How To Choose The Right Insulation For The Job


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It’s oft en been said that the key to success is getting the right people to do the right job at the right time. This adage holds true for building materials as well — we need to make sure we have the right building materials installed, the right way, in the right location in order for the home to be successful at delivering a healthy, safe, comfortable, and durable shelter.

When faced with an existing home, the renovation choices are oft en limited by budget considerations and the scope of the project. Will this be a gut-rehab, is the owner going to remain in the home during the work phase, or will you only have access to basements, crawl spaces, and attics? Is there a cost-benefit to increasing the level of insulation in certain locations?

The first step should be to hire a certified HERS rater to perform an evaluation of the home. Make sure you interview them and talk to past clients — most HERS raters only work in new construction, some are almost exclusively in the existing homes market, and some do both. Based on their evaluation of the building envelope, they can make defensible recommendations for insulation levels and specific locations that need it.

The second step is to evaluate, based on the cost-effectiveness report from the rater, the budget considerations. If it isn’t in the client’s budget, it doesn’t matter how much sense it makes, if it can’t be paid for, it won’t get done.

Insulation Types

Fiberglass, whether blown-in or batts, is still one of the least expensive insulation options available. However, it can be extremely difficult to get installers to install it correctly.

Cellulose, an environmentally friendly blown-in option, is next on the lower cost side, but there are essentially two different types of cellulose available in the market. One is treated with a borate solution to provide fi re and insect retardation (and is slightly more expensive), while the other is treated with an ammonium sulfate solution to provide those same benefits (and is slightly less expensive). The ammonium sulfate treated cellulose, however, can release ammonia gas when it gets wet, and what are the odds that a roof leak may occur at some future point? I’ve worked with several projects where the walls had to be ripped open and the insulation removed because water got into the assembly and then started to off -gas. In addition to the vile odor, ammonia gas is extremely unhealthy to inhale.

Spray polyurethane foam is one of the more expensive options available. If the HVAC system is in a vented attic space, it may be cost effective to encapsulate the attic with spray foam by applying it to the underside of the roof sheathing. By moving the building envelope from the attic floor to the roof deck, the HVAC system is placed in a much more hospitable environment that will be much closer to the house temperature and will dramatically reduce the amount of heat loss and gain on the duct system. However, the foam will need to either be covered with a thermal barrier or treated with an intumescent coating that is approved by the spray foam manufacturer, driving the cost up even more. Installation can become extremely difficult in low-slope roof spaces, and the home owner will need to leave the home for a couple of days during the installation to avoid any environmental sensitivity issues while the foam is being installed.

Other Considerations

In an existing home where access is limited to the attic, any existing insulation material should be removed to reveal the holes between the house and the attic. Those holes (plumbing vent stack penetrations, duct boot penetrations, electrical wiring penetrations) should be sealed with a fi re-rated caulk or foam prior to any insulation installation. This should be done even if (perhaps particularly if) the chosen insulation is spray foam and the chosen location is the roof deck. The attic space should be isolated and not “communicate” with home.

Attic kneewalls, those walls that separate unconditioned attic space from conditioned space, are a particularly thorny problem. If they are insulated, that insulation material would need to be removed and the holes in the attic kneewall sealed. These holes are typically electrical outlet boxes and duct boot penetrations for side-wall distribution. Next, insulation must be installed in the framed cavities and completely fill the cavity, then an air barrier (unless spray foam is installed at an appropriate thickness) installed on the attic side. R-19 batts hanging out in space in a 2×4 wall are useless and when compressed and covered with an air barrier, will perform worse than an R-13 batt correctly installed and covered with an air barrier.

How do you pick the right insulation? Ask three questions: 1) is it in the budget? 2) is it safe? 3) is it easily installed correctly?


Posted In: ACCA Now, Building Performance

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