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We have a customer who needs their roof replaced, and we had suggested they consider a metal roof. They have a hip roof with a 4' skylight at the top, 2000sf roof surface, and 2x8 rafters with 8" fiberglass insulation installed between the rafters. The majority of the underside of the rafters have a polyethylene vapor barrier and wood paneling. They have been dealing with moisture problems since they purchased the house some years ago, and now that it is time to reroof, we want to deal with all the problems. Because of the design, it is not possible to install enough ventilation at the top of the roof, and in talking with Air-Vent, they are recommending a 3" air space between the insulation and the underside of the roof deck. I have read a few articles about "hot roofs", and one mentioned the idea of installing 2" foam board on top of the rafters, and then the roof sheathing. Do you have any recommendations for us to consider? I am concerned that we cannot get enough ventilation, and therefore am seriously considering the hot roof method. I would appreciate a response as soon as possible, as the owner would like to get this resolved as quickly as possible. Thank you!
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A requisite for "hot roof theory" is icynene insulation which is a sprayed product that fully seals the attic space in order to prevent moisture from penetrating and reaching the bottom side of the roof decking. In essence, it acts like a complete vapor barrier as well as an insulator.
Unless you have a way to install a complete vapor barrier, I am going to stand by my thought that you need to build a new roof over the existing one, allowing for a vented airspace. An engineer should be consulted for load calculations. Additionally, the International Building Code calls for this airspace to be a minimum1" but that really is quite minimal in my opinion.
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I have been reading articles and talking with ventilation/insulation experts about how to deal with venting(cold roof model) or not venting(hot roof theory) my standing seam metal roof, and have gotten very different answers. The metal roof is installed on a renovated barn(cathedral ceiling); the metal roof is screwed to purlins attached to the roof decking, with 2" solid insulation laid down between the purlins next to the metal. There is Grace water and ice shield applied directly to the decking under the purlins and insulation. A ridge vent and soffit vents were installed. But the internal wallboard ceiling had pre-existing 6" fiberglass insulation (without propervents) above it, so the air space is minimal. I have a new kitchen addition and unheated entryway tied in to this building, and have not yet intalled soffit vents or a vented ridge to the entryway roof. The new space does not yet have ceiling or wallboards or a vapour barrier and I have noticed moisture on the screws behind the fiberglass bats.
We need to make the right ventilation decision in order to go any further with the renovation. Because I live here and do not want to tear out the interior ceiling, and taking the exterior roof and facia out is not an option, I am hoping there is a correct solution.
I was told that I could take out the ridge vent and cover the soffit vents and seal up the wallboard ceiling with a vapour barrier paint, to create a solid ceiling or hot roof. But there is fiberglass insultaion in the ceiling not high-density foam. I was also told that I needed to tear out the ceiling, start over with propervents, etc, to create 2" air channels from soffit to ridge, but the rafters are only 2 x 6, and there may not be enough space to insulate properly for the needs to the New England climate, and this will be very expensive and disruptive. The building is heated with a wood stove, so the temperatures fluctuate; I try to keep the dew point below 45.
Do you have an opinion about what I should do, given the way the installation of the metal roof has been done? Thank you.
Megan
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The building code calls for a minimum vented air space of 1" in the US above the insulation on sloped roof assemblies. The differance between residential and commercial buildings is the use, where residential activities create much more moisture in the air.
Condensation comes from two sources. The most dangerous one is warm moist air leaking through the ceiling assembly. The best method of control is a monolithoic plastic barrier under the drywall finish.
In your assembly you have installed an ice and water shield over the existing deck and if installed properly it becomes an air barrier. You had an R20 of fiberglass insulation and then added 2" of ridgid which if a blue or pink board could be R16 for a total of R36. Provided there is limited air space, I doubt that this is a problem from the inside.
You then need top look at the cold trying to eter the assembly from the outside. The proper roof assembly requires a moisture barrier over the insulation. This is ice and water shields principal purpose. The problem can start when you place it in the insulation layer as he condensation can freeze and start to destroy the roof assembly. While the US code is behind on this the Canadian code says that when it is required, that it should to be located at least 40% into the assembly to not freeze. If the ridgid is as outlined then you are OK from this point.
You are on the right track in trying to control all the moisture elements possible. Inside I would open up all the ceiling penetrations such as electrical boxes and inject foam through the holes to seal the outside of the boxes to the drywall. At all terminations of the drywall such as at a bean, caulk the joints with a flexible long lasting sealant and then paint the ceiling with two coats of a high quality oil base paint. At the same time look at your exterior walls and do the same thing.
Without knowing the type of vertical rib panel you have I would try and vent at least the flutes by installing a proviled vented closure strip at the eaves and under the ridge cap. This will at least allow for some passive air flow.
Last I would look at interior moisture sources and try to keep that under control at least in the winter months.
Now to discuss what started this is that any new construction creates moisture and metal screws transmit cold so you see condensation without the completed assembly so what you are seeing is normal at this phase given the time of year. Hope this helps.
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Thank you for this lengthy reply, which I did not see when I had sent another question. I will email you directly as you suggested. It sounds like I already have the passive venting you are suggesting.
Megan
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