A collection of experts working on deep-energy retrofits recently attended a brainstorming session to share design tips and propose topics for further research.
The conference, formally titled the “Expert Meeting for Details for Deep Energy Retrofits,” was held in Boston on March 12. The meeting was funded by the Department of Energy’s Building America program and hosted by the Building Science Corporation.
Several experts — including two principals of the Building Science Corporation, John Straube and Joe Lstiburek — gave presentations. Straube discussed retrofit options for walls; Lstiburek covered roofs; and their colleague Kohta Ueno discussed basements. Paul Eldrenkamp, a remodeler from Newton, Mass., shared his experience with several deep-energy retrofit projects, while energy consultant Marc Rosenbaum shared questions designed to stimulate new approaches to reducing residential energy use.
I’ve mined the published report of the proceedings for the following tips and pithy quotes.
If you’re adding rigid foam to your wall, put it on the exterior
Dr. Straube noted the advantages of exterior over interior wall foam. Thick walls without exterior foam usually have cold OSB sheathing. That’s bad: cold OSB is a potential condensing surface — and damp OSB can rot fast. (By encouraging drying, a ventilated air gap between the OSB and the siding can go a long ways toward reducing the risks associated with cold OSB. But adding exterior foam is the best way to eliminate the risk of condensation.)
Among Dr. Straube’s points:
After his presentation, Straube answered questions.
Q: “Can a Larsen truss exterior wall assembly filled with cellulose insulation be used to provide significant insulation to the exterior of a structure?”
Straube’s response: “With a Larsen truss, cellulose, and sheathing on the outside, the sheathing is actually colder than ambient air because of radiation transfer.” (Straube was referring to nighttime radiation transfer, when a wall’s heat radiates into the night sky.) “Therefore there is a condensation risk for the exterior sheathing in this situation.”
Q: “Is this type of condensation risk bad news for SIPs?”
Straube’s response: “It would be very risky to have cedar siding directly against a vapor-open layer — i.e. housewrap — which is directly against sheathing with low drying potential — i.e. OSB with foam directly against it. In such situations, the OSB will tend to rot.”
Q. “Doesn’t the foil facing on insulating sheathing, which is a vapor barrier, make you nervous?”
Straube’s response: “Yes, it is risky if the insulation board is thin. [But] with a 4-in. layer of [polyisocyanurate] insulation there is very low risk due to the elevation of the condensing surface temperature. However, there are risks associated with bulk water control, if it is not correctly addressed during the retrofit.”
Q: “Is there a concern about the longevity of tape on the face of exterior insulation in light of the fact that the panels expand and contract?”
Straube’s response: “Yes, this is a concern. That is why it is important to have a good air flow control system and drainage plane behind the exterior insulation.” (In other words, Straube likes to see housewrap installed behind the exterior foam, not on top of it.)
For a cathedral ceiling, either open-cell foam or closed-cell foam will work
Dr. Lstiburek shared advice on adding insulation to roofs:
- “Closed-cell spray foam insulation does not elevate the risk of damage from roof leaks relative to open-cell spray foam. The vapor permeability of open-cell spray foams does not appear to be a factor in mitigating damage from roof leaks.”
- “In climate zones 6 and above, a vapor control layer must be added to [roof] assemblies that employ open-cell spray foam in a configuration with all of the insulation below the roof deck. Open-cell foams in compact roof assemblies may require supplemental vapor control layers in climate zones 4 and 5, depending upon the interior conditions, construction of the roof and other factors.” (In other words, install a vapor retarder on the interior side of open-cell spray foam in cold climates.)
Q: “Does an unvented roof assembly necessarily employ spray foam insulation?”
Lstiburek’s response: “No. For example, with insulation exterior to the roof deck, virtually any insulation works underneath the deck (that is, insulation below the roof deck need not be air impermeable or offer significant vapor diffusion control in this configuration).” (That’s right — even fiberglass batts can be used, as long as you have enough rigid foam on top of the roof sheathing.) “Board foam in conjunction with sealants or tapes may also be used as the air impermeable insulation below a roof deck. It is also appropriate to use a ‘flash’ application of spray foam below the deck (for airflow control and vapor control) and then provide the balance of the insulation with an air permeable insulation material.”
Q: Do compact roof assemblies employing open-cell foams require a vapor-control layer?
Lstiburek’s response: “Yes in Zones 6 and higher, probably 5 as well. Direct-applied vapor retarder paints have not provided rated perm ratings in lab trials.” (That’s news. In other words, if you apply vapor-retarder paint to cured foam, and you test the assembly in the lab, the vapor-retarder paint doesn’t work.) “I recommend installing drywall in contact with the foam” — (that’s tough if the foam is recessed below the framing) — “and painted with a vapor-control primer. In this configuration, the drywall must cover and be in contact with the insulation over the entire area of insulation application, including behind kneewalls and above flat ceilings.”
Q: “Is it better not to vent a roof?”
Lstiburek’s response: “I like to vent whenever I can. But when roof venting cannot be done so that it will be effective or when it is difficult (e.g. with hipped roofs or multiple dormers) it is better to have an unvented assembly.”
Lstiburek gave a provisional thumbs-up to those who attack roof overhangs with chainsaws. “The chainsaw retrofit strategy (removing roof projections to allow a membrane to wrap from the wall onto the roof) is very robust in terms of air flow control because it offers a very effective means of transitioning the wall air control layer to that of the roof,” said Lstiburek. “However, some will object to removing character-defining details even if they are to be rebuilt or replaced.”
If a builder doesn’t want to lop of a building’s roof overhangs, the integrity of the air barrier at the wall/roof intersection has to be maintained by installing spray polyurethane foam from the inside of the building. This only works when installers are meticulous. In other words, this approach requires careful supervision.
For more information on this strategy, see “The History of the Chainsaw Retrofit.”
Kohta Ueno gave a presentation on foundation insulation.
Q: “When using spray foam insulation against the foundation wall, is it preferable to use closed-cell or open-cell foam?”
Kohta Ueno’s response: “Open-cell [foam] gives some drying to the interior but entails a risk of condensation/frost layer in cold climates. Closed-cell [foam] is less vulnerable to moisture.”
Marc Rosenbaum shared his questions
Rosenbaum’s presentation was mostly in the form of open-ended questions, including:
- “When is a cocooning strategy sensible (isolating and superinsulating part of a house, providing it with its own heat)?” As energy prices rise, many families may not be able to afford to keep a large home at 70°F all winter long.
- “At what point is the gas meter disconnected to save the monthly meter charges?” After a deep-energy retrofit, gas consumption may drop to the point that the meter-reading charge exceeds the charge for gas usage. At that point, an all-electric house makes a lot of sense.
More information needed
The official summary of the deep-energy retrofit meeting identified several “research gaps” worthy of further study. These included:
- Details for attaching siding, porches, and roof overhangs through thick exterior foam.
- Details to guarantee the continuity of drainage and airflow control functions at windows, at the roof-wall interface, and at the interface between an above-grade wall and the foundation.
- Details for the structural attachment of windows in high-R walls.
- The identification or development of coatings that significantly reduce the wetting of brick walls without overly reducing either the ability of the bricks to dry or the aesthetics of the brickwork.
I’ll let Paul Eldrenkamp have the last word: “Almost all remodeling decisions are emotional decisions. The $100,000 kitchen is going to look 20 years old in 20 years, while the deep-energy retrofit is going to look better and better over time.”
Last week’s blog: Energy Efficiency Retrofits: Insulation or Solar Power?