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What is the best combination to renovate my 1950 modern home?

Our one story 2,000 sf modern home was built in the Boston area in 1950 with a thoughtful floor plan stretched east-west, with lots of glass facing south, and a small amount facing the other orientations. We plan to add a study, half bath, screened porch and bring the laundry up from the basement so that we can more easily "age in place" for twenty years. A few years ago we replaced the tar and gravel roof with a white TPO membrane on PIC roof sheathing. The original Hopes-type steel casement windows continue to grow mold despite quality storm windows and most of the cranks are broken. I plan to replace them with fiberglass windows, triple pane if possible. The 2x4 stud walls on the south, west and north sides support a mono-pitch 2x10 "cathedral ceiling" roof. Those walls are sheathed with 1x8s and covered with 1x4 T&G fir vertical siding. The east wall was built of exposed 8" CMU. All rest on a 10" CIP foundation without insulation. The basement has been dry for the 16 years we have lived here. There is a three foot tall crawl space under two bedrooms and a bathroom, and the master bedroom, bathroom and family room are SOG. The house was built with reasonable length roof overhangs above the south-facing windows. I plan to fill the rafter bays (unvented roof) with CCIPF, for R60-R65, including the new sheathing above. What I can't decide is how much R-value and type of insulation to put in the walls and on the foundation. Due to some irregularities and numerous perforations, I plan to seal the rim joists with 3" CCIPF. One source thinks it is possible to very carefully fill the 2x4 walls with CCIPF for R23. If the GWB and sheathing aren't then blown out, should I then add 3" PIC to the exterior to get to R42, or instead should I do the two layers of 2" PIC outside dense-pack cellulose or OCIPF for R40? If the triple pane windows are just too expensive, should I similarly scale back the walls to R26-R30? After adding board insulation outside the sheathing, I plan to install vertical siding to maintain the original design of the house. What are people's experience with fiber cement siding vertically? What would be the best air barrier/drainage plane to use? I can't use vertical strapping under vertical siding, so I am considering using Home Slicker instead. I am concerned that this will be squishy to nail through the PIC, so I assume a layer of 3/4" plywood would be needed outboard of PIC sheathing. Are there better solutions to vent vertical siding? On the CMU wall, I'm thinking of using EIFS, but should I pour Perlite in the voids and add insulation on the interior side? If so, how much? Do I need a vapor barrier there? Thanks in advance for your insights.

Asked by Timothy Oldfield
Posted Tue, 06/24/2014 - 16:19
Edited Tue, 06/24/2014 - 16:24

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1 Answer

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Timothy,
You use lots of confusing abbreviations. In the future, try to spell things out.

Q. "If the gypsum wallboard and sheathing aren't then blown out, should I then add 3 inches of polyisocyanurate to the exterior to get to R-42, or instead should I do the two layers of 2-inch polyisocyanurate outside dense-packed cellulose or open-cell spray polyurethane foam for R-40?"

A. You are listing lots of options. Most of these options will result in a high-performance wall, but the cost will be high. My own preference would be for dense-packed cellulose between the studs, and either 2 inches or 4 inches of polyisocyanurate on the exterior side of the wall sheathing.

Q. "If the triple-pane windows are just too expensive, should I similarly scale back the walls to R-26 to R-30?"

A. Obviously, we can't determine your budget or your specifications. Perhaps you are asking, "Which measures will be the most cost-effective"? If that is what you want to know, you should hire an energy consultant (I suggest Paul Eldrenkamp and Mike Duclos) to run some energy modeling software and to make construction cost estimates. The software and cost estimates will help you make these decisions.

In general, R-40 walls and triple-glazed replacement windows aren't cost-effective in Massachusetts for a retrofit project. That doesn't mean that they don't work.

Q. "After adding board insulation outside the sheathing, I plan to install vertical siding to maintain the original design of the house. What are people's experience with fiber cement siding vertically? What would be the best air barrier/drainage plane to use? I can't use vertical strapping under vertical siding, so I am considering using Home Slicker instead."

A. I would suggest that you either (a) install horizontal furring strips over the rigid foam, and stop worrying about drainage (you can trust evaporation to work), or (b) install vertical furring strips followed by horizontal strapping.

Q. "I am concerned that this will be squishy to nail through the polyisocyanurate, so I assume a layer of 3/4-inch plywood would be needed outboard of polyisocyanurate."

A. Furring strips are better than plywood, because the air gap is beneficial.

Q. "On the CMU wall, I'm thinking of using EIFS, but should I pour Perlite in the voids and add insulation on the interior side?"

A. No. With EIFS, all of the insulation is installed on the exterior side of the concrete block wall. Perlite in the voids won't do much.

Q. "Do I need a vapor barrier there?"

A. On an EIFS wall, the rigid foam is an adequate vapor retarder.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 06/25/2014 - 05:32
Edited Wed, 06/25/2014 - 05:34.

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