There’s a growing demand for high performance roof retrofits. For many homes it can be most practical and economical to improve the airtightness and insulation from the interior. Often the homeowners also don’t want to leave their homes during the retrofit work. That takes spray foam off the table, since chemical exposure to spray foam during installation is a serious health risk. (Those haz-mat suits and gas masks aren’t just for show).
Dolphin Insulation, a cellulose insulation contractor based in Littleton MA, offers interior roof retrofits that are both high performance and foam-free. Over the years they’e refined their approach, and now provide unvented roof retrofits that exceed code requirements for insulation and vapor control. They do this using dense-pack cellulose along with Intello Plus airtight, smart vapor retarding membrane.
Beyond Roof Weatherization
Dolphin is pushing the industry far beyond conventional weatherization and into Deep Energy Retrofit levels of performance. By using cellulose insulation, they also complete the work without danger to themselves or the building occupants – and with minimal inconvenience – so life can go on largely without disruption in the living areas below.
These days Dolphin takes on a lot of “hot roof” retrofit projects that aim for minimum R-40 insulation. By including the attic in the conditioned space, it becomes useful for storage and HVAC. For HVAC systems in particular it’s a win-win scenario. The airtight and super-insulated enclosure makes an HVAC system more efficient, but also reduces heating and cooling demand so the homeowner can save money on a downsized system. David Posluszny, Dolphin’s Energy Engineer (recently featured in Home Energy Magazine’s “Thirty under 30“), notes: “We worked to an R-40 because [the owner] was having an air conditioner installed in the attic. However, the house has existing baseboards for heating. If the HVAC was going to be a furnace in the attic we prefer going to an R-60.”
They start by furring inward from the 2×6 rafters with 2x4s to create a thermally broken “double roof”. They also fur in the gable end walls. Then they install Intello Plus by stapling to the inside of the studs. They tape horizontal and vertical seams with Tescon Vana 75 (3″) or Vana 100 (4″) solid acrylic tape for robust airtight connections. Whenever possible they use double-wide (~10′) Intello Plus to keep horizontal connections down to a minimum. For this project they used 10′ wide membrane over the gable and 5′ wide membrane for the remaining areas on each side down to the eaves.
They follow with vertical plywood strapping over the rafters and studs. This strapping ensures that they avoid staple tear-outs when dense-packing to levels above the industry standard 3.5 pounds per cubic foot. For maximum strength they also position vertical taped seams to coincide with strapping. (Note from 475: we typically recommend horizontal strapping perpendicular to studs/rafters. But in cases where higher density is desired, vertical strapping can help prevent staple blow-outs.)
Beyond Typical Dense-packing
Once the strapping is in place, they like to dense-pack to as much as 4.5 pounds per cubic foot. At this density, the Intello feels quite hard (for comparison, at 3.5 psf the density is often likened to a “firm mattress”). David notes: “It is a great product. Our crews like using it because of the strength. Dolphin Insulation packs cellulose to 4.5 pounds per cubic foot. We have tried several products on the market to contain this density, but all seemed to pull out, give way, or stretch too much. The Intello Plus stands up to the pressures.”
After several projects, Dolphin has optimized their dense-packing approach. They prefer to allow the bays to “communicate” and pre-fill all of them. Then they go back and dense-pack one at a time. The pre-filling in the adjoining bays serves the function of separating the bays. David: “With our machines (extremely powerful and expensive) we find the best results with this method.”
They cut one opening in each bay for blowing in. After the bays are packed, they patch the blow holes with 6″ squares cut from Tescon Vana 150. David adds: “Of course there are plenty of spots thicker where the Intello Plus bulges out a bit with the massive cellulose. Those areas have higher R-values, but we don’t count those [in the R-40 total].”
With Intello Plus, Dolphin achieves high performance insulation, airtightness and smart vapor control in essentially one step.
Dolphin also makes improvements at the wall connections. They extend Intello Plus all the way to the wall airtight layer with resilient stapled and taped airtight connections. In practice, it’s challenging to get those connections perfect. By dense-packing into the attic joist bays – a physically demanding job (see photo) – they balance the air and vapor control benefits of both Intello Plus and hygroscopic cellulose at that important transition.
Vapor Variability = Venting Not Required
The Dolphin team notes “there is no venting in the assembly, and we don’t need any thanks to the Intello Plus.” 475 High Performance Building Supply’s best practice recommendation is to vent cathedral roof assemblies. But there are times when roof venting is either impossible or impractical (ie the house has a new roof and there is plenty of attic space to interior insulate). In those cases, Intello Plus provides the robust airtightness and vapor control needed to keep moisture levels safe, because of its best-in-the-industry vapor variable properties. This has been documented in our project spotlight on the recent Middlesex VT Passive House, which takes full advantage of Intello Plus for its unvented and super-insulated flat roof. In combination with Tescon Vana tape, with 100 year age-tested performance, this resilient system can be expected to far outlast any foam-based solutions while providing more reliable and predictable air and vapor control.
Most building codes or inspectors do not fully understand the safety that smart vapor control can bring. The route to choose in those cases is to work with the 475 technical team and convince the inspectors that:
- Intello airtightness verified with a blowerdoor test is equal to or better than foam inboard of the sheathing – and certainly more durable.
- Intello’s vapor variable properties allow inward drying of unforeseen moisture (caused by rain, construction moisture, or new air leaks from occupants’ puncture holes). With Intello these can dry out in summer – something that is not possible with foam. By request, we can provide a worst case WUFI Pro study for your location to press the point.
For best practice interior roof retrofits, we like to advocate a hybrid approach that combines Intello Plus at the interior with back-venting at the exterior. This can be done with or without a new roof, but we acknowledge it does take more effort. With Intello at the interior, in most cases the back-venting is not required.
For a detailed discussion of retrofit approaches, see our blog post Foam-Free DER: Roof Assemblies, get our free downloadable CAD details here, and take a look at Alex Wilson’s interior roof retrofit case study: Can’t vent at valleys and dormers? Use a smart vapor retarder (not foam).