Replies

1. Expert Member
   Dana Dorsett | Jan 31, 2020 03:08pm | #1

   How much roof overhang, and how tall is this building?

   Is there any window flashing?

   In order:

   1>"Should we remove the fiberglass from the walls?"

   Yes, but that's because the R9 in a steel studwall is nearly useless due to the extreme thermal conductivity of the steel. It's probably performing at no more than R5, possibly less. The steel studs need to go.

   2>" Is there any benefit in keeping the lath wall under the drywall? "

   The plaster & lath wall between the existing fiberglass & steel was probably performing as a capillary break, but it's probably worth taking it down to the brick and installing an intentional wall stackup on any renovation rather than layering over it. On a house that vintage exposing the brick may even find places where the mortar may need repair.

   3>"If we decided to completely gut the walls, would it be OK to go with Cavityrock Rockwool using the same installation process as for rigid board."

   No- it absolutely needs an air space between the brick & rock wool to provide at least some amount of capillary break/drain space.

   With sufficient roof overhang and robust flashing details it may be worth installing blown cellulose right up against the brick. Unlike rock wool cellulose will buffer and redistribute the moisture burden, which is fine as long as it's not dealing with a lot of bulk water.

   Scroll down to the sub heading "Can I use cellulose?"

   https://www.greenbuildingadvisor.com/article/insulating-old-brick-buildings

   If rock wool, I'd be inclined to install a mesh type rainscreen underlayment between the rock wool and brick as both a capillary break & drain space. Even 1/4" RainSlicker (tm) is better than putting the rigid rock wool tight to the brick.

   4."Could wall insulation impact the quality of a brick building to stay cool in the summer and warm in the winter."

   Yes, but in a GOOD way, assuming you don't have a massive amount of unshaded west facing window area.

   5>"I’ve read somewhere that the old buildings in New England are built with semi-continuous air space between the outer wythe and the inner wythe and with insulation on the interior, the exterior wythe still sees the same environment it always has. Can I get your thoughts on this?"

   That's called a "cavity wall", though not as common in New England as it is in the UK. The cavity serves as an excellent capillary break, which dramatically reduces the amount of moisture reaching the interior wythe, and provides a drying space for both the interior & exterior wythes. Cavity walls are some of the earliest examples of what's now referred to as a "rainscreen".

   6> " Considering the diminishing returns of adding more insulation and the potential increase in the exterior wall maintenance, is wall insulation really necessary in our case? "

   HELL yes it's necessary (to meet code, if for no other reason). For a mass wall insulated on the interior side that takes R17 continuous insulation, or a total U-factor of U0.082 (= R12.2 "whole wall"). See TABLE N1102.1.2 and TABLE N1102.1.4 :

   https://codes.iccsafe.org/content/IRC2015/chapter-11-re-energy-efficiency?site_type=public

   Clearly going for it on a U-factor basis would be easier. The 8" of typical soft brick is good for about R1.5 , 2" of CavityRock is good for R8.4, an empty wood stud 2x4 cavity with half-inch gypsum on the interior and housewrap on the exterior is good for about R2.5, so even without counting the R-value of any mesh rainscreen you'd get there in about 6" of total depth. Going with only 1" of Cavity Rock and a 2x3/R8 wood studwall would do it with less depth.

   Alternatively, if the bulk water management details are up to snuff, setting a 2x3 studwall with the interior edges of the studwall 4.5" from the brick would also deliver code-min on a U-factor basis with dense-packed borate-only cellulose. (~R16.5 at center cavity, but with ~R7.5 of thermal break on the studs.)

   Air tight drywall painted with "vapor barrier latex" would reduce interior side moisture drives from reaching the brick, but the bigger problem for the brick is direct rain/dew wetting from the exterior. It's probably safer to use a smart membrane type vapor retarder on the interior side like Intello Plus, detailed as an air barrier.

   7>"Where can I find a building science consultant who works on small projects like ours."

   Building Science Corporation
   70 Main Street
   Westford, MA 01886
   978.589.5100

   https://www.buildingscience.com/service/building-performance-and-enclosure-consulting

   They're not cheap and not always available, but they are constantly re-writing the book on this topic.

2. 
   gdbf | Feb 01, 2020 08:54am | #2

   Thank you Dana for your detailed response as always.

   It's a 3-story building so wood studs are allowed. We're reluctant to use cellulose before we do the exterior maintenance (water management, brick repointing, flashing). But that's another project and we need a building science consultant to guide us through the process.

   Back to the interior wall insulation, the lath wall, like you mentioned, is probably performing as a capillary break. So we're thinking to keep the lath, remove the old plaster and then install Rockwool between the wood studs. Would that be an option?  And from what you know, is there a minimum space between the brick and the insulation/wood lath to make the capillary break/drain space. We really like Rockwool - it's fire resistance and it doesn't hold moisture so a lower risk of a buildup of mold/mildew. 

   I tried to contact BSC about a week ago with no success, they are probably too busy. This website is really the main source of information for homeowners like us who want good & healthy homes - I can't thank you enough for what you do here.

   Gabriela

3. Expert Member
   Dana Dorsett | Feb 01, 2020 11:32am | #3

   >" So we're thinking to keep the lath, remove the old plaster and then install Rockwool between the wood studs. Would that be an option?"

   Code minimum in MA would be 2x6/R20 or 2x4/R13 + R5 continuous insulation. If the wood studs are tight to the old plaster (make sure the old plaster is air-tight, or install a layer of housewrap detailed as an air barrier), a 2x6/R23 rock wool wall gets you there. Going with continuous 1.5 inches of rigid rock wool against the old plaster and an 2x4/R15 rock wool studwall would get you there too.

   In most 19th century plaster & lath on brick walls in MA there is 3/4"-1" furring on the brick, with the lath nailed to the furring. A cavity depth of 3/4" will usually be enough. In most brick veneer new construction 1" is more typical. It's worth doing a bit of archeology on the wall assembly to verify that. In other parts of the country I've seen a few houses with the plaster applied directly to the brick, but never in MA.