Adding insulation to a 1930s home with a slate roof
Location (New Jersey)
Attic is only for storage with pull down stairs above 2nd floor of the home. Attic has original wood floor. Ceiling in attic is about 7 feet in the center. Home has insulation (Cellulose loose fill) in the attic under the floor, about 6 inches deep. AC and Heat mechanicals (air handler and duct work) are in the attic serving the second floor living space below. Wet heating coil is fed from a basement boiler. Similar system on first floor in the basement. Roof is the original slate from 1930, on boards. Gable windows at at each end (Summer open/Winter closed). Duct work is R4.2 flex off of insulated main fixed duct. Duct work sitting on the floor. Rafters have no insulation under slate.
Overall energy consumption is reasonable, but I would like to improve winter performance. Home near coast, so AC is not used a lot.
Is it worth trying to create a conditioned attic? Add insulation to rafters (rigid boards?) Replace duct work with higher R value ducts?
I would like to keep the floor for storage and not add loose fill above the current wood floor.
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How deep are the rafters? How deep are the studs on the gable ends?
Is the slate mounted on skip-sheathing or a full deck?
Coastal NJ is all within zone 4A. The IRC code min is R49, but the 5.5-6" of fluff is good for maybe R20, and the ducts being in the unconditioned attic are an efficiency issue.
I'll await more detail before making specific recommendations.
Rafters are almost 6" deep. Gable end walls are 3.5" deep; Slate is on a full deck of boards. One additional detail; There is a layer of "Celotex" brand black coasted boards (3/4") on underside of rafters. Looks like a wood/paper fiber board. The boards are not in great shape and not on all rafter surfaces with a few missing pieces. Thanks for your timely response to my question.
Celotex is asphalted fiberboard, which is fairly water-tolerant, and water vapor permeable when damp.
What's between the rafters and the slate, skip sheathing, or tight planks? Skip sheathing looks like this:
http://verroneroofing.net/wp-content/uploads/2013/10/cedar-roof-ventillation.jpg
Full deck-tight planking-no visible light; I think it is 3/4" by 6" boards. Other facts: Slate is in good shape. Attic floor is about 1,000 sq feet.
With the full plank decking you 'll need a minimum of 1" air gap between the planking and the next layer. It's just fine to make that next layer cutn'n'cobbled re-used Celotex, air sealed to the rafters with can-foam using furring strips or other stand-offs to maintain the 1" spacing. If you run out you can used unfaced 3/4"- 1" EPS, which is at least semi-permeable to water vapor. At 3/4" fiberboard is about R2 and 5 to 15 perms, and 3/4" EPS is about R3, and 4-5 perms adding to the center-cavity R value while providing the necessary air-barrier for fiber insulation to work.
Adjust the ventilation gap deck such that you have exactly 3.5" of space between the exterior air-barrier material and the interior rafter edge, (the depth of a milled 2x4 stud), to allow a perfect depth fit for R15 rock wool batts. Friction fit the batts snugly into the cavities- scuplt them with a 10" bread knife or insulation knife so that the fit is as perfect as possible. You can then install 2" thick fire-rated Thermax polyiso on the underside of the rafters with 3" cap nails, taping the seams with a high quality aluminium duct tape, that ends up being R29-R30 at center cavity but performs about as well as R38HD batts in 2x10 rafters. In most areas you won't need to install a fire rated thermal barrier on the interior if you use the fire-rated polyiso. Seal the bottom edges of the foam to the top of the exterior walls &/or ceiling plane with can-foam.
If you use unrated polyiso you'd need to install 1x4 furring perpendicular to the rafters 24" o.c., through-screwed to the rafters with 4" screws, and hang half-inch wallboard on the foam.
You'll have to figure out how to best insulate & air seal at the top of the exterior walls, depending on how it's built, but it's do-able. The foil facers on the polyiso are true vapor barriers, so you're counting on the ventilation channel to keep the roof deck dry. If cut any blocking at the soffit end so that the vent channel is fully open to the outdoor air at the exteror- put in some of the roll mesh used for ridge venting under asphalt shingles to keep the vent channel from becoming a critter-condo. You'll probably have pull out some of the cellulose at the soffit end to be able to make the exterior side air-barriers perfectly air tight, and sculpt in some cut'n' cobbled batt & polyiso.
Sealing the gable vents and batt insulating those walls with R15 rock wool with an inch of continuous fire rated polyiso on the interior would bring the wall-R fully up to IRC code-min on thermal performance.
Dana-thanks for all your advice. I was thinking the same type of solution after reading a lot on Greenbuildingadvisor. I am a "handy" home owner but interested in this topic. The issues i will have with a ventilation air gap channel is the lack of any soffet ventilation at all. The only attic opening are the gable windows (True windows) and a louvered vent on the gable end. Adding an air gap channel will will be difficult to vent to the outside without a lot of construction/moditication on the soffit. If i understand the suggestion this air gap needs direct exterior ventilation (covered to protect against animal intrusion). Would a simpler/acceptable approach be to blow in cellulose behind the Thermax? Would there still be a need for the air gap with the cellulose? I was thinking the wood/slate roof would breath OK since it is not a plywood sheet. Also, the spacing of the rafters is not standard. More than 16" on center, but less than 24" on center. Fitting batts will require a lot of hand crafting to fit well. Rock wool versus Fiberglass? What is advantage of rock wool? Slightly higher R value? If I leave up the fiberboard on the rafters and cover with 2" Thermax (without any fill behind), other than overall lower R value is that a "good enough" solution? It seems adding the ventilation will be a lot of extra labor. I will need longer fasteners to pass through 2" of Thermax, then 3/4" fiberboard, but that seems OK if i can find the hardware.
More comments:
The current fiber board is not flush against the rafters at the peak, but rather horizontal-about 1foot from the peak. Do you suggest keeping the Thermax off the roof peak and in a similar horizontal plane? Thanks again! My wife thinks I am crazy to do all this.
The cut " ...cut any blocking at the soffit end so that the vent channel is fully open to the outdoor air..." is intended to mean create at least some reasonable venting at the soffit end.
Venting wood roof decks on the exterior side doesn't cut it, since it's often wet on the exterior side, and it the wood is blocking the interior moisture drive with it's own 1-5 perm vapor retardency. When vented from below the vent gap intercepts the interior moisture and moves it out, it doesn't have to get through the wood to get out.
If venting is not possible, you may have to bite the bullet and install ~2" of closed cell polyurethane on the underside of the roof deck (R12) and blow the remaining ~ 4" with cellulose in netting (R15) to fully fill the rafter bays, and use semi-permeable 2" unfaced EPS (R8.4 & 1.2-1.5 perms) on the interior. That would be R35-36 center cavity, with more than 30% of the total being closed cell foam on the exterior side, which is sufficient for dew point control in zone 4. With EPS you'll definitely need to install the thermal barrier to meet fire code.
The advantages of rock wool over fiberglass is that it's somewhat easier to trim and it's completely fireproof (whereas fiberglass melts at wood-burning temps). It's my perception (but no hard data to point to) that there is less fiber floating in the air from trimming, and that it's less likely to end up creating suspended particulate indoor air problems in the even of air leaks.
David,
I disagree with Dana on the need for venting. Slate is a wonderful, forgiving roofing. It is vapor-permeable and air leaky. If you create an air space under the roof sheathing boards, as Dana suggests, there is no need to have any soffit vents or ridge vent. The underside of the roof sheathing will dry rapidly to the exterior, through the slate, due to ventilation drying and diffusion.
For more information on creating the needed air gap under the roof sheathing, see Site-Built Ventilation Baffles for Roofs.
I'd be a lot happier going with just an unvented gap as a capillary break if it were skip sheathing with minimal or no underlayment, which would give it a huge capacity to dry toward the exterior.
In coastal areas wind-driven wetting of the sheathing on the exterior happens regularly, all seasons, and there is likely to be multiple layers of asphalted felt between the slate & deck, but you can probably find out what it is. (Worst case, drill a 1" hole through the deck from the interior with a hole saw to figure it out, and glue/seal the plug back with expanding foam or polyurethane glue, when you're done.)
Even without ridge venting providing a tiny bit of soffit venting delivers SOME capacity to dry toward the interior. Right now it has very significant volumes of air to dry into, dropping that to an inch of unvented air gap is more than a 99% reduction.
We had 2x6 rafters, left a 1" air gap, and then filled in using a combination of 1" & 2" rigid insulation.
It took a while to rip the sheets to fit, but we got a tight fit, and didn't end up using any caulk and/or foam.
While doing this we did notice that the tar paper under the slate roof was breaking down.
I really liked that the rigid insulation provided a path for the occasional small leak to be diverted to the eaves.
It's been 5 years since I was thinking of starting this project and other home improvements took priority. A few things have changed. Hopefully it's not to late to update an old question.
1. The Thermax sheets I used previously in my basement seem harder to acquire (not sure if this is only in my area)
2. My desire to do this job personally has decreased; so hiring a contractor now seems to make sense.
I have 2 contractors coming to do onsite inspection soon:
1. First contractor over the phone is proposing applying closed cell polyurethane foam to the roof deck (thickness to be determined). Risks of this approach? Not sure how he will install since bays are covered by the old Celotex boards; will he remove the boards to apply or not. I am assuming the old boards will be removed.
2. Second contractor is primarily a cellulose installer. If he wants to blow cellulose into the rafter bays what type vapor barrier is needed? (If old boards stay intact). Can the cellulose fill the bays completely?
Thanks for the advice
Sorry to resurrect an old thread, but submitting questions appears broken for me (they have been waiting for approval for over a week).
Do you have any advice on sealing the fiberboard to the beams? With only access from the back, my fear is trapping water between the edge of the fiberboard and the beam.
Also, is there a tape that is effective in forming an air-tight joint between asphalted boards?
Thanks in advance,
Peter