How to Insulate an Attic Floor

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How to Insulate an Attic Floor

If you’re building a new home, remember to seal the air leaks before you insulate

Posted on Mar 25 2016 by Martin Holladay

Maybe you are building a new home with an unconditioned vented attic. The house is framed, sheathed, and roofed. The drywall contractors have finished their work, so now you’re ready to insulate the attic floor.

If you are an owner-builder, this may be the first time you’ve done this. So what do you need to know?

Air sealing comes first

Although it’s common to talk about installing insulation on “the attic floor,” most attics don’t really have a floor. They have floor joists (or the bottom chords of roof trusses) with drywall below. When you are in the attic, you’re usually stepping carefully on top of the floor joists, looking down at the back side of the drywall ceiling.

You’ll be installing insulation on top of this drywall ceiling. (In some high-performance homes, builders install OSB rather than drywall in this location, and then install a service chase below the OSB. If you’re insulating the attic, however, it doesn’t matter very much whether there is a drywall ceiling below or an OSB ceiling below.)

If you care about energy performance, now is probably a good time for a blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas.. You certainly can’t insulate your attic until you’ve finished your air sealing work — once the insulation is installed, tracking down air leaks is a real pain.

In most cases, air-sealing workers enter the attic after the drywall is installed, and seal the obvious leaks. The blower-door test occurs after this work is done. On the day of the blower-door test, it’s a good idea to have a few people on site, equipped with caulk, tape, and canned spray foam, so that any unexpected leaks discovered during the blower-door test can be quickly corrected.

Inspect the attic

So you are up in the attic, before the blower-door test. You have good lighting, so you can see what’s going on.

First, because this is a newly built home, there are no big holes — right? From the attic, you can verify that there aren’t any goofy dropped ceilings or unsealed soffits where huge volumes of air can escape from the home. If there are any problems like that, now’s the time to seal them.

Next, verify that the attic hatch is well insulated with rigid foam, and equipped with weatherstripping. (For more information on this issue, see How to Insulate and Air Seal an Attic Hatch.)

Next, verify that the bottoms of the rafters are elevated at the eaves, or that raised-heel trusses were specified. You want plenty of room above the top plates of the perimeter walls — enough room for R-38 to R-60 insulation, depending on your climate zone — plus a few extra inches for the ventilation baffles and the air space under the roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. . If there isn’t enough room near the eaves, there’s not much you can do about it now… but at least you assessed the situation.

While you’re inspecting the eaves, see if there is some light coming in through the soffit vents. If so, it’s a good sign. Now look up at the ridge — you want to make sure that the roofers remembered to install a ridge vent.

Seal the air leaks

Next, focus on sealing all the usual penetrations and leaks:

  • Air leaks around chimneys;
  • Air leaks near bath exhaust fans;
  • Air leaks near plumbing vent pipes;
  • Air leaks at ceiling electrical boxes;
  • Air leaks at electrical cable penetrations.

Hopefully, there aren’t any recessed can lights in your ceiling, nor are there any heating or cooling ducts in the attic (because you are a GBA reader). If for some reasons you have recessed can lights, you had better come up with a plan to address them. For more information on this issue, see Recessed Can Lights.

[Photo credit: Rook Energy Solutions, Yarmouth, ME]

The leaks that beginners often miss are the leaks between partition top plates and partition drywall. Left unsealed, these cracks can leak tremendous quantities of air. So it’s important to use a foam gun and to seal all of these cracks (see photo at right).

For more information on air sealing, see Air Sealing an Attic.

Install the ventilation baffles

The next job is to install ventilation baffles under the roof sheathing near the eaves of your roof. These baffles ensure that you’ll have a clear ventilation channel so that air can flow from the soffit vents into the attic. If you want to use commercially available baffles, choose high-quality one like the SmartBaffle or the AccuVent.

If you want to make your own baffles, follow the instructions in this article: Site-Built Ventilation Baffles for Roofs.

The baffles should begin above the ventilated soffit — just to the exterior side of the plane of the wall sheathing — and should extend far enough into the attic that they terminate above the top of the insulation on the attic floor. The deeper the insulation, the longer the baffles have to be.

The blue rectangle in this photo is a wind-washing dam; the air leaks at the perimeter of the dam have not yet been sealed. [Photo credit: Christopher King]

You’ll also need to install insulation dams (also called soffit dams or wind-washing dams) between the top plate of the exterior wall and the underside of the ventilation baffles. These wind-washing dams need to be installed in the same plane as the wall sheathing. (That way, you can be sure that the attic insulation covers the wall’s top plate.) The easiest material to use for these dams is foil-faced rigid foam; some builders use OSB or plywood. As each dam is installed, seal the perimeter with canned spray foam so that the installation is airtight.

[Photo credit:]

Install a dam around the attic access hatch

Don’t forget to build an insulation dam around the perimeter of your attic hatch, using plywood, OSB, or 2x12s (see photo at right).

Make sure that the dam is high enough; for example, if you plan to install 16 inches of cellulose, build a dam that extends at least 18 inches higher than the drywall ceiling.

Install the insulation

In a new house, you’ll probably be installing either R-38 insulation (appropriate for a code-minimum house in Climate Zones 2 or 3), R-49 insulation (appropriate for a code-minimum house in Climate Zones 4 through 8), or R-60 insulation (the depth recommended for a “pretty good house” in a cold climate).

There are three common types of insulation used to insulate attic floors: fiberglass batts, blown-in fiberglass, and cellulose.

Cellulose is the best choice. The second-best choice is blown-in fiberglass. The worst choice is fiberglass batts.

Once you have determined the depth of the insulation you’ll be installing, make several marks (using a wide marker that leaves a bold, easy-to-read line) on the framing members in the attic — the rafters or chord webs — indicating the desired depth of the insulation.

This attic was insulated with blown-in fiberglass.

When it’s time to blow in the insulation, remember to wear a dust mask. If you’re an owner-builder, you may want to read this article: Borrowing a Cellulose Blower From a Big Box Store.

If you’re the person up in the attic holding the blower hose, don’t skimp on insulation depth. Aim the hose at the corners and edges of your attic; look for crannies that may need special attention; and be generous. A little too much does no harm at all. Once all of the needed insulation has been installed, rake the high spots and fill the valleys.

If you are a builder or an insulation contractor, remember that you’re required to provide the homeowner with documentation detailing the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the insulation you have just installed. R-value claims must meet the requirements of the Federal R-Value Rule (16 CFR Part 460, “Trade Regulation Rule Concerning the Labeling and Advertising of Home Insulation”), so don’t exaggerate.

Martin Holladay’s previous blog: “Plan Ahead For Insulation.”

Click here to follow Martin Holladay on Twitter.

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Image Credits:

  1. Image #1: Buildipedia
  2. Image #2: Code Poet / Flickr

Mar 25, 2016 10:02 PM ET

Gabled Luxury
by Paul Kuenn

How we long for something other than 4:1 hip roofs. Certainly not any in NE WI where almost every house built in the 60s was a bird cut rafter-ed hip roof. I'm dreaming of someday seeing something like those photos you like to use for this Web Site. Need another Peshtigo fire to clear the ruins.

Mar 27, 2016 4:17 PM ET

When is R-38 the same as R-49?
by Douglas Horgan

Answer: in Zones 4 through 8, if you have full-height insulation over 100% of the ceiling including the top plates (as shown in your excellent guide here), the IECC allows R-38 rather than the R-49 that is required if you can't achieve full depth insulation over 100% of the attic floor.
It's an interesting exception...
See section 402.2.1:[RE].html
This 2015 language is updated a bit from the 2012 version, but both are supposed to say the same thing: if you have full depth insulation everywhere, you don't need as much.

Mar 28, 2016 7:04 AM ET

Response to Douglas Horgan
by Martin Holladay

Of course, as I'm sure you know, R-38 is not the same as R-49.

The intent of the code is to encourage builders to extend the insulation out so that it covers the top plates of the exterior walls -- an excellent goal. It's unfortunate, however, that the code writers decided to encourage what should be a standard practice by allowing builders to skimp on insulation depth.

Here is the language of the code exception:

"R402.2.1 Ceilings with attic spaces.
Where Section R402.1.2 would require R-38 insulation in the ceiling, installing R-30 over 100 percent of the ceiling area requiring insulation shall be deemed to satisfy the requirement for R-38 wherever the full height of uncompressed R-30 insulation extends over the wall top plate at the eaves. Similarly, where Section R402.1.2 would require R-49 insulation in the ceiling, installing R-38 over 100 percent of the ceiling area requiring insulation shall be deemed to satisfy the requirement for R-49 insulation wherever the full height of uncompressed R-38 insulation extends over the wall top plate at the eaves. This reduction shall not apply to the U-factor alternative approach in Section R402.1.4 and the total UA alternative in Section R402.1.5."

Mar 29, 2016 10:59 PM ET

Wind Washing Dams
by jim elliott

Greetings- Martin.
I understand the reason for installing the wind washing dams to protect the insulation below the insulation baffles. The part I am missing is the requirement for these baffles to be caulked and airtight around the perimeter. If the junction of the wall OSB and the top plate is sealed like in the picture, then what value is the caulking around dams?

Mar 30, 2016 4:33 AM ET

Edited Mar 30, 2016 10:12 AM ET.

Response to Jim Elliott
by Martin Holladay

Of course, many builders don't bother to seal the perimeter of their wind-washing dams. However, if wind is allowed to enter through these unsealed cracks, the performance of your fluffy insulation will be degraded at this crucial location. (When cold winter wind blows through fluffy insulation, it reduces the insulation's performance, just as a cold wind blowing through a wool sweater will make you feel cold during the winter. If you put a windbreaker over the sweater, you'll feel much warmer.)

If you live in a region with winter snowfall, the performance of the insulation at the perimeter of your attic is very important. Heat loss near the eaves leads to ice dams.

Mar 31, 2016 2:18 PM ET

Bad link
by Bruce Lepper

For info, the link " Borrowing a Cellulose Blower From a Big Box Store" goes to the wrong subject.

Mar 31, 2016 2:37 PM ET

Response to Bruce Lepper
by Martin Holladay

Thanks very much for pointing out our error. I have fixed the broken link.

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