View Full Version : 70's Raised Ranch....Unvented Attic
05-14-2012, 09:34 AM
We are hopefully going to get a new roof in the near future, and I'm wondering if we should leave the attic unvented like it is. I mean it has the original gable vents, but does not have a ridge vent or soffit vents. The previous owners had central ac installed in 2005, and I've read that maybe I should spray-foam the rafters and make it a conditioned space so it takes the load of the AC in the summer. I added 9 inches of fiberglass insulation over the older insulation on the floor two years ago. I know I would have to cover the existing gable end vents with plywood to make it truly unvented.
I guess my question is, should be stick with an unvented attic since we have the Air handler and duct work in the attic?
Stick with an unvented roof, and DO spray foam the roof deck with either open cell foam, or 2" of closed-cell + batts. Besides the additional R-value, using foam is better due to it's superior air-sealing, which reduces stack-effect heat losses in winter, and limits the amount of exterior air that can get into the space and condense on the outside of the ducts during muggy weather. It also reduces infiltration driven by duct-leakage and duct-system imbalances, since it puts the ducts and air handler inside the PRESSURE boundary of the building envelope.
If it can work (usually does) installing rigid foam insulation over the exterior of the roof sheathing improves performance in greater proportion than it's R-value, since unlike rafter-bay insulation, it insulates over the thermal-bridging of the rafters. This reduces ice-damming potential and boost the performance to better-than code.
There are several outlets for used 2-4" polyiso or EPS roofing (http://www.insulationdepot.com/) foam reclaimed from commercial building re-roofing and demolition at something like 25-35% the cost of virgin stock, which takes the financial sting out of it. Held in place with 2x furring through screwed to structural roof deck 16" on center you can then put 7/16" OSB up for a nailer deck for the shingles. At the ends, put in something like Cor-A-Vent (http://www.cor-a-vent.com/s400.cfm) to keep the critters from setting up a condo in there and you'll have the benefits of a vented roofing for shingle-longevity, and a semi-permeable path through which the structural roof deck can dry 24/365.
You can leave the insulation in the attic floor, but if you do you may want to monitor the relative humidity in those spaces in winter. There is some (but not a huge) potential for condensation in there in winter when you split the total-R that way. Monitor the temp & humidity- if it never drops below 40F in there you're pretty safe even if the RH peaks are high, but if it's constantly ~40F with 80%+ RH in January there is at least some mold potential. In southern New England that pretty much goes away with R20 or more at the roof deck even if you have R38 or more at the attic floor. With 2x4 rafters you'll get about R10 (R13, center cavity) out of open cell foam, and if you put 2" of reclaimed polyiso brings it up to R22. With 2x6 rafters you get about R20 center-cavity out of open cell foam, but the thermal bridging reduces that to under R15, so you'd want at least an inch of exterior foam to be absolutely assured. But even without the exterior foam, if the average January temp is too low/RH too high, peeling off a few inches of the fiberglass would take care of it without a huge energy-loss penalty.
If you insulate & seal at the roof deck, insulate & seal the gable-ends with foam too, or you risk wintertime condensation at the sheathing on the gable ends. Even 3.5" of open cell foam makes a HUGE difference. While open cell foam is semi-permeable to water vapor, it's impermeable to air, and the rate of moisture transfer to the cold sheathing via vapor diffusion is orders of magnitude less than if exposed to air. Yet it still dries rapidly to the interior during summertime weather, and the risk of problems is quite low.
05-14-2012, 12:40 PM
You just gave me a ton of good info........Thanks!
05-15-2012, 07:39 AM
If I went with spray foam on the roof deck, how much of the ceiling insulation should I remove? There is a nasty 2" compressed layer from 1973. A dirty pink layer of maybe 3" that is compressed, and then the 9" I added two years ago.
Don't remove ANY of the existing insulation. Pull it back at the soffits so the foam installers can get a good shot at it for a good seal, then re-install.
Then monitor the temperature and humidity of the attic space when the colder weather arrives. The greater the R value of the roof deck & gable end foam, the warmer the average temp will be in winter, and the cooler it will be in summer. A lot of other things affect the winter average temp too, the color of the shingles, the amount of shading from hills, nearby buildings, trees, etc. If the average temp in the attic stays above 40F even in January (which it very well might), you're pretty much golden, even if it's running 90% RH @ 45F up in the attic.
The key is that the mean temp in the attic stays above the mean dew point of the conditioned space air. If the conditioned space is not be allowed to rise above 30-35% RH @ 68-70F during the coldest weather the attic would be fine at an average 40F, since the dew point of that air is 37-40F. It takes a fairly tight house or a lot of unventilated cooking & showering activity for the humidity to go higher than that when its 20F outside, but keep track. If your house tends to be drier than that the attic is fine even at colder temps, but 30% RH is at the low end of the perfectly healthy range- below 25% RH @ 68F humans become more susceptible to airborne viruses, etc. If you run humidifiers to combat winter dryness, hold the upper bound at 35%, not 40-50%. The higher humidity isn't dramatically more comfortable, and it's fine for humans, but increases the mold risk in walls & attics, etc.
Only if it turns out the average temp in your attic is running below 40F would it be worth considering thinning out the floor insulation. In my home in Worcester MA I have R20 between the rafters and 8" of rock wool between the floor joists in my attic spaces behind the insulated kneewalls in my house and it stays in the 50s even on the shady north side of the house. I have more heat leakage through the kneewalls to raise the temp than you'd get through the ceilings of a simple gabled roof attic, but I also get effectively zero solar gain (shaded by hills & trees.) The only way to know for sure is to measure it. You have ~R45 nominal on the attic floor, and if you put R20 on the roof, it's likely that you'd still be averaging above 40F in there in winter.
With your zip code we could look up weather data and run a simple-math R/ratio guesstimate, but roof temperature averages run warmer than outdoor temp averages due to direct solar heating, so the model isn't really that simple.
05-15-2012, 10:31 AM
I'm in Tolland, CT 06084
I sort of wish I ripped out the old insulation in the ceiling before putting the new stuff in. I vacuumed out as much mouse activity as I could. Pretty nasty up there. Do you know of any professional spray foamers?
Eyeballing it with cursors (http://weatherspark.com/#!dashboard;a=USA/CT/Tolland), the mean January temp on Tolland is about +29F. So even if you had R50 on the attic floor and R20 between the rafters, with an average ceiling temp of 70F, with a 29F outdoor temp you have a 41F delta-T over a total of R70. The attic is 20/70, in from the outside and will average (41 x 20/70=) 11.7F warmer than the outside. The mean temp will then average (29F + 11.7F=) 40.7F, which is over 40F, but close. If the attic floor's actual performance is less than R50 (which it is), the temp in the attic will be warmer. And this is assuming ZERO solar gain. If it's more like R40 on the floor, and you put R10 on the roof deck (R50 total) you'd be (29F + (41 x 10/50)= 37.2F for a mean attic temp, but the solar gains would likely raise the average by at least 5F, but it's worth monitoring just to be sure.
There are many operating in central & southern MA, and I'm sure in CT too. You can find them in google searches (https://www.google.com/#hl=en&sclient=psy-ab&q=spray+foam+insulation+tolland+ct&oq=spray+foam+insulation+tolland+ct&aq=f&aqi=&aql=&gs_l=hp.3...1610.10236.0.104188.8.131.52.10.10.1.726 .6743.2j11j2j5j1j3j1.25.0...0.0.alTp-n8KGu0&pbx=1&bav=on.2,or.r_gc.r_pw.r_qf.,cf.osb&fp=3bab4e5a695f4992&biw=1173&bih=773) and online. Get proposals, but check references- experience counts.
To hit R20 with open cell takes about 5-6" of depth at 30-40 cents per inch of depth for every square foot. Call it ~$2/foot- it adds up. There may or may not be subsidies available, but if you're already at code-min (which you are) it may limit what you can actually get. If you just want an air-seal and will be putting rigid foam on the exterior 3"/R10 at about a buck a foot would do it, but make sure they understand your air-sealing goals.
Leaving the mouse-nests in place and blowing fiberglass over them is fine- the insulation value remains.
Low density fiberglass without a top-side air barrier doesn't perform anywhere near it's R rating in a bottom-side warm configuration, because air freely convects up from the warmed bottom layer into the cooler attic space, allowing the cold air to displace it. The colder the attic is, the bigger this convective loss becomes. So air sealing the attic and putting a stable-R at the roof deck will improve the average performance by more than the simple-model R values might indicate. With a now warmer attic the convective loss of R in the fiber layer at the temperature extremes doesn't drop as far. Cellulose is denser than fiberglass and more air-retardent and doesn't have this issue so long as the layer is at least 3" deep. If you weren't going to insulate the roof deck it would be worth blowing cellulose over the whole thing to give it an air-retardent cap. Even laying a sheet of housewrap (Tyvek, Typar, et al) over the top would improve the cold-weather performance of the fiberglass layer.