What would be the best way to insulate a roof that is placed on top of an existing roof to increase its pitch to meet an add-on?
I am rehabbing a lake house that was built in the late 70’s. It is a timber frame 2 story and we are adding a two story addition to include a master bedroom/bath and family room. The house initially had a 3X12 pitch with a mansard style roof. The mansard sides have been removed. My framer wants to increase the pitch to a 6X12 using a scissor truss over the existing roof/cathedral ceiling. My roofing contractor said that if the new roof was spray foamed, it would void the warranty unless it was vented with a baffle. The space between the existing roof and the new roof makes it difficult to work. Any suggestions on a cost effective/energy efficient solution?
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Replies
Scott,
I'm not sure I follow you completely. I sounds like you are setting new trusses on top of an existing roof without removing the old roof, thereby creating a very tight attic -- too small for someone to crawl inside. Is that correct?
Certainteed shingles are approved for use over unvented roof assemblies. Many others are too.
Much of what you desire to do depends on exactly where you live both in elevation and zone and even whether you are in forest or on an open high altitude wind swept plateau. I know of a builder that built for the first time in an open wind blown area and he was surprised to have 15% of the shingles blown off the build just after install late in the Fall.
Many factors come into play when building.
aj
Scott,
If you are installing new roof trusses on top of an existing roof without removing the old roof, then there are many possible locations for the insulation layer.
Where is the existing roof insulation? How thick is it? Will it stay in place, or will it be removed?
I'm working on a house right now that has a newer roof framed over the original. The result is two too-small-to-work attics, one with a sloping floor. Since this was done, a mess of plumbing and electrical work has been done in both spaces. Two thumbs down.
How is your framer proposing to connect the new trusses to the existing top plate? Won't he have to cut thru the existing roofing to do this, thereby exposing the building to the water intrusion that he's probably trying to avoid with this strategy?
The the existing insulation has been removed. It was 6 inches of fiberglass batting. There will be cuts needed to the existing roof. The trusses would sit on the existing exterior walls and the interior timber frame where it intersects with old roof. For efficiency purposes, is it best to insulate over the top of the old roof and leave the the space above vented or to insulate the new roof in the rafter cavity?
Scott,
Where was the old insulation on the floor of an attic? Or was the insulation installed between the rafters (as it would be in a cathedral ceiling)?
The fiberglass batt insulation was was installed in the rafter cavities of the cathedral ceiling.
Scott,
If the access is easier, why not just install the new insulation against the underside of the old roof sheathing, from the interior?
The difficulty with a job like this one is maintaining the continuity of the thermal barrier and air barrier, especially at the intersection of the walls and roof, and where a roof makes a transition between an addition and the old section of the house. This type of question can't easily be answered over the Internet.