Insulating angled roof rafters or laying insulation on the attic floor?

Submitted by haxxer_4chan t3_127pp82 in DIY

After some critter damage to current loose-lay insulation, we have to remove and replace all insulation in the attic of a new (to us, 1919 build) house. Suggestions have fallen into two camps: replacing the loose-lay with similar insulation on the floor, or spraying the angled roof rafters with foam insulation. We probably won't use the attic for storage either way, but there is an AC handler up there that we've got to access every once in a while. I see lots of pros and cons to each insulation method, so I'd love any thoughts and suggestions folks have on which is better for an old, small attic. We are in Chicago, so the roof will be exposed to weather extremes of pretty much every type. Thanks in advance!

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bassboat1 t1_jef73y9 wrote

If there's no need to pay to heat the attic, insulate the ceiling and save the $$.

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haxxer_4chan OP t1_jef7mnb wrote

Thank you, out of curiosity, how does that save money if we don't heat the attic (which we do not)? I would have assumed that keeping insulation closer to the floors we do heat would have been more efficient, but I am asking here because I admittedly know very little about it.

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bassboat1 t1_jef9lhv wrote

You'll be heating less surface area and volume.

In my area (zone 5), ceiling R-value requirement is R-49. The job would require one run of tunnel vents at the eaves, some fiberglass batts to block them up and 18"-20" of blown insulation on the flat ceiling. To get the same R-value on the rafters, you would need either a hybrid of spray foam/batts/rigid board insulation, or build down the rafters for batts. You'd also have to apply R-19 to the gables, and consider a venting solution.

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haxxer_4chan OP t1_jefab37 wrote

Thank you, this makes a lot of sense. Much appreciated!

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shootdang167 t1_jef900u wrote

Foam is expensive compared to batts or cellulose

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haxxer_4chan OP t1_jef995g wrote

Ah right, I totally misread ceiling as meaning rafters and thought they were suggesting the opposite. Makes more sense now.

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DarkLink1065 t1_jefgbix wrote

There are actually some very good reasons to include the attic in the heated space (e.g. if your hvac conduits are inside your conditioned space rather than running through a blistering hot or freezing cold attic, you can prevent fairly significant losses and actually save money even though you're heating a larger space), but you have to make sure your attic, roof, and HVAC systems are set up for it. You shouldn't just slap insulation on the bottom of the roof deck and assume it will work, it may not and it also has the possibility of creating things like moisture damage problems in your roof depending on your climate.

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BullOak t1_jefgtu9 wrote

This is one of those counterintuitive building science things. Done properly, a house with a conditioned attic will require less HVAC energy than a house with a vented attic. Generally speaking, there's enough gains in latent loads, stack effect control, and keeping the ducts in conditioned space to offset the additional conditioned area.

but vented or conditioned is much less important than getting the details for either correct.

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shootdang167 t1_jef90ed wrote

Ceiling

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haxxer_4chan OP t1_jef9d2r wrote

Decisive, I like it. Thank you! Just to save on installation and materials?

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shootdang167 t1_jef9sob wrote

Yep. I presume your 1919 house doesn’t have soffit vents or ridge vents in the roof, so you would have to use closed cell spray foam which is a minimum of $2.50/sqft for 2” thickness (R14). This is per sqft of roof deck, not floor space.

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Doctor_Frasier_Crane t1_jefazl5 wrote

I would assume that the HVAC unit in your attic is the furnace & vent pipes. Do you have a separate AC unit outside on the ground somewhere?

The problem with having HVAC in the article is the heat/cooling loss from improperly insulated vent pipes. So if you enclose the space with spray foam or other method on the underside of the roof sheathing, your HVAC system doesn’t have to work as hard.

https://buildingscience.com/documents/building-science-insights-newsletters/bsi-119-conditioned-unconditioned

https://buildingscience.com/documents/bareports/ba-0305-why-it-s-so-important-and-troubling-to-keep-ducts-and-equipment-in-conditioned-space/view

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haxxer_4chan OP t1_jefbaq7 wrote

It's actually just a SpacePak air handler, so no heating equipment or AC compressor, just the intake, handler, and some ducts, though it seems those might also be important to keep in a relatively conditioned space

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Doctor_Frasier_Crane t1_jefc3r2 wrote

Agreed. Ducting tends to be leaky, and any insulation on them is likely maximum R-6 or R-8. So heating/cooling loss and negative pressure on your living space.

What do you have for heating & cooling then? Baseboard heaters, mini-split, or something else?

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haxxer_4chan OP t1_jefkmp9 wrote

Radiator heating with a boiler in the basement and separate AC systems for upstairs and downstairs

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MoSChuin t1_jefisn8 wrote

I live northwest of you. By 8 hours. So it gets super cold up here. Going with the foam is more expensive, but in the hear savings it pays for itself in 5 years. So, if you're going to live there for more than 5 years, get the foam. If you're planning to live there less than that, get the blown.

In both cases, go with the ceiling insulation, not the rafters. The rafters won't give you enough thickness, and you'll need an air chute the entire length of the rafter, to prevent ice dams. You might have venting issues because of the way roofs were constructed back then with the center spine acting as a venting block.

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