Insulation, Greater Seattle Area
How to Properly Install Radiant Barrier Insulation
A radiant barrier is reflective aluminum foil stapled foil side down to the underside of the roof rafters or trusses, with gaps left at the ridge and soffits for ventilation. It blocks radiant heat from a hot roof. Around Seattle, where summers are mild, attic insulation to code usually comes first.
Below is how the foil goes in, the mistakes that cancel it out and what the climate data says for Puget Sound homes. To have us look at your attic, Book a Consultation.
What is radiant barrier insulation?
A radiant barrier is a layer of reflective aluminum foil placed in the attic next to an air space. The DOE Building America Solution Center guide Attic Radiant Barriers (updated 2022) says it blocks radiant heat moving from a hot surface, such as the underside of the roof deck, to a cooler one, such as the insulation on the attic floor.
It works differently from insulation. The same 2022 guide says the barrier "acts as a barrier against radiated, not convected, heat." Foil sheets come single-sided or double-sided on a backing of kraft paper, mylar or polypropylene. Some add fiber webbing, because the 2022 guide says unreinforced foil tears very easily.
The 2022 guide lists four common types: foil sheets, foil-faced roof sheathing, radiant barrier "chips" blown onto the attic floor, and foil-faced insulation, which it recommends for walls.
How do you install a radiant barrier in an attic?
In an existing home, the foil is stapled up from inside the attic. These steps follow the method in the Building America Attic Radiant Barriers guide (updated 2022):
- Measure from peak to soffit, then cut and reroll every length below.
- Lay plywood or a 1x12 across two truss members as a work platform, without crushing the insulation.
- Leave 1 or 2 inches open at the peak for ridge vent airflow, then staple the foil to the underside of the truss top chord, foil side facing down.
- Staple down the truss every 6 to 12 inches and stop 2 to 3 inches above the ceiling insulation so the soffit vents still feed the attic.
- Staple the other edge to the next truss and repeat until only the strip at the peak shows.
The 2022 guide says the air space above the foil "is not critical to the system's performance," so stapling it straight to the underside of the roof deck is acceptable. A sag between rafters keeps the shingles slightly cooler. For a new roof, the 2022 guide names four methods:
| Method | When it is done | Guide note |
|---|---|---|
| Foil-faced roof sheathing | Goes on like regular decking | Perhaps the easiest way in a new house |
| Foil attached to the decking first | Before the decking goes on the trusses | Handle the panels carefully to avoid damaging the foil |
| Foil or low-emittance paint under the deck | After the decking is installed | The paint is an interior radiation control coating |
| Foil draped across the trusses | Before the decking goes on | Quick, but a breeze can wreck the job |
Source: DOE Building America Solution Center (PNNL), Attic Radiant Barriers, updated 2022, read October 2026.
What radiant barrier installation mistakes should you avoid?
The Building America Attic Radiant Barriers guide (updated 2022) points to four mistakes:
- Sheet foil on the attic floor. The 2022 guide recommends against laying sheet foil on the ceiling insulation because dust will accumulate on it. Blown radiant barrier "chips" are the exception.
- Blocked ventilation. The foil stops radiated heat, and air still has to move. The 2022 guide says "a gap should be left at the soffit and at the ridge."
- Stapling over wiring. The 2022 guide says to "never staple through or over electrical wiring," and to take extra care at junction boxes and older wiring.
- Stepping off the framing. Step only on the trusses, rafters and the work platform. The 2022 guide warns that a misstep can drop you through the ceiling drywall below.
Is a radiant barrier worth it in the Seattle area?
Usually not as a first upgrade. The DOE Building America Solution Center guide Attic Radiant Barriers (updated 2022) says a radiant barrier cuts the heat flowing into the house from the attic by about 40%, which "typically equates to 8% to 12% of the annual cooling costs." The same 2022 guide adds that "if your attic is insulated to R-30 or better, there is very little heat flow to reduce." It adds that "in climates where summer heat gain from the attic is minimal, the advantages of a radiant barrier diminish." Ducts in the attic are its exception.
King, Pierce and Snohomish counties are climate zone 4C, the marine zone, in Table R301.1 of the Washington State Energy Code, WAC 51-11R-30100 (read October 2026). The DOE Climate Zones page (read October 2026) defines a marine climate as one with a warmest month mean below 72 degrees F.
For zone 4C, the ENERGY STAR table Recommended Home Insulation R-Values (read October 2026) lists R-60 for an uninsulated attic and R-49 added where 3 to 4 inches are already down. ENERGY STAR calls these levels cost-effective. With summers that mild, the money usually does more in insulation than in foil.
What attic insulation do we install instead of a radiant barrier?
Our insulation installation services name three attic methods for up to 1,500 sq ft, and a radiant barrier is not one of them: a blown in top up with air sealing from $5,000+. Remove and replace runs from $8,000+ and spray foam from $15,000+. Washington code asks for R-60 in the ceiling (WAC 51-11R-40213, read October 2026). What each level includes is in How Much Does Attic Insulation Cost?
Can you install a radiant barrier yourself?
You can. The Building America Attic Radiant Barriers guide (updated 2022) gives a do it yourself method and says "this work should be done with two people, for ease as well as safety." Its 2022 safety list says to work early in the morning or in cool weather and to wear a hard hat, because roofing nails poke through the underside of the roof. Around mineral fiber insulation, the 2022 guide says to wear goggles, long sleeves and a particle mask. It also says to pick foil with a UBC Class I or NFPA Class A fire rating, as building codes require.
Before deciding, read Can You Install Attic Insulation Yourself? It lists the signs that call for a pro.
How do we start an attic insulation project?
We inspect the attic and take photos, then price the work in writing. An attic takes 1 to 4 days on site, after any permits the job needs. Every insulation job carries a 2-year workmanship warranty, which can be extended to 5 years for an added charge.
In 2024 we rebuilt a carport in Auburn with open wood trusses under roof sheathing, the same framing a radiant barrier is stapled to, shown above. We work across the Greater Seattle Area from offices in Bellevue, Seattle, Monroe, Tacoma and Renton. See Home Remodeling Contractors in Auburn, WA.
Frequently Asked Questions
Is double-sided radiant barrier foil better than single-sided?
Not under the roof deck or on the rafters. The DOE Building America Solution Center guide Attic Radiant Barriers (updated 2022) found "no significant performance difference" there, because the first layer of foil cuts radiant heat gain by at least 90%.
Do small tears or holes ruin a radiant barrier?
No. The 2022 Building America Attic Radiant Barriers guide says "small tears and holes will not significantly lessen the performance," so you can cut and patch around vent stacks and truss supports.
Will a radiant barrier void my shingle warranty?
It should not. The 2022 Building America Attic Radiant Barriers guide says shingle warranties "should not be subject to cancellation" because of a radiant barrier. It still advises reading the warranty and getting any change from the manufacturer in writing.
