Trying to insulate a historic home
We are trying to add insulation to a 2800 s.f. federal style timber framed home built c.1800 on a stone foundation that may be as old as the 1630s. The property is located within the city limits in Boston and is protected by Historic Landmark status, which means we need to preserve any exterior features that are original to the house, or interpreted to be consistent with the era. There is much more flexibility with the interior. Non-negotiable elements include: old clapboards, rebuilt period-style windows and doors (a topic for another post), repointed stone foundation, rebuilt brick chimneys, and a wood shake roof – yet to be installed. We are not planning on winning any awards with this house, we just want to bring it into the 20th century. (Yes, we will settle for the 20th century.)
There are two details I would love some input on.
1. The main section of the house has a hip roof, approximately 5 over 12 in the main section, with an attic about 4′ high at the ridge. And while in its current condition plenty of air is circulating through there, there are no “official” soffit or ridge vents, and it’s unlikely the historical commission will approve the addition of these features. I’m assuming that raising the entire roof line a few inches by installing exterior insulation is also out of the question. Fortunately the attic is relatively clear and accessible, and I am confident in our ability to create a complete air barrier between the attic and the conditioned space, including proper detailing around the chimneys. We are also installing idiot-proof ventilation in the kitchen and baths, which should help prevent excess humidity. The plan at the moment is to blow in maybe 16″ or so of cellulose into the attic, although batt insulation is also a possibility. My question is, must we vent this attic that has gone unvented for 200 years? And if so, how would you recommend doing this?
2. Again, this is a timber frame home, two stories. The exterior wall assemblies are what you’d expect: rough cut studs, 3/4 board sheathing, building paper and clapboards, with horse hair plaster and lath on the interior, currently uninsulated. Some of the walls (maybe 1/4 or less of the exterior face) have brick nogging in them – a feature charmingly described to us as “brick insulation”. Basically, some of the stud bays are filled with a layer of mortared bricks, right where the insulation should be. It’s unclear to me what purpose the bricks served to the original builders – some combination of pest deterrent, wind break and insulation is the theory. The historical people consider this a feature of the home to be saved. Given that the exterior clapboards are not to be touched, our initial thought was to strip the interior of all exterior walls down to the studs, and build in a second wall, with staggered studs, to provide space for insulation. This invasive strategy was quickly abandoned, however, on both preservation and budgetary grounds. At this point, our next option is blown-in cellulose (spray foams are not an option due to “reversibility” restrictions). Again, the one part of this we have some control over are the air sealing details, and although we will have to go to great lengths with caulk and spray foams, I am confident we can provide an adequate air barrier. And so my questions here are: would you go ahead with blown in cellulose in this wall assembly? What do you know about this nogging business? Why would a builder put their thermal mass in the exterior wall in this climate? Does it have something to do with heating with wood? We are actually planning on putting a wood stove in the room that has most of the nogging, and while we expect the bricks will soak up interior heat and radiate it outside, we can actually see how it would make the room more comfortable by tempering the heat from the wood stove, in addition to the thermal mass from the two large chimneys.
Well, I could go on and on but that should give you all enough information to chew on. This is a very interesting challenge for us, I hope you think so too.
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Ah, to clarify. The blown-in cellulose would only go in stud bays without bricks. Also, while spray foams are not an option to completely fill stud bays, we can get away with using them for air sealing around penetrations.
1. I think you can safely install cellulose in the stud bays. Weatherization crews do this all the time in historic homes.
2. I think you understand the bricks correctly. They were a way to slow down the wind and make the walls more airtight in an era with few materials to choose from.
Thanks Martin, what do you think about that roof venting?
I don't foresee any problems if you don't vent the attic.