then up and out of a ridge vent at the peak. As mentioned earlier, some metal roofs would require full decking above the airspace and some wouldn't. Inquire with manufacturers of the products you like. Todd Miller
Todd Miller
Isaiah Industries, Inc.

9/1/2001
The new building codes are now requiring that a 1 1/2" air space be provide on cathedral ceilings which becomes more critical as you add insulation. You should be installing an air barrier first, then the insulation, then allow for the air chamber, then a moisture barrier and finally the roof system. Metal is ideal for this type of retrofit as it is light weight. There are many systems out out there designed for this and I would suggest that you chose a couple of product styles then contact the technical departments of the manufacturers.
Allan Reid
Dura-Loc Roofing Systems, Inc.

9/4/2001
If I put a vapor barrier between the T&G and the insulation such that from inside out the order was T&G - Vapor Barrier - Insulation - Air gap - Metal roofing (no decking) and provided ventilation in the soffit and at the ridge would this keep condensation from forming and damaging the T&G?
Guest User

9/9/2001
In your assembly you described you are still missing a moisture barrier between the insulation and the metal roof. As well we generally do not referr to the interior barrier as a vapour barrier rather than an air barrier. The idea is to stop the warm in winter air from movingfrom the interior through the assemblt to the exterior and taking marm, moist air with it that would then condensate at some point in the assembly. The idea of the moisture barrier on the outside of the assembly is to allow air to pass through in either direction,while shedding moisture that condensates on the exterior. The 2x6 wood deck has an insulation value in itself. A general rule is to place the "air" barrier as close to the interior as possible but no more than 40% into "R" rating of the assembly from the inside. Condensation will happen at the dew point which becomes a complex formula between the inside and outside temperature along with a number of other factors. The "air" barrier was wants to place where the temperature is above the dew point so as not to collect on the wrong side and second where it could freeze and destry the assembly. By placing an "air" barrier directly against the 2x6 decking which may have a natural R value of 3,you would want to add at least an R12 to the assembly. In more northern climates I would tend to add substantially more which is why most codes now call for an R40 assembly. I would strongly reccommend using a self sealing and self adhesive Ice and Water sheild as your "air" barrier. This will allow your contractor to tear off sections and protect them immediately. As well it will self seal around each fastener penetration providing a very good barrier. Then we suggest using ridgid insulation board layed continuous which stops the thermal bridging. Then strapping is layed vertically, fastened through the insulation board, followed by the moisture barrier and horizontal strapping to hold the metal roof system. The energy savings and increased comfort level will be well worth the investment.
Allan Reid
Dura-Loc Roofing Systems, Inc.

9/11/2001