Insulating attic with R-15 and R-23 Rockwool Comfortbatts
Hey everyone,
We’re preparing to replace old fiberglass insulation in the attic of a 1 story ranch, climate zone 5, Massachusetts. The insulation is paper faced, breeding silverfish and the fiber is saturated with years of a mice infestation.
We renovated 2 rooms and insulated the exterior walls with Rockwool Comfortbatt R15s. I really like the rigidity and density of the Comfortbatt products and am trying to plan the best strategy for an attic install.
The rafters and joists are 2×6 lumber spaced 16″. I understand we need to allow ventilation though the soffit and should insert manufactured or diy soffit baffles. When creating a 3D model of the structure I noticed an R15 batt installed along the rafters could maintain an air gap of 2″. Though not required by code, I began to wonder if insulating between the joists and rafters would create a more efficient system.
Question 1: Soffit baffles seem necessary for spray-foam, blown-in and even overly fluffy fiberglass batts. However, if we’re using rigid batts are soffit baffles even necessary? My only thought is they help carry the cold air up and over the ceiling, creating a guided path for the air to flow.
Question 2: Because the joists are 2×6 inches I wanted to maintain a similar height for crawling across the joists for visual inspection, repairs or modifications. I thought we could install 2×6 comfortbatts along the joists, and 2×4 comfortbatts along the rafters for a combined R value. If doing so, would we leave a 3-6 inch gap across the ridge for venting? Or, should we butt the batts at center and convert the attic into an un-vented space?
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I’m assuming this is a vented attic since you're talking about vent baffles. If you put all your insulation in the attic floor, you only need baffles at the eaves to prevent wind washing. You can’t add insulation between the rafters to add to the total R value for code purposes — you have to put full code minimum insulation either in the rafters or on the floor. You can’t split the total the way you’re describing.
That said, you can add some extra to slow heat loss a little, but I doubt it would be worth it. You can get blown mineral wool insulation. I’ve never used, or even seen, the stuff, but it’s out there. If you don’t want to use cellulose (which is usually the preferred option for blown in insulation in an attic), you’d probably be best off using blown mineral wool instead of batts. Blown insulation is really best for an attic floor, and it’s certainly the easiest to apply. You also get the advantage of no issues with thermal bridging since you bury the floor joists in the insulation.
Bill
Thanks Bill,
Yes it's presently a vented attic. If we choose Rockwool I think we will need to install 2 layers of batts. R23 in between the joists and R15 overlapping on top, perpendicular to the joists.
1 R23 - 5.5x14x47 ($6.50) + 1 R15 - 3.5x14x47 ($4.30) = $10.80 for two layers.
Hi Justin,
I agree with all that Bill said and hope that you are not overthinking this. In attics like yours there is a straightforward and cost effective approach: Pull out the old insulation, put a lot of effort into air sealing the ceiling from the attic, and blow in as much insulation as you can afford, with blocking and vent baffles at the eaves to prevent wind washing and maintain air flow to the ridge vent.
This is messy and sometime uncomfortable to do, but also very DIY-friendly work.
Check out these links for more info:
Air Sealing an Attic
How to Insulate an attic floor
One more thing, if your attic has a hatch, or pull-down stairs, you'll need to make sure that isn't a big hole in the air and thermal barriers. Here's some more info on that:
How to Insulate and Air Seal Pull Down Attic Stairs
Thanks Brian,
I'll read up on those articles.
My next step is to decide on already made or diy baffles. When looking at Home Depot and Lowes I saw they don't carry 16" baffles like Berger Products Accuvent. See illustration. The ones they do carry are 12" or 24" wide. Any option will require cutting around the rafter and joist assemblies. I see some articles listing plywood as an option.
I'm leaning towards custom baffles cut from 1/2" or 1" extruded polystyrene. I'm thinking I would cut the lengths to 32" or 48" to maximize the material cut from the 4x8 sheet.
Hi Justin,
A lot of builders go with site made baffles and rigid foam is often the material of choice. I have done that myself, but I had a bunch of scraps to get me started. If I was buying material to do it, I'd consider plywood, because I could efficiently fasten them into place with a nail gun. In other words, you can work through the process, and then decide what makes sense for you.
>"The rafters and joists are 2×6 lumber spaced 16″"
The typical solution usually subsidized by MassSave would be to install a layer of 2x8 joists 24" o.c. perpendicular to the 2x6 joists for a 12.75 cavity depth, and fill it to the top with cellulose. While that as a tad shy of a code-min R49, it will meet code on a U-factor basis due to the much lowered framing fraction and thermal break of the cellulose above/below the joist edges. If you are under a MassSave utility, letting the pros do it will be substantially cheaper than doing it DIY (due to the 75% subisdy).
MassSave will not subsidize insulating at the roof deck.
https://www.masssave.com/
>"I'm leaning towards custom baffles cut from 1/2" or 1" extruded polystyrene"
Extruded polystyrene is the least-green insulating material in common use today due to the polymer type but more importantly the HFC blowing agents used, the predominant component being HFC134a, which has a global warming potential of ~1400x CO2. Doing it with foil faced polyisocyanurate (a nice polymer, blown with low impact hydrocarbons) would be a lot greener, and it's easier/cleaner to cut too:
https://www.finehomebuilding.com/2009/01/29/theres-a-better-way-cutting-rigid-insulation
If you're in central MA, Green Insulation Group in Worcester always has factory-seconds foil faced polyiso in stock, priced at about half the typical distributor cost, as well as reclaimed goods at even lower price points. The Insulation Depot/Nationwide Foam in Framingham is another potential source for used & surplus foam board. There are a few smaller operators too, but those two are the most consistent and largest.
https://www.greeninsulationgroup.com/
https://boston.craigslist.org/gbs/mad/d/boxborough-3-polyiso-insulation/6987998685.html
https://boston.craigslist.org/bmw/mat/d/new-factory-seconds-polyiso-insulation/6995113116.html
https://boston.craigslist.org/search/sss?query=rigid+insulation
https://westernmass.craigslist.org/search/sss?query=rigid+insulation
https://worcester.craigslist.org/search/sss?query=rigid+insulation
Thanks Dana,
I will look at those resources when selecting foam.
Question, (ignoring code requirements for a moment). See attached image. Some insulation is placed along the joists and some along the rafters. All while leaving a 2" air gap to mitigate ice melting in winter and asphalt shingle overheating in summer. Wouldn't the hybrid system of 2 layers of insulation, separated with a large air gap in between, and air gap above offer better insulating properties then a single layer along the floor or ceiling?
>"Wouldn't the hybrid system of 2 layers of insulation, separated with a large air gap in between, and air gap above offer better insulating properties then a single layer along the floor or ceiling?"
No. It would offer some benefit, but not the same benefit at any given combined total-R.
The big air gap between the roof insulation and floor insulation are a large thermal bypass, with it's performance being undercut by the gable venting and other losses out the gables. Whatever the R-value of the roof deck insulation is, a well insulated wind-tunnel provides little benefit.
That's why only the R-value of the floor insulation counts toward code compliance in a vented attic.
+1 for Dana’s suggestion to use reclaimed or surplus polyiso for gent baffles. Polyiso is going to be easier to work with here than XPS (polyiso is easier to cut, a little more rigid, easier to tape and seal, etc.). This is a PERFECT application for reclaimed polyiso because a squish or nail hole doesn’t matter here, and If the insulation is manufacturer seconds that ok too — you aren’t really using it for insulating value when you’re building a battle with it.
A large air space between insulation layers actually hurts you. Once an air space gets wide enough (over around an inch or so), you start getting convection currents which help to move heat around. The point of insulation is to PREVENT heat from moving around. Insulation generally works best in a single, thicker layer and not two layers separated by an air space.
Bill
Another reason not to separate the layers of insulation is that it doesn’t effectively address heat loss through the ceiling joists. The air gap allows heat lost through the joists to distribute and then transfer through the rafters. If the insulation layers were monolithic, the gap wouldn’t make much difference, but they are not due to the framing. You want to block the heat flow path through the more conducting elements.
I would go with Dana’s plan and let let MassSave have at this. That is as long as there is not duct work in the attic.
I don’t know what MassSave does for sealing, but I would pull all the insulation, use box fans to pressurize the rooms below, and look for leaks with a thermal camera. I can borrow a thermal camera from my library.
>"I don’t know what MassSave does for sealing..."
Contractors who work under the MassSave programs are required to air seal the assembly they're insulating, or prove that it's tight enough. Some (but not all) of them use blower door & IR imaging verification as part of their process.
MassSave also has third party inspections of the details after the fact- I'm not sure that's true on all projects, but it's true on at least some. A co-worker of mine had some dense packing of the walls and a secondary joist system to allow them to hit R49 in the attic done on his 1920s bungalow in Arlington, MA a couple of years ago. A MassSave inspector failed the attic job due to insufficient weatherstripping on the attic access hatch, which had to be rectified for the contractor to get paid for the attic portion of the project. The contractor's IR imaging of the walls showed that they didn't miss any blocked-off cavities- it's a reasonably tight house now.
I'm sure this has been addressed somewhere, but why run another set of joists to reach cellulose depth? Why not just let it pile?
The joist are not required, but do allow putting down some floor over everything. Some people like to store things in hot/cold attics. At least some minimal floor and joists allows future attic work to proceed much easier. Finding where to step in a deep pile of insulation is not easy.