Insulating old 2×4 walls with rigid insulation
Our home is located in western Maryland, (zone 4?) and was commercially built to increase profits without regard as to results. It is a 2800 sq. ft. brick faced story and a half salt box style with full basement and set back into a hill. Foundation is essentially a monolithic pad poured without benefit of steel reinforcement of any kind with extra depth along the rear ground level side. Walls are 2 x 4s with fibreglass insulation stapled inside the stud bays. Front roof 2 x 10s have 6″ fibreglass as do the upper bedroom ceiling joists. Heating is baseboard hot water and seems to require 650 gallons fuel oil per year. Windows are cheap and drafty but I’ve upgraded several through the years. As you can imagine we require a full time humidifier upstairs during the winter and dehumidifier in the basement the rest of each year. Presently I’m renovation to downstairs bedrooms into one large one with a walkin closet and have stripped the walls to the outer covering which appears to be fibre board and in this process I discovered the insulation near the outer roof edge stopped short of the front side wall with essentially allowed outside air coming from the roof inlet vents to enter the floor joists between upper and lower level living spaces. I’ve corrected that by adding 1 ½” EPS foil and plastic faced foam between front roofing joists and connected to the top wall plate. The first floor walls now have 1 ½” EPS sealed between the studs and I plan to add another 1 ½” of XPS followed by ½” EPS between the stud facing and wall board panels. I’ve done a lot of reading on upgrading insulation materials and being careful to allow drying to prevent moisture from causing mold growth and it seems the best solutions recommend an external air and vapor barrier, some with rigid insulation but most of these examples are for walls with external siding. My walls are brick faced with the space between brick and stud wall open into the roof venting area so will this allow the walls to dry to the outside? And what should I change in my intended wall assembly while they are open?
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Replies
User-6818892,
1. Just so we can define our terms: You have decided to insulate your walls using the cut-and-cobble approach. For more information on this approach, see Cut-and-Cobble Insulation.
2. Assuming that you do a careful job, your planned approach will work, and will not cause moisture problems.
3. One of your statements -- "it seems the best solutions recommend an external air and vapor barrier" -- is mistaken. While exterior air barriers are a good idea, exterior vapor barriers can cause problems, unless the vapor barrier in question consists of an adequately thick layer of rigid foam or spray foam. For more information on these concepts, see these articles:
Questions and Answers About Air Barriers
Do I Need a Vapor Retarder?
-- Martin Holladay
Asphalted fiberboard is highly moisture tolerant, but the studs aren't. Using a fiber insulation would allow far greater drying capacity toward the interior. R15 batts (rock wool or fiberglass) are sufficiently high density & air retardent to nearly completely mitigate convective moisture & heat transfer, provided they are sculpted to a near-perfect fit, but vapor open enough to allow excellent drying toward the interior.
In the walls with the 1.5" EPS you now have 2" of cavity to fill, which could be filled with compressed R11 fiberglass (unfaced or kraft faced). A crummy low-density R11 will compress fairly easily to 2", and at the higher compressed density at 2" would deliver ~R7.8-R8 performance (about the same as the foil/plastic faced Type-I EPS, inch for inch). With only 1.5" of stud poking into the foam layer it will still dry at reasonable rates into the interior despite the low permeance facers, and fiberboard isn't damaged by being trapped in the now higher-moisture zone next to the brick.
If you wanted to bump up the performance of the wall to near current IRC 2015 code minimum you could install 3/4" polyiso foam laminated to 1x furring (or strips of 3/4" plywood) to give it an R6 thermal break between the stud and furring to add 1.5" of depth, and use R15 batts to fill the now 3.5" deep cavities. The cut'n'cobbled R6-ish EPS foam offers huge dew point control margin at the foam/fiber boundary for R15 fiber, and no interior side vapor retarder would be needed. There's a Fine Homebuilding article detailing how that's done but I couldn't dig it up with a quick search- perhaps Martin can point you to it.
11:40a 4/14/17
Many thanks to Martin and Dana for their practical and sound advice. I’ve just spent most of this morning reading all the posts on “Cut and Cobble” and greatly appreciate this information too. Being retired for some time now I felt it better to spend the time with this approach as opposed to dealing with the expense and possible negative aspects of spraying foam.