Help with prioritizing in a 50s Cape Cod
Hey all,
I’ve been doing all the reading I can to get more knowledgeable around home efficiency and building science. I’m hoping for some advice on where to prioritize energy efficiency improvements to gain the highest impact on the initial investment.
Home background:
50′ long Cape Cod built in 1950
Zone 4/5 (just north of Philadelphia)
1.5 floor:
– currently insulated with r13 fiberglass at roof but in really rough shape (2×6 rafters)
– full length soffit vents, can see daylight from knee wall crawl space
– only partially finished out, majority is really crappy drywall
1 floor:
– 8″ block wall construction with little to no wall insulation
– some newer windows but not sure of the type
– wood burning insert located at one side wall
– wood floors over whole level, no insulation under floor joists
Basement:
– half is 4′ tall crawlspace with low quality poly vapor barrier
– other half is 6′ tall concrete floor with French drain around exterior
– whole basement is open, no walls
– no existing rim joist insulation or wall insulation
Overall HVAC:
– oil fired boiler, baseboard/radiators, sized plenty big but uses lots of oil (~300-350 gals per month in winter)
– typical AC system, run through one knee wall space. Usually only 1 month a year with high electrical costs ($300ish)
I was thinking it made the most sense to start with a good roof/attic insulation given the massive air leaks and rather low r value. What advice might you all give a young homeowner trying to stretch his investment?
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Replies
That’s got to hurt the wallet! How old is the AC?
No doubt! The AC was installed in 2009 but it's actually "affordable" compared to oil usage in the winter... we're spending on the order of $3000-$5000 per winter in oil since we moved in two years ago.
What’s your electricity rate? Ditching oil might be the easiest and quickest path to savings.
That's definitely the long term plan but feels like we need to get the overall heating load down to make that a real possibility. Our current electric rate is right about ~21 cents per kWhr.
You should still insulate and air seal, but that doesn't preempt you from switching the AC to a heat pump. $.21 electricity is 30% cheaper than $3.5 oil.
There are two elements to insulation: air sealing, and insulation. In general you want to do your air sealing first, as an unsealed house is drafty and uncomfortable, poor air sealing in one part of the house affects the rest of the house, and the insulation is less effective if air is leaking. Once you've got air sealing under control you can insulate the house incrementally, each surface is effectively independent of the others.
In a house like this you really have to think about where the building envelope -- the boundary between heated space and unconditioned space -- lies. You want a continuous building envelope, the adage is you should be able to draw the building envelope on a plan of the house without ever lifting your pencil from the paper. When houses of this vintage were built they had no concept of the building envelope -- nor of air sealing -- and you get spaces like the basement and behind the knee walls which are kind-of inside the building envelope and kind-of outside. You need to pick a side for them.
My recommendation would be to include the basement and crawl space in the building envelope, which means the walls and rim joists need to be sealed and insulated. On the second floor it's a little trickier, you can either run the building envelope across the floor of the knee wall area and up the wall to the roof -- excluding the knee wall area from the building envelope -- or run the building envelope along the roof to the exterior wall, including it. I'd have to know more about how it's constructed to know for sure, but if there's HVAC equipment in there it probably wants to be inside the envelope.
If it's block wall from the ground to the eave that's probably pretty airtight already. Most of your effort is probably going to be around the roof.
When it comes to insulating the roof and walls, the thing you're going to run into is insulation is thick. And thinner insulation is a lot more expensive.
Yeah I agree with everything about air sealing being the most important first step. Seems straight forward enough in the basement through rigid foam and great stuff at the rim joist. Following that mentality, the building envelope would follow the interior side of the block wall to the roof eaves. This area seems like my biggest air leak with the soffit vents wide open to the second floor joists at this point.
I've read up extensively on Martin's cathedral roof guidance and the most applicable design at this point feels like a vented assembly with site built vents from rigid foam. I based this on the basically new roof and my aversion to spray foam. Would you agree that that decision should get made to air seal the roof would require an all out reinsulate of the roof?
Thanks for the response!
The first question is will you still own this house on 20 years when all the work you are contemplating is likely to recover its costs. The fact is most people are gone in 7 years or less and almost nothing ROIs that fast.
I will start by saying I hate spray foam but if you are bound and determined to air seal an old story and a half home it is your only real hope.
Walta
You raise a good conversation about ROI... I am in the group where we locked in a stunningly low mortgage rate so the odds are higher of staying put for 7-10 years minimum. The balance point for what makes sense to invest in insulation from a cost standpoint is about 6-7 years with some napkin math of 20-25% efficiency improvements with existing oil prices but money isn't the only story right?
I'm thinking from your description that the place to start is to rip out the "really crappy drywall" and insulation on the second floor and spray foam to the tops of the exterior walls. That shouldn't be a big job, the demo you could do yourself if you rent a dumpster.
Then how about this for a plan: fur out the rafters to an overall thickness of 7.5". Apply 4" of closed cell spray foam and 3.5 inches of open cell, that would give you an overall R-38 in addition to sealing the entire top of the house. If the end walls are stick construction do them too. Then get a drywall crew in to redo the second floor.
Figure on about $2/board foot for the closed cell and $1 for the open cell, or $11.50/square foot. Drywall should be about $3/square foot. The foam needs to be covered in drywall to be fire-safe.
Then as time allows work on insulating the block walls on the first floor and basement. You can either build 2x4 walls on the interior and fill them with batts or blow-in, or put sheets of foam directly against the wall and drywall over them.
Oof, at 11.50 a square foot the spray foam is a tough pill to swallow. That's about $15,000 for the ~1400sq.ft of roof I'd have. I've built out a calculator to estimate a vented roof with eps baffle, r30 mineral wool (with furring strips), and a layer of polyiso for nominal r35-40 at about $7500 in materials + some sweat equity. Understandably the air seal would be better with spray foam but is the increased cost really worth it?
Another option over spray foam in these older houses is to dense pack the rafters in the lower attic and the sloped ceiling section and vent only the upper attic. The existing baffles even if partially collapsed would still work well enough.
Dense pack cellulose does a decent job of air sealing and relatively cheap material. Much less work than dealing with foam baffles.
You still need a decent air barrier in the lower attic. This can be one of the fancier membranes that will hold up to dens packing (ie intello-plus) or interior rigid insulation over the rafters with the seams taped. You still have to deal with sealing your floor joist area at the soffits.
If you want extra R value, you can always strap out the rafters with 2x2 or 2x3 on edge to get more depth or go the rigid insulation route. Under most codes, the area with the HVAC would need rated rigid insulation (ie Thermax).