As we outlined in Foam-Free DER: Series Introduction (here), we are looking at the recommended assemblies provided in the Mass Save Deep Energy Retrofit Builder Guide (the Guide), and offering foam-free alternative approaches. Also as noted in the introduction, we are starting on the “white board”, literally – and look forward to comments as we elaborate and refine these options.
We follow the order and categorization of the Guide for easy comparison:
- Vented Attic
- Unvented Attic with Insulation Exterior to Roof Sheathing
- Unvented Attic with Insulation Interior to Roof Sheathing
Note: All suggested approaches below are only a starting point in the process, and must be carefully considered by the project professionals in relation to all the interrelated general, as well as project specific conditions. Airtight high performance assemblies must maintain airtightness to function properly and require robust detailing and blower-door verification.
1. Vented Attic
Page 49 of the Guide shows a vented attic condition with closed cell spray foam forming the airtight layer on the top side of the existing ceiling with loose fill fibrous insulation above.
Instead of spray foam at back side of ceiling (which is a sacrificial layer), make an air-tight layer with OSB sheathing at the top side of the ceiling joists. Tape the sheathing joints with TESCON Vana tape. This forms an enduring air barrier with insulated service cavity below. Install loose-fill insulation above as needed. Consider laying interlocked sheathing over the loose-fill to prevent wind-washing.
2. Unvented Attic with Insulation Exterior to Roof Sheathing
Pages 50 and 51 of the Guide show an unvented attic condition with foam board insulation exterior to the existing roof sheathing. The foam boards prevent condensation at existing sheathing but inhibit the natural outward drying tendency of the assembly in winter when condensation is a concern. No inboard airtightness is indicated and therefore thermal bypass air currents, that can degrade the insulation’s effectiveness, are a greater possibility.
Instead, use vapor open fibrous insulation like mineral wool or wood fiberboards. Put the airtight, waterproof and vapor open membrane (SOLITEX Mento 1000) on top of the fibrous boards, with battens on the membrane. You can then apply cross battens and a metal roof or sheathing and roof shingles – both back vented and cold. For additional protection or at roof pitches less than 30 degrees, apply self-sealing double sided butyl tape (TESCON NAIDECK) to back of battens before fastening through the membrane and insulation boards. Surround the fibrous insulation with airtightness at the interior with air tight and vapor intelligent membrane (INTELLO). Battens over the membrane at interior hold dense-pack in place and provide a protective shallow service cavity.
Now if you want to expose the existing roof deck to the interior and put all the insulation to the exterior, the Guide doesn’t offer a suggestion, but we do here. On the existing roof deck install an air and vapor control membrane – DA or INTESANA. DA is an airtight, waterproof, Class III (1.43 perms) vapor retarder, and INTESANA is an airtight vapor intelligent membrane like INTELLO but suitable for exterior exposure during the construction process. Place fibrous insulation boards on the membrane. Place exterior air control and waterproof membrane (SOLITEX Mento 1000) on top of insulation boards with battens and roof, similar to above.
Note: R60 can mean 16 inches of fiber insulation board and although it is possible to achieve this in a number of ways (long fasteners, built-up bridging, modified larsen truss), it may prove impractical to reach R60. Therefore if exposing the existing roof and structure is a project priority then a reduced R value should be considered.
3. Unvented Attic with Insulation Interior to Roof Sheathing
Pages 52 and 53 of the Guide show an unvented attic condition with closed-cell spray foam to the interior of the existing roof sheathing – one without interior finish (p52) and one with (p53). In this case the existing roof can be left untouched and should be considered vapor closed. The closed-cell spray foam is serving as air, vapor and thermal control.
Instead, we offer two possible foam-free alternatives: one providing back-venting to the existing roof and one that doesn’t.
Back-venting can be provided by applying airtight and vapor open membrane (SOLITEX Mento 1000 or SOLITEX Mento Plus) to the underside of the existing sheathing with center and side spacing battens and staples and airtight adhesive caulking (CONTEGA HF). Note that the venting gaps created must extend to the exterior to do their job. Rolls of membrane can be saw–cut to typical widths. Below the membrane and between the joists, to depth necessary, install fibrous insulation. If dense-packing, use SOLITEX Mento Plus membrane above, and if using batt insulation use SOLITEX Mento 1000 membrane above. Extend depth of joists as required for insulation levels with either cross-battens or wood plate extenders (a modified larsen truss for the roof). At interior of insulation place an airtight and vapor variable membrane (DB+ or INTELLO). Add cross-battens for protective service cavity.
The non-back vented solution involves simply installing dense-pack fibrous insulation (to a density that is self-supporting, preventing separation from sheathing above) with “joist extenders” similar to above. Inboard of insulation install airtight vapor intelligent membrane (INTELLO). Because in this assembly we conservatively assume that the existing roofing is vapor closed it is important to have air-tight insulation provided by the INTELLO. The INTELLO is a vapor barrier in the winter months protecting the assembly from wetting and is vapor open as needed in the summer months – to ensure maximum protection from moisture damage.
Note: As this assembly has an unvented, vapor closed roof, it should be checked with a WUFI calculation – because local climate (shading, orientation) can dramatically effect the drying potential of the assembly. We can help with this.
Next in this series is Foam-Free DER: Wall & Foundation Assemblies.
- 475 Presentation: Foam Free High Performance Building Enclosures
- Foam-Free Wall Assembly Guidelines by Climate Zone
- Foam Fails
- Foam Fails – Reason #1: Dangerous Toxic Ingredients
- Foam Fails – Reason #2: Unacceptable Fire Hazard
- Foam-Free DER: Series Introduction
- Historic Masonry Retrofit Details and Notes (cold/mixed climates)
- How to airseal a (rowhouse) roof
- How to airseal a Brownstone to PH standards (party walls)
- How to install dense-pack cellulose with Intello Plus airtight membrane
- Integrating a window in the airtight layer – brick/brownstone retrofit case
- Joe Lstiburek: air seal both sides of the insulation<
- Tighthouse – Park Place Brownstone renovation
- 475 Video: Historic Brownstone Passive House Retrofit in Brooklyn, NY