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Insulation Discussion Forum • View topic - Insulating exterior wall top plate from attic

Insulating exterior wall top plate from attic

Our "how to" video library showing you how to insulate and air seal your home. While you may decide to hire a professional be sure you understand the importance of doing the job right.

Insulating exterior wall top plate from attic

Postby SIM » Sat Jan 02, 2010 9:20 pm



The exterior wall top plate is an area that is difficult to insulate in older homes.

Typically there is a 3-4" space above the attic floor and the roof decking. Most homes have fiberglass batts rolled into the joist cavity that extends out onto the exterior top plate. The problem being that air from the soffit is free to circulate through the fiberglass batt, thus greatly reducing the R-value. Some estimates project the R-value of un-faced fiberglass batt that is exposed to horizontal air flow to be less than 1.0. Other homes with blown in cellulose or fiberglass insulation have next to no insulation near the exterior wall top plate. This is usually the result of the insulation being tapered off near the top plate, or because it fell into the soffit area.

A solution to this problematic area is to dense pack a mesh bag with cellulose and stuff it down between the attic floor joists and roof rafter channels. This will assure maximum the thickness of insulation while greatly reducing the amount of air flow through it. Remember, you can pack cellulose but you can't pack fiberglass. Packing fiberglass reduces its R-value to almost zero, especially if there is any kind of air flow through it.

You can purchase the bags from Energy Strategies: 1-920-207-9481 for about 55 cents a bag.

Many people, primarily roofers, oppose this practice of blocking a roof rafter channel because it stops the air flow in and out of the attic. But with proper air sealing of the attic floor insulation always trumps ventilation. With that said, some homeowners insist on having ventilation even at the expense of less insulation. In this case we keep the existing baffles in some channels (or even replace attic baffles with a more rigid type so the dense packed cellulose bag does not crush it when put into place).
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Re: Insulating exterior wall top plate from attic

Postby focusonenergy » Sun Jan 17, 2010 11:50 am

Nice video. Too bad so many homes still have the exterior wall top plates insulated with fiberglass batts. Foam and these dense packed cellulose bags are the only way to do the job right.
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Re: Insulating exterior wall top plate from attic

Postby Jackofalltrades » Thu Jan 21, 2010 12:18 am

Sweeeeeeeeeeeeet! Nice idea for a truly difficult area that most people can't appreciate.
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Re: Insulating exterior wall top plate from attic

Postby catchley » Mon Feb 01, 2010 5:00 pm

I'm an amateur DIY'er and watching this video truly has me confused with the information I've been given. I understand the importance of insulating the exterior wall top plate but I've also been admonished to be sure to leave the soffit open to provide proper air flow in the attic. Is there not a way to achieve both? What effect does sealing off the soffit in this manner have on the goal of providing adequate air flow in the attic. And then there's the question of what constitutes "adequate" air flow in the attic...how much air flow is needed? Doesn't the method in this video produce the possibility of warming the roof at the point of contact - some heat will undoubtedly transfer through the dense pack bag and warm the roof?

I've already air-sealed our attic and later had cellulose blown in - with the baffles installed to keep soffits clear. If I go with your method, do I need to clear out that area first (there's cellulose on top of fiberglass) or can I pack in a bag right on top of the existing insulation (understanding, of course, that it will compress the existing fiberglass, rendering it useless)?

Questions, questions... Details, details...

Thanks so very much for your thoughts/opinions/suggestions,

Chell
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Re: Insulating exterior wall top plate from attic

Postby foamer » Tue Feb 02, 2010 12:31 pm

This is not a question you are going to get a lot of agreement, but it seems like you have all of the important issues in mind. There will definitely be some heat loss via conduction through the roof deck, and some melting will occur. Will it cause ice damning? We have not found ice damning to be an issue when foaming the space between the top plate and roof deck, or when using onion bags.

It may be better to have a 5% energy loss straight to the roof deck than an unnoticeable 20% energy loss that goes into the attic via convection and out the roof vents.
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Re: Insulating exterior wall top plate from attic

Postby mistascott » Thu Nov 18, 2010 10:21 am

I don't think that blocking the eave ventilation is ever a good idea because it increases the likelihood of an ice dam. Even with an airtight attic seal and proper insulation, heat energy still reaches the attic. If the eaves are blocked, there is no where for this heat to go except to the rafters where it can melt snow which can then refreeze once it reaches the eaves. So, in theory at least, this is not a good idea. The best way to prevent ice dams is a combination of air sealing the conditioned space, ventilation to remove heat in the attic, and proper insulation.
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Re: Insulating exterior wall top plate from attic

Postby socold » Sun Jan 30, 2011 11:15 pm

Well Mistascott, that is the textbook answer isn't it. How about commenting on the reality that all those ridge vents do nothing when there is 8 inches of snow on the roof or how much heat is lost to the attic when there is only 2" of loosely blown cellulose insulation around the perimeter of the typical low pitch roof?

We just haven't seen any "ice daming" and we always pack cellulose tight (in onion bag or not) up against the roof deck around the entire perimeter, minus wherever there is an active soffit vent. Do you really recommend every rafter channel be wide open to the soffit system?
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Re: Insulating exterior wall top plate from attic

Postby dlarrivee » Sun Feb 06, 2011 3:25 am

Lets get one other thing straight here.

You should always have fresh air entering the soffits, and those onion bags are a terrible idea...

Ice damming from heat entering the attic is one thing... how about humidity leaking through to your attic during the heating season, do you not believe you're going to see condensation at some point?

All you guys are talking about so far is the heating season.

How about when the sun is beating down on your asphalt shingles... this is the second purpose of fresh air soffit vents and ridge/peak vents, and other roof vents.

By the way, spraying foam (proper 2lb. foam) up against a soffit baffle would not be a problem at all in that low pitch roof, whoever says it can't be done, hasn't sprayed foam before. (I'm not talking about DIY foam in a box kits either)
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Re: Insulating exterior wall top plate from attic

Postby SIM » Sun Jan 20, 2013 11:59 am

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