Our 1927 single story brick bungalow in the mid-Atlantic (climate zone 7, 60-90 days above 86 F). The existing insulation is rolls of 6" John's Manville gold insulation fiberglass stapled laterally across the underside of the rafters which has been a bitch trying to keep attached and needs to be replaced.

The attic floor joists have a deck installed and we have an AC air handler for a SpacePak system in the attic which has lines which run under the floor deck to returns in each room ceiling, so blown in insulation appears to not be an option. I estimate the area of the roof needing insulation to be a bit more than 1,300 square feet. The attic is unfinished and the has something like a 8/12 pitch.

The roof is 5 year old asphalt, dark gray. The roof has a functioning ridge vent installed when the new shingles were installed. There are no soffit vents. The attic gets very hot during the summer (even with the ridge vent) and I believe this leads to the AC not doing that great a job cooling the house, though I mitigate that somewhat by venting with a box fan out through one of the windows at either end of the structure.

The rafters are 2 x 6 (5 1/2" x 1 3/4" actual) and are 24" on center and appear to be consistent in spacing. I considered having closed cell foam sprayed up under the eaves, but have pretty much rejected that due to concerns for off-gassing and the proximity of our AC system. I've been following the threads on this and agree there is room for disagreement, but don't want the risk.

So I am leaning towards doing a DIY, installing R22 Roxul batts between the rafters, with the possibility of building out the rafters with 2 x 4's later to add another R14 layer later. My question is on soffit vents. The rafters extend out from the exterior wall and the space between the top of the brick wall and the underside of the roof deck is "sealed" with a 1" vertical board between each set of rafters.

My plan is to install soffit vents and rafter baffles, perhaps something like the Durovent.

Specifically:

1. Should I run the rafter baffles all the way up the rafters to meet up with the ridge vent? To this layperson that would seem logical, but I am told that 3-4 foot from the vent up is sufficient.

2. Soffit vents, how many? Should I bite the bullet and put one in between each set of rafters, or is one vent every three rafters good? Assuming my plan is sound and I go forward installing the insulation, vents and baffles, do I need any sort of vapor barrier between the Roxul and the attic space?

Appreciate any feedback, ideas or criticism that you all would like to share. Thanks in advance!

Am I reading that correctly in that you have your existing insulation at the roof line in the rafters AND you have the attic vented via a ridge vent? If so, no wonder its not working like you want.

If the attic is vented, the insulation needs to be down at the ceiling joist level.

If the insulation is at the roof line, then the attic needs to be un-vented and everything in the attic needs to be sealed tightly.

If the insulation is at the roof line, then vent to attic to the interior. Or put a dehumidifier up there.

Posted By arkie6 on 11 Dec 2011 01:34 PM
Am I reading that correctly in that you have your existing insulation at the roof line in the rafters AND you have the attic vented via a ridge vent? If so, no wonder its not working like you want.

If the attic is vented, the insulation needs to be down at the ceiling joist level.

If the insulation is at the roof line, then the attic needs to be un-vented and everything in the attic needs to be sealed tightly.

Yes, you read correctly. We have a vent and the insulation is at least partially attached to the underside of the rafters.  The space never had insulation at the floor level of the attic.  The space has always been cold in the winter (even when there was no vent) and very hot during the summer. the ridge vent helps a bit during the summer, but the space still gets very hot.

I understood that as long at the venting is on the outside of the insulation then it was OK.

With no air-barrier on the outside of the batt insulation facing the ventilation gap it's thermal effectiveness in a soffit & ridge-vented rafter scenario will be nearly-worthless (half or less it's rated R when at the temperature extremes.

Soffit & ridge venting will have almost no effect on the comfort levels or heat transfer through the roof, but it provides a drying path for any moisture that finds it's way to the roof deck (which can accumulate moisture in winter, if unvented.)

As a DIY you might cut some 1x1" spacers from 1" XPS sheathing and attach sister them to the rafters, and mid-way between them, reducing the span to under 12". Cut'n'cobble some 22.5" wide XPS from the sheathing to form the exterior air-barrier, seal the edges with 1-part foam, and tape any seams with housewrap tape. That way you'll have an exterior air barrier that can still dry toward the exterior (1" XPS has ~1.2 perms of vapor diffusion) and R5 of moisture-tolerant insulaion. (You could use also use 1" UNFACED EPS, which would be more like 3- 5 perms.) That would leave you the code-min 1" vent gap, and a 3.5" cavity to fill.

Blown is definitely better than batts, but if the spacing is very consistent and you do it carefully batts designed for 2x4 24" o.c. framing should fit pretty well. With R4+ of foam and a vent gap on the exterior you don't NEED an interior vapor retarder, but if the batts come cheaper with kraft facers than unfaced, put vapor retarder on the interior side. At that point you'd be up to ~ R18 center cavity, but more like R15 after thermal bridging. Rock wool is less IR translucent than fiberglass, outperforming it in the cooling season. (Blown cellulose is IR-opaque, and more air-retardent, but difficult to do as a DIY.)

On the interior you then have the option of fitting in rigid fire-rated foil-faced polyisocyanurate (eg Dow Thermax) on the interior as a thermal break, and just leave it, some other foam board + 1/2" gypsum as an ignition barrier or a 2x4 lateral stringers, etc as both thermal break and additional R. A 2" layer of iso would bring the center-cavity R up to ~R30, (which is probably code-min), but since it would be adding R12 of thermal break over the rafters it would outperform a code-min between joists. A 2x4 lateral layer with R13 cavity fill would be similar.

In many areas it's possible to find great deals on reclaimed rigid foam from commercial re-roofing (I can usually find 3-5 people selling use foam board within 40 miles of me just looking on Craigslist, but I'm also close enough to InsulationDepot.com yard to just drive there.) It's typically at a discount of 65-80% over virgin stock, and for a cut'n'cobble application dinged corners and edges aren't much of an issue, you can go with some pretty low-grade goods and get the same result, since you don't need perfect sheets.

Durovent is designed to establish a vent clearance between attic floor insulation. If using a product like that it must go from soffit to ridge, but stuffing batts behind it would leave gaps and voids, cutting into performance, which is why the cut'n'cobble XPS solution is better (but can be a PITA to install in tiny spaces behind kneewalls, etc.) Having at least some amount of soffit venting per rafter bay is preferable, but even if only ridge-vented it'll have at least some capacity to dry. The tougher areas are typically in valleys, where no soffit venting is possible.

Things to watch out for: If it's a brick veneer over a structural studwall, you have to be sure the cavity between the sheathing & brick is properly vented, and this is easy to interfere with when insulating the roof deck. If it can't vent directly into your roof-venting scheme and must of necessity be blocked at the top, cut top vents to the exterior every 3-5 bricks in the vertical mortar to let it vent to the exterior under the eaves.

Dana1, Thanks for the response.

I am researching sources for XPS sheathing and will likely go with your suggestion. I happen to have a large supply of 1x1" stock I scrounged a long time ago which I can apply to this project. There are no knee walls, so installation using your scenario should be doable.

Thanks again!