Roof Top Rigid Foam — Taking Efficiency Through The Roof — Fine Homebuilding

Roof Top Rigid Foam - Taking Efficiency Through The Roof - Fine Homebuilding

Video Length: 2:04

Comments (6)

ltarnack writes: Dreamcatcher, You mentioned getting XPS reclaimed foam in Detroit. I live in Detroit and am building a house in which I would like to use foam to insulate my roof deck and the underside of porches with living space above. However, rigid foam is super expensive. Can you forward the contact info for your source? I would greatly appreciate it. Great video!

Posted: 8:22 pm on November 10th

Yes, XPS is on the inside just under the drywall. No real penetrations to go around on the cathedral ceiling but all XPS is foam glued to the rafters and at all edges to fill any possible gaps then taped prior to DW installation.

Matt Risinger writes: @DanMorrison: Thanks for catching that answer for me on the dew point. The inside of the house will get 5.5" of open cell spray foam which is a great air barrier & a total fill insulation so I’ve not had any condensation issues with this type of assembly over the years I’ve been building this way.

@DArcyM: This may seem like a small R factor but it’s not all about R values here in the hot/humid south. We’re most concerned about heat gain on 100 degree days and by moving the thermal envelope to the outside of the roofline I’ve moved all my HVAC equipment into the "conditioned" attic. My attics are usually 5-7 degrees hotter than the house on a typical July day. Compare this to most Austin attics that are vented and have HVAC equipment and R-6 ducts running through a 130 degree attic space. This house had a Manual J showing 800 sf per ton of cooling so it’s going to be a very efficient little house. -Matt Risinger

Posted: 5:49 pm on September 24th

DanMorrison writes: Dreamcatcher, Great for you on building a high-performing roof. Is the XPS on the inside?

It is often a LOT harder to get a continuous air seal on the inside of a house because of all of the obstructions — interior walls, stairs, floors between 1st and 2nd floor, kitchen cabinets, etc. And if you don’t have a good air seal, the air is still moving through the fiberglass batts, which slashes the effective R-value and may actually cause condensation problems.

Of course, it’s hard to speculate on an assembly without seeing a drawing.

If Matt did a good job air sealing the outside, which it sounds like he did with the zip system, then he probably could have used fiberglass batts. But, fiberglass batts only work when they actually fill the cavity — no twist to the lumber, no wires, no pipes, exactly 14-1/2 in. cavity, etc. The spray foam may have been overkill, but I’m not going to complain about someone doing too good of a job.

Posted: 8:30 am on September 24th

Dreamcatcher writes: Seems like a lot of time and money to achieve a measly R28.6

I just designed and built a cathedral ceiling using 1/8" foam air baffles, R38 fiberglass batts, 2" XPS, and 1/2" drywall to achieve R49.1 and still have an air cooled roof deck. It’s a cheap, easy, and highly efficient method.

While spray foam is a great product, the price is too prohibitive for me and most of my clients. Such a shame that high efficiency products have to carry a premium price tag.

Posted: 12:08 am on September 24th

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