1947 Cape Cod Attic Insulation Advice Needed

[Dana]

I have minisplit ductless electric units. There is literally zero insulation in the walls. About 2100 sq ft plus an unconditioned basement.

Is it a 3- zone multi-split tied to a single outdoor compressor, or 3 separate mini-splits? (Model numbers might be useful here too.)

With the house tightened up a bit an with some insulation stuffed in the walls you should be able to get the whole-house heat load down to something like 35,000 BTU/hr @ +15F (NYC's 99% outside design temp- yes I know it gets colder than that) even without insulating the foundation walls. In the process that will lower the room-to-room temperature differences between those rooms with ductless heads and those without. Just about any combination of three mini-splits would be suboptimally oversized for the "after upgrades" picture of the house, even if they struggle to keep up during Polar Vortex disturbance cold snaps now.

If it's a poured concrete or CMU (concrete block) foundation it can be pretty easy to insulate the basement too. Even though it's not actively being heated, the heat loss through the foundation walls is responsible for a double-digit percentage of the heating bill, most of it for the above grade portion of the foundation. While important at Con-Ed pricing, insulating the foundation is still lower on the priority list :

#1: air sealing the house (particularly the foundation sill & band joist + attic)

#2: insulating the walls

#3: insulating the attic

#4: insulating the foundation walls

As a DIY, in my area R15 fiberglass costs about the same as R15 rock wool at box store pricing, and rock wool is preferable on a number of different accounts, not the least of which is fire-resistance. Fiberglass is somewhat translucent to infra red, and can reach melting temperatures relatively quickly. With rock wool the whole house could burn down and the rock wool would still survive.

 

[Lordoftheflies]

A nice buried junction box.

 

[Sponsor]

 

[Lordoftheflies]

Two outdoor units. One 3 zone, one two zone, 5 interior units. Two zone is 20k btu, three zone is also 20k. 4 interior are 6k and one in living room is 15k.

Block foundation, not filled (hollow), 2x8 joists, water penetration on one side. It never ends.

 

[Lordoftheflies]

On this side at least there is blocking.

 

[Dana]

How do I deal with this? Insulation goes down to the crawl space. Should I cut some foam board and spray foam seal it? Stuff with insulation (guessing this is bad)? The wall below does not have an access panel so I'd have to do more demo to get back there.

View attachment 54218

By "crawl space" do you mean another mini-attic without an access hatch? If yes...

That space and the intervening roof deck still needs air access to the vented attic as a drying path, even if there isn't a large convective drive to move air. It doesn't look like it would be easy to slip in some 2" foam board the full length of it, but you don't want to dam it off completely. Installing 1" foam board as a vent chute bent into an air dam where the floor batts would bump up to it is probably the right approach. It's sort of like the vent baffle this soffit venting detail, but instead of a soffit it's a connection to a mini-attic below:

On rafter bays connecting the crawlspace to the main attic, maintain a 2" gap to the roof deck, not the code-min 1". On the side with the blocking I assume that's at an exterior wall (?), in which case the code-min 1" is enough.

 

[Dana]

Two outdoor units. One 3 zone, one two zone, 5 interior units. Two zone is 20k btu, three zone is also 20k. 4 interior are 6k and one in living room is 15k.

Block foundation, not filled (hollow), 2x8 joists, water penetration on one side. It never ends.

Not all multi-splits have the same heating capacity/efficiency at colder temps or minimum modulation levels during the shoulder season temps, so model numbers (including those of the heads) would be useful in trying to analyze roughly how efficiently they're operating now, vs. how they would do after some air sealing and insulating.

I assume the 15K head is on the two-zone compressor?

 

[Lordoftheflies]

Yes 15k is on the 2 zone. BTW $650 includes gas and electric. So our tankless water heater, gas dryer, and super duper 22k (2-burners other two are 10k and simmer).

We paid about the same when we had central air. That system was one zone and leaked like a sieve. When we took out the return ducts there was about 100lbs of dirt in them.

I wi get models later. About done with half of removal.

 

[Lordoftheflies]

They filled up two bays with r19. Good job!

And it's a mini attic just like your picture. 2nd floor of cape is the roof. So I guess I can put the mini rafter, slip it down from the top and just prevent the insulation from touching the roof.

 

[Lordoftheflies]

Over here at the side of the roof near the vent the exterior wall isn't sealed. I can the insulation does go down the wall a bit here. BTW it's rockwool insulation. And I can use rockwool but it doesn't come faced, correct? Is it ok to lay unfaced in between 2x4 joists then?

 

[Dana]

Since the space heating is only on the electricity portion of the bill, see if you can't separate it out. Take your lowest month of the year's power use as the presumptive background use, and divide by the days between meter readings as kilowatt-hours per day ratio. Then take the winter months electric meter readings and compare power use in excess of the background use to heating degree days for those periods, which should fall into a reasonably tight range. From that we can use the efficiency numbers on the ductless to estimate the actual design heating load using the methods outlined here.

 

[Lordoftheflies]

I was expecting to find treasure. So far I found:

One prybar
One Schaefer bottle cap
One piece of fusili pasta (someone's spilled lunch no doubt
One Kent half smoked cigarette.
A few pieces of roof shingles.

And no treasure.

 

[Lordoftheflies]

Short video of current status.

 

[Dana]

Over here at the side of the roof near the vent the exterior wall isn't sealed. I can the insulation does go down the wall a bit here. BTW it's rockwool insulation. And I can use rockwool but it doesn't come faced, correct? Is it ok to lay unfaced in between 2x4 joists then?

View attachment 54222

Wow! So this house is balloon-framed, with the wall cavities extending from the basement to the attic unobstructed (except for window & door framing)? That was pretty common prior to 1930, but fell out of use during the post WW-II era. That's an air-infiltration express!

When you get a chance snap a shot of how it's framed at the foundation sill.

It also looks like fiberboard sheathing, not plywood or planking(?). Fiberboard sheathing is good for about R2 if 3/4" thick, R1.5 at half-inch, but it means for blowing insulation into the wall you won't be able to dense pack it without risking bowing (or worse) of the fiberboard. Cellulose is still very air retardent, far more air retardent than 1lb fiberglass, and would be the right choice here. In the mean time, cutting up cardboard boxes to use as air-dams at the top of each stud bay would be a worthwhile (if slow going) endeavor. Cutting it so that it can be folded 3.5" x 14.5" with 1" tabs for stapling/caulking on all 4 sides makes it a bit easier to install and air seal.

On many balloon-framed houses the partition walls are open at the top and continuously convecting air into the attic, and those would need to be air sealed too.

 

[Lordoftheflies]

Here's the paper of the rockwool blanket.

 

[Lordoftheflies]

Literally everything that one. Ould do wrong has been done wrong in this house. In my first post look through the pictures of my bathroom gut and you will see the 2xi joist notched down to 1"...which subsequently just snapped in half when I leaned on it. You may have posted in that thread.

 

[Lordoftheflies]

Yes I think it is fiberboard sheathing. When they installed aluminum siding, they went right over the wood shingles.

 

[Dana]

The "BLACK VAPOR BARRIER" is asphalted kraft paper, and not a true vapor barrier.

Regarding those slope ceiling rafter bays, do NOT seal them up completely- leave at least a 2" for air to flow in those rafter bays. If/when it's time to bite the bullet and demo the sloped ceiling below would be the right time for improving the insulation there. It may even be worth drilling 2-3 holes in those bays that have the blocking if they're on a north facing pitch (where the roof deck temperature averages are lower, and roof deck moisture normally higher.)

 

[Lordoftheflies]

Over on the size where the 2x6 were used they used some very crunchy yellow unfaxed very skinny insulation as well. Seems newer. Barelly 2" tall.

 

[Dana]

Yes I think it is fiberboard sheathing. When they installed aluminum siding, they went right over the wood shingles.

It's worth discussing that issue with a few insulation contractors who do cellulose. The higher the density the better air retardency, but you won't be able to hit 3lbs, which is about what it takes to eliminate settling over time. At 2-2.5lbs it can sag as much as 6"after a decade or two (and can be topped off when it does), but even 7.5' out of 8' of wall being insulated would be huge, especially given the air retardency. If there are the 2 inch "econobatts" still in the cavity, it's still possible to install blown fiber over it using a dense-packing hose, snaking it between the batt & sheathing while running just air until the hose is all the way in, then turning on the fiber. They would have to adjust the blower settings to keep the pressures low enough to avoid bowing the fiberboard.

 

[Lordoftheflies]

The "BLACK VAPOR BARRIER" is asphalted kraft paper, and not a true vapor barrier.

Regarding those slope ceiling rafter bays, do NOT seal them up completely- leave at least a 2" for air to flow in those rafter bays. If/when it's time to bite the bullet and demo the sloped ceiling below would be the right time for improving the insulation there. It may even be worth drilling 2-3 holes in those bays that have the blocking if they're on a north facing pitch (where the roof deck temperature averages are lower, and roof deck moisture normally higher.)

Those blocks are on the south facing side.