Is this worth the money?
I have a large 2nd floor Master with a double gable vaulted ceiling. The interior ceiling is beautiful wood that I want to keep intact. I’m about to start a small dormer construction project that includes reroofing the entire west facing roof. Presently, this 2nd floor room suffers from high radiant heat temperatures and that’s something I would like to cure. I considering creating a hot roof from the exterior by removing the roof sheathing and filling the 2×10 joist bays with closed cell foam insulation. I’m in Zone 5. Is this my best option? I’m not sure this will do much to solve my heat problem, as the joists will still be connected to the roof deck and will transmit heat to the wood ceiling. To the best of my knowledge, the roof is currently insulated with R30 fiberglass batt.
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Replies
stek,
The three things that do the most to reduce radiant heat gain through roofs are:
- Light coloured roofing.
- A vented air-gap above the sheathing.
- A radiant barrier facing up onto that air gap.
Malcolm.
Thanks for the reply. I live in a historic district and light-colored asphalt shingles are not an option. If I’m removing the sheathing, would there be a benefit to adding the reflective barrier over the foam filled joist prior to installing the new sheathing? Also, is there a benefit to using a vented underlayment under the shingles, similar to a rainscreen setup?
I think it's a mistake to assume that radiant transfer is happening through the roof and I don't think a radiant barrier within the roof will do much. Radiant transfer is driven by temperature difference. The outer surface of the roof is getting radiation from the sun, but the surface of the sun is at about 10,000F. Anything that blocks light is a radiant barrier, the surface of the roof is itself a 100% barrier to radiation from the sun and the insulation and drywall in the ceiling are radiant barriers as well. The underside of the roof is at maybe 150F, so there will be minimal radiant transmission to the interior.
What you're seeing is mostly conduction, and the solution to that is more insulation.
What you're calling a "hot roof" sounds like an unvented roof, you have to be extremely cautious to prevent moisture from accumulating under the sheathing leading to the roof rotting from below.
I recommend this article for a discussion of the issues and techniques with an unvented roof:
https://www.finehomebuilding.com/project-guides/insulation/five-cathedral-ceilings-that-work
Yes, I'm talking about an unvented roof. Filling the 2x10's with closed cell foam will double my insulation but does nothing to reduce the thermal bridging through the joists connecting to the wood ceiling. Putting rigid foam on the exterior sound like a great solution, but would require redoing the entire roof structure and that's not in the budget.
Two questions: Is this worth the money and the effort of filling the bays with closed cell foam? And, would a rainscreen type underlayment reduce the temperature of the sheathing and thermal bridging?
While opening the roof from the exterior, are there other options that might be better. Currently, this is a vented roof. Would it make more sense to leave it vented and not fully fill the bays with foam?
If the roof is currently r30 and vented, how big are the rafters? I'd have to assume at least 2x10?
So you have 9-1/4" to work with. If you leave it vented, you have to allow 1" for venting, you can have 8" of insulation. My opinion is a vented roof with 8" of insulation works better than an unvented roof with 9" of insulation.
You don't say what kind of closed cell foam you're considering, spray or board. Each has drawbacks. The problem with closed cell spray foam is that it cures hard and it's hard for the applicator to control the thickness. If it rises beyond the framing it's a real chore to trim it back flush. So the tendency is to spray it thin, so 9" turns into more like 7".
Boards have consistent thickness. The issue there is sealing the gaps along the edges.
DCcontrarian:
Yes the joists are 2x10's. I would like to spray closed cell foam rather than deal with boards. My ultimate goal is to reduce the transfer of heat into the room below.
I had a vented 2x10 4:12 pitch shed roof (so half cathedral) and had massive heat gain/loss. I came here for advice and used Martin's 5 ways as a guide, along with various experts who weighed in. I found the existing fiberglass insulation poorly installed, and the "venting" was pointless because there was solid blocking halfway up the roof. I chose the cut/cobble method of using 8" of polyiso sheets cut to fit with canned spray foam at the perimeters and seams and I fixed the blocking to restore a 1" channel under the sheathing. I was working from above (did two sheets of plywood removal/replacement at a time so I had surfaces to work from). Anecdata: my air conditioning suddenly improved dramatically and the surface temps of the ceiling drywall (measured by Flir camera app) were down from upper 90s to low 100s to mid 70s on a sunny mid 90s day (live in the DC area with high humidity). For me it was worth the money and time to get a significant comfort increase and some energy savings. Thanks to Martin and others again for the guidance.
mikelion,
Thanks for your input. You've answered my, is it worth the effort question.
I'm in climate zone 5/6. I tried to find a good way to insulate a low pitched dormer roof, and did a "cut and coble" using foam sheets and spray foam to close and seal the gaps. But I was still concerned about air leakage leading to roof failure. Two years later, I ripped out a section of the roof - sure enough the sheathing was soaked. At that point I ripped it all out, let the roof dry, and spray foamed the roof (CCSF). One other roof venting issue - I wanted to try blown in cellulose insulation, but the only installer, from another state, did a wet spray method. In most applications it worked fine. Most of the house had an insulated, vented attic, but in the garage, where the owner wanted a finished room, we installed premanufactured vents, and they dense packed the rest of the rafters. A few years later the owner called. The dense packed installation had flatened out the plastic vents, and the cellulose was damp and damaging the drywall. We re-did the roof with a sealed plywood vent which we built on site and used on every job after that, and dense packed under that. Good solution. - Bob Irving
concerning the "double gable vaulted ceiling" - I'd suggest leaving the interior insulation alone, strapping the roof sheathing - on the outside - vertically with 1x3's, adding a layer 0f plywood and roofing over that. It'll vent the roof and add maybe 1-1/4" but you can make that disappear pretty easily. Solves the venting issue. If you want a better interior venting method - the sealed 1-1/2" deep plywood vent in each bay works great. - Bob Irving
I would be very concerned trying to "cut and coble" using foam sheets and spray foam to close and seal the gaps in an unvented roof. Instead, I'm considering using CCSF and filling 8 inches of the 2x10's and keeping a 1" channel for heat venting. My roofs are currently vented. Since I’m filling from the outside, I'm thinking this is my best option. The CCFS will seal and prevent moisture from reaching the sheathing and increase my R-value significantly. I'm hoping the increased R-value will reduce the transmission of heat to the room below and reduce heating and cooling costs. I know this isn’t cheap, but isn’t it the safest option?
Bob,
Maybe I have the wrong image in my head, but don't you have the same concern with moisture getting to the bottom of the sealed plywood vent?
The only way to make an unvented roof from above is with exterior rigid insulation. Spray foam for an unvented roof needs to be applied to the roof deck, when you are spraying it from above, that is impossible.
You can still do a spray foam roof but it needs to be vented. At that point it would do an in-between of the cut and cobble suggested above which saves you the cost of spray foam. You could open the roof, squish the existing batts down to about 5.5" with a pieces of 2.5" rigid foam leaving a 1.5" vent gap. Seal this in place with canned foam. I would also air seal any device boxes or wall plates before you do any anything after opening the roof.
The other option is to dense pack over the existing batts and install exterior rigid. In zone 5 with 2x10 rafters you need about R20 above the roof deck. Over this rigid install a new roof deck and roofing.