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Community and Q&A

Insulation and mechanical plan critique for 115-year-old retrofit

twoodson | Posted in Mechanicals on

Currents State:
– Zone 4A (st louis).  Heating design temp 10F / Cooling 93F
-1700 sq foot 2nd floor rental apartment
-3 wythe thick brick walls (12 inches)
-Low slope roof with 4 inches polyiso and TPO membrane
-Small “false” roof in front 5 foot high peak
-10 foot ceilings
-14 large (72″x34″) original windows windows (retrofitted with silicone and polypropylene seals, low-e storms)
–  1 5/8″ steel framing to the inside with min 1″ gap from brick.

Insulation Plan:
Wall – 1.5 inches of closed cell spray foam (Icynene ProSeal).  Trying to balancing the risk of freeze/thaw damage risk and interior comfort/equipment sizing.  Improved water management on exterior parapet, windows to keep brick dryer.  Exterior parapet bricks were in good condition, so I’m not overly concerned.
Ceiling – R38 fiberglass batts between roof joists (4 inches of exterior poly already in place)
False Roof  – 4 inches of spray foam on interior with R38 batts (framed out on inside to accommodate)

Mechanical Plan
-CoolCalc Manual J attached
-Careful Manual D done to ensure correct airflow/low static pressure
-Fujitsu 9RLFCD ducted mini-split for back rooms/bathrooms
-Fujitsu 18RLFCD for front kitchen, living rooms, front bedroom
-Panasonic WhisperComfort ERV
-Panasonic WhisperGreen bath fans

I know that I am slightly oversized, however, I was 1) concerned about undersizing for cold/hot snaps with tenants complaining.  2) concerned about airflow (the 1.5 ton unit can move more air)

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  1. Expert Member
    Peter Yost | | #1

    Hi Tom -

    Mechanical system sizing and calcs is not my strong suit but I can tell you that your concern regarding freeze-thaw for your climate: no worries. Even with making your 3-wythe brick colder with interior insulation, you should not have issues in your climate.


  2. twoodson | | #2

    Thanks Peter, that may convince me to add a bit more insulation. It would be nice to have R20 walls and r60 ceiling.

    Any opinions about the depth of the foam on the walls? Can i decrease to 1 inch and add r8 batts to fill? That would get me about the same as 2 inches of foam and I'm guessing cost substantially less.

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