Hello,
Looking for advise in proper techniques for insulating the ceiling in a Gambrel roof. The rafters are 2×4’s spaced on 19″ centers. The rafters do not hang over or have any vents.
Currently the insulation has big gaps (see in pictures). One idea I had was to leave current insulation and put sheets of foam board on the under side of the rafters then drywall. I’m concerned about condensation. What would you do?
Thanks,
Bob
Replies
Here's a resize on the pic
Vince Carbone
Riverside Builders
Franklin,NY
Assuming you are planning at some point to finish the interior, I would insulate the ceiling of the floor below from the outside wall in to the kneewall (note that this area may require vapor barrier before insulating). Doors for future access to the space behind the kneewall might be worth considering. Then I would insulate the kneewall up to the gambrel roof. I would then insulate the gambrel roof. Perhaps 1 piece rigid insulation between the rafters (foamed in place) with additional insulation below the rafters. If headroom is not a concern I would put all the insulation below the rafters (easier, faster and more ventilation).
Vapor barrier should be installed on the warm side where applicable and as practicable. At the very least the penetrations in the ceiling of the lower floor (outside the kneewall) should be sealed. I would try to establish continuous ventilation connecting all airspaces from eave to ridge under the roof sheathing. Of course you will want to address any structural, mechanical or electrical issues before insulating.
Some people would prefer to just foam the underside of the roof. Which has the merit of simplicity and is arguably superior.
Thank you sisyphus,I would like the simple and superior technique. When you say foam the underside of the roof, are you talking spray foam? And does this method eliminate the need for ventilation??Thanks,
Bob
Whether ventilation is required or not is debatable. Some jurisdictions, apparently, insist on it and some do not. The approach I detailed reflects my experience (and conservatism) and I am confident it will produce reasonable results.
Spraying the underside of the sheathing with foam is a technique which I have no experience with. It may well be superior. I have, however, questions about the longevity of such a roof system. Specifically what is potential for trapped moisture from condensation or leaks to cause rot? I suspect that time will tell that it depends on the local climate, quality of installation, maitenance and lifestyle of the home owners etc.
No doubt someone who uses spray foam will chime in.
Hi Bob,
Looking for advise in proper techniques for insulating the ceiling in a Gambrel roof. The rafters are 2x4's spaced on 19" centers. The rafters do not hang over or have any vents.
One option.....closed cell spray foam between the rafters and trim even.....cover with 2" polyiso foam boards temporarily tacked into place, foam joints while hanging and tape all seams and breaches and hang 1/2" drywall with 3 1/2" screws......time consuming but worth the insulating value including a great thermal break greatly reducing condensation. Use a metal roof or other similiar heat reflective roofing material and forget about venting it.
Pedro the Mule - Staying dry and comfortable in my barn
Thanks Pedro,The roof is already shingled so I'm not doing anything with that.
I want it simple and good. Thanks for the help.
Hi kneetwoods,
The roof is already shingled so I'm not doing anything with that.I want it simple and good. Thanks for the help.
Some folks still spray foam right up to the back of regular shingles but I personally have found that standard shingles curl prematurely when this is done. Check with your shingle manufacturer to verify.
Since the shingles are in place, I'd create mini-soffit vents and cover with screening to keep bugs out. Cut strips of wood 3/4x3/4" and attach along the inside edges of your rafters along the sheathing edge. This will create a stand off from your sheathing. Use foil reflective foam board and create your own channel for air to move through utilizing a radiate barrier method all the way up to the peak and install a ridge vent. Fill in the rest with foam, whether you cut your own panels and seal extra good with can foam or whether you have someone come in and spray foam. Finish with another layer of foam board over the rafters to create the thermal break and drywall to complete.
Any "simpler" than this and you sacrifice the "good" level of quality you're looking for. Spend the time and $ recommended and you'll find it to be more comfortable year round and less costly to heat/cool when you need to.
Pedro the Mule - Have fun
Pedro,Thanks for the help! Insulating is not fun for me....but doing it right the first time has its rewards and means I can do other fun things.
Pedro, I like your foam with ventilation concept. I was thinking about doing something similar on a place I own because while I admire the thermal, structural and audio advantages foam offers I am reluctant to abandon ventilation. I got the impression, perhaps erroneously, that Kneetwoods has no soffits so he might have to vent through the fascia or ???
Edited 7/6/2009 11:01 pm ET by sisyphus
Hi sisyphus,
I like your foam with ventilation concept. I was thinking about doing something similar on a place I own because while I admire the thermal, structural and audio advantages foam offers I am reluctant to abandon ventilation. I got the impression, perhaps erroneously, that Kneetwoods has no soffits so he might have to vent through the fascia or ???
Yeah, I'm not completely certain what he's fully starting with. Sometimes a building is constructed simply as a shelter and was never intended for it's new purpose, thus some reconstruction may have to occur to accomodate the new usage.
I did some pretty serious research prior to building my last home and with some sort of metal roof, I would never bother with ventilation. We're planning our retirement home now and I'll do the same thing for an insulated roof again......I am going to add to the R value but I'll still insulate all the way up without ventilation. To me, metal roofing is a one time expense that pays for itself over the years. Awesome investment.
Pedro the Mule - Metal Roofs now and forever
Hi Pedro,
I am a big fan of metal roofs too. Climbing up on a roof to deal with problems becomes less and less appealing as one gets older. As the saying goes, "if you have a good roof and a good foundation the rest of the house pretty much takes care of itself ".
Hi sisyphus,<!----><!----><!---->
I keep watching the building codes around here requiring higher SEER ratings and such to supposedly cut energy usage. For pretty close to the same upgrade money you can switch from arch. shingles to metal shingles. Same labor, life time roof and "if" it does every wear out.....recyclable "and" it will cut AC bills noticeably. We average a month of extra "No AC" vs. the neighbors because the roof reflects so much heat.<!----><!---->
Got my last months power bill yesterday......had a very unusual June....some of the coolest days I've seen for June and two of the hottest weeks I've seen so I expected the bill to run higher this month. 4600 sq. ft. AC set to 74, due to some portions of business in the home I have a minimum of 7 computers running 24/7 sometimes more, including propane costs for cooking and water heating.....total energy bill comes to $167......I hope to cut that in time with a solar water pre-heater, passive solar collector for space heating in winter, etc.<!----><!---->
Bottom line is that the metal roof makes a noticeable difference in our cooling costs.<!----><!---->
Pedro the Mule - Do the agencies "really" want the conservation they're calling for?<!----><!---->
<!----> <!---->
Hi Pedro,
You make a great point about saving energy with a metal roof. It looks like when you consider all the savings metal vs shingles (ie. energy, maintenance, fire risk, resale and longevity) it is the cheaper option. Peace of mind is also valuable.
The big roof issue where I am is snow. The roof should be designed with the covering in mind. We get problems with metal roofs when the snow builds up and the slides off in a thunderous avalanche shearing pipes off, smashing lower porch roofs and potentially giving pedestrians an "experience which is seldom pleasant". Snow stops are a potential solution but they have issues as well. Entrances on the gable end are the best. Deck and ancillary structure locations should be well thought out etc.
As you know good design should integrate all facets of a building to produce a harmonius whole. It is great that you have the opportunity to design a home from scratch rather than have to try to do a retrofit with the inherent compromises.
Hi sisyphus,
The big roof issue where I am is snow.....thunderous avalanche
Check out http://www.decra.com/index.php?option=com_expose&topcoll=51
Click on Residential then Shingle out to the right.
Still get snow buildup but the shear slide is less of an issue
Pedro the Mule - I've sent them so many customers they ought to pay me
Edited 7/8/2009 7:15 pm ET by PedroTheMule
Hi Pedro,
Thanks for the link. I have bookmarked it for later reference.
Metal shingles intrigue me because they would seem to address some of the flashing issues which the corrugated roofing which I am familar with has. In fact a couple of the corrugated metal roofed houses I worked on had problematic skylights (fortunately not of my doing) which were subsequently removed. I would expect that a metal shingle/skylight tandem would be fine.
I could understand the old nail/lead washer combo failures but the propensity of some neoprene washered roofing screws to work loose puzzles me. Gremlins? Concealing fasteners and protecting their heads from the brunt of the weather seems like sound practice to me. I bet repairs and modifications are easier with the shingles as well and certainly easier to do solo.
Thanks again for the insight.
Sisyphus