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Marine Zone 4 — Wall insulation — addition

Zone: Marine 4
Location: Seattle, WA
House Built: 1915
Square footage: 900 ft2 single story with 900 ft2 finished basement.
Wall height over basement: 12 ft
Current siding: Cedar, Paint in poor condition.
Insulation: not much, some fiberglass bat stuffed in nooks and crannies as they opened walls in the past. And dirty so there is plenty of air gaps.

GOAL: Comfort

During winter my thermostat reads 70F but still feels cold because the cold walls/ceiling suck the heat out of my body. After reading through GBA articles and forums I thought adding external insulation might be a good idea. Current plan of action:

Existing Walls: Sheet rock > 2x4 framing > some insulation, no vapor barrier > sheathing (plenty of air gaps)

New Addition: Tyvek HomeWrap > Two layers 2" Roxul ComfortBoard 80 (old house, I fear fire) > 2x4 furring strips > fiber cement siding.

Windows: New fiberglass, inside installation.

Not adding ceiling insulation yet. That will come later after I replace electrical wiring.

Does this plan look reasonable, disastrous, wasteful?

Thank you

Asked by Brad Steeg
Posted Oct 11, 2017 6:12 PM ET
Edited Oct 12, 2017 3:28 PM ET


15 Answers

Select your preferred way to display the comments and click "Save settings" to activate your changes.

Is there any framing or framing cavity insulation on the addition? Or is the 900' of 2x4 framed leaky space what you're adding the comfort board to?

Adding insulation over an air-leaky wall isn't the best investment, since the air leaks can be anywhere. Whether you add insulating sheathing or not, fixing the air leaks is the first order of business.

The existing wall cavities can probably be dense-packed from the exterior with fiberglass or cellulose, either drilling through both siding & sheathing, or popping off a clapboard or shingle for an easier cosmetic fix (if keeping the siding), which would tighten things up (a lot!). At that point you'll have a ~R10 whole-wall wall, before adding any exterior insulating sheathing. Adding just one layer 2" of rigid rock wool would cut the heat loss from the walls nearly in half.

If the existing windows are single panes + clear glass storms or clear glass (no low-E) double panes, weatherstripping the windows and replacing the storms with tight low-E storms can also make a huge difference in comfort. The low-E storm on the exterior raises the temperature of the interior side glass, reducing the convection drafts and raising the radiant temperature sligthly. That might make the most economic sense if you don't have to pull the windows when adding the exterior insulation.

If you're pulling the windows and replacing them, a U0.25-ish double-pane with low-E coatings surface #2 (the usual low-E glass unit) and surface #4 (the inside facing surface glass) can make a major uptick in comfort. The low-E on #4 lowers the surface temperature of the window a bit, but it reflects body-heat and room heat back toward the source, improving the mean radiant temperature (the primary source of comfort) by quite a bit.

If the foundation isn't already air sealed and insulated, the cold floor and drafts could be a major source of discomfort.

Answered by Dana Dorsett
Posted Oct 11, 2017 6:36 PM ET


First of all, can you tell us your name?

Like Dana, I'm confused about this addition. Are you planning to add a new addition to an old house built in 1915? Or are you describing an old 1915 addition that you want to fix?

Assuming you are adding a new addition to this 102-year-old house, what stage are you at? The design stage? Or is the addition already framed?

I agree with Dana that air sealing work always comes before insulation work. An old 102-year-old house like the one you describe is a good candidate for blower-door-directed air sealing. Hire a home performance contractor or a weatherization contractor equipped with a blower door and focus on air sealing.

If you are still at the design stage for a new addition, it's odd that you didn't mention whether you will have wall sheathing, or where your air barrier will be located, or whether you will be installing any insulation between your studs. I recommend that you include sheathing (OSB or plywood) with taped seams -- that will be your exterior air barrier -- and that you select some type of insulation to install between the studs.

The approach you describe -- adding all of the insulation on the exterior of the wall -- is more appropriate for retrofit work on an older house than it is for a new addition, which is one reason why I'm confused about where you intend to install the semi-rigid mineral wool panels. Will they be installed on an older wall or a new wall?

Answered by Martin Holladay
Posted Oct 12, 2017 6:07 AM ET
Edited Oct 12, 2017 6:34 AM ET.


My name is Brad. When I go to "My GBA" there's no option to add a user name or personal information, only option is to add a project. So I'm stuck with the weird User-#### handle.

It's just a retrofit to existing walls. The addition is 1) Tyvek HomeWrap, 2) 4 inches total of continuous Roxul ComfortBoard 80 insulation added to exterior of existing walls, and 3) new windows.

I have 4 original single pane wood framed windows and all the rest have been replaced with single pane aluminum framed windows. Intention is to replace all of them with fiberglass double or triple pane -- undecided because I don't know where the sweet spot is on window value.

Dana, thanks for the surface #4 tip. And thanks for the terminology: I want to improve mean radiant temperature.

I will be removing all my siding so it will be easy to drill holes and add insulation to existing walls. One concern is not knowing what I'm filling over -- e.g. old wiring. If I decided to insulate the wall cavity I'd lean toward removing the sheetrock. That way I could remove all existing bat insulation (most likely incorrectly installed, just like it is incorrectly installed in the attic) and I can inspect the wiring. With an old house like this I don't want to do anything without seeing it first.

At present, the finished basement is the warmest part of the house. It looks like they may have framed a wall inside the cement foundation and insulated it. Primary living space above the basement is the comfort problem.

Martin, regarding a blower door air test, at this moment I can tell you exactly where the air would leak. It would flow out of the old aluminum framed windows. When I remove the siding to install the Tyvek and external insulation I will be able to see my sheathing. I could seal cracks and seams in the sheathing at that point. In addition, the Tyvek will also be properly installed and sealed over the sheathing. I could place a membrane over the sheathing to completely air seal the walls but in an old house like this I'd be nervous. Rainscreen over four inches of Roxul over taped Tyvek over my sheathing seems like the least risky approach to insulating the walls of my home. But maybe I'm wrong and that's why I'm asking.

Thanks for your help,

Answered by Brad Steeg
Posted Oct 12, 2017 2:59 PM ET


I'll address your problems in two separate posts. First, concerning your screen name problem: you should follow the directions posted in this article: How the GBA Site Displays Readers’ Names.

If you are still having trouble after reading the article, let me know and I'll see what I can do to help. My email address is martin [at] greenbuildingadvisor [dot] com.

Answered by Martin Holladay
Posted Oct 12, 2017 3:06 PM ET


The confusion arises because the word "addition" (in construction terminology) means a new room or rooms added to an older house.

You aren't really building an addition. You are performing retrofit work (or implementing energy upgrades) on existing walls.

OK, we've cleared up that confusion.

Here's my advice:

1. I don't think that adding rigid foam to the exterior of your walls increases the fire risk. But if rigid foam makes you nervous, you can certainly use semi-rigid mineral wool instead if that's what you want to do. Just be aware that installing semi-rigid mineral wool isn't as easy as installing rigid foam, because mineral wool is squishier. That makes fastening the furring strips a little trickier -- it's harder to get the furring strips co-planar with mineral wool than it is with rigid foam.

2. Make sure that you do a good job of installing window flashing (including sill pans) in your window rough openings. Your decision to replace the windows is a good one; it gives you a golden opportunity to get the flashing right. But you don't want to screw up these details -- once the windows are installed, it's fairly difficult to correct flashing errors.

3. Your instincts about the location of air leaks in your home may be accurate -- but it may not. Many homeowners notice air leaks near windows, while ignoring air leaks into a basement or at the top floor (through the ceiling into the attic). Basement leaks and attic leaks aren't noticeable -- but they represent the worst leaks from an energy perspective. I suggest that you read up on the issue. Here are links to two articles to get you started:

Air Sealing an Attic

Air-Sealing a Basement

4. As you contemplate different approaches to fixing your walls, remember that you want to pay attention to airtightness every step of the way. If possible, your walls need an exterior air barrier as well as an interior air barrier.

Answered by Martin Holladay
Posted Oct 12, 2017 3:20 PM ET
Edited Oct 12, 2017 3:22 PM ET.


Hi Martin, there's no screen name in the upper right. Here's a screen snip.

Answered by Brad Steeg
Posted Oct 12, 2017 4:18 PM ET


You can get to the My Account page and Update Profile link by clicking the user name in the forum posts. I made those changes but don't see my name yet.

Answered by Brad Steeg
Posted Oct 12, 2017 4:23 PM ET


My guess is that you have to wait 24 hours for the changes to take effect. Let me know if I'm wrong.

Answered by Martin Holladay
Posted Oct 12, 2017 4:45 PM ET



1) Good to know. Maybe I'll switch to foam. After watching that building in UK burn up I got leery of foam board http://www.telegraph.co.uk/news/2017/06/22/grenfell-tower-victims-poison...

2) Correct. I replaced one of the old windows. No window flashing at all on the original. But it was easy to add flashing when installing the new window. Although, no evidence of past water leaks either. I'm sure I'll find leaks as I work my way around the house.

3) I'm sure I have attic and basement leaks too. I'll read that article.

4) Do you agree with this statement by BuildingScience.com? "Rigid materials such as gypsum board, exterior sheathing materials like plywood or OSB, and supported flexible barriers are typically effective air barrier systems if joints and seams are sealed." If so, sealing the sheathing should supply the external air barrier on the walls. Ceiling and attic will need to come later. And it looks like I have different options to install an internal air barrier depending on the next steps I take.

- Brad

Answered by Brad Steeg
Posted Oct 12, 2017 5:15 PM ET


Ceiling and basement air sealing will need to come later, I mean.

Answered by Brad Steeg
Posted Oct 12, 2017 5:18 PM ET

Answered by Brad Steeg
Posted Oct 12, 2017 10:57 PM ET


OK, leaning back toward mineral wool now. Carpenter ants are a problem in the PNW.

"Ants like wet wood because it is soft and easy to chew. Carpenter ants like insulating foam too."


Answered by Brad Steeg
Posted Oct 13, 2017 3:32 AM ET


Q. "Do you agree with this statement by BuildingScience.com? ‘Rigid materials such as gypsum board, exterior sheathing materials like plywood or OSB, and supported flexible barriers are typically effective air barrier systems if joints and seams are sealed.’"

A. Yes. If your old house has board sheathing, you can't use the sheathing as an air barrier, because there are too many seams to tape. You can either cover the old sheathing with new OSB or plywood and tape the seams of the new sheathing, creating an effective air barrier that way; or you can install a layer of rigid foam on the exterior side of the old sheathing boards and tape the seams of the rigid foam.

On the interior, drywall or plaster can be an effective air barrier, but only if the electrical outlets and other penetrations are carefully sealed.

Answered by Martin Holladay
Posted Oct 13, 2017 4:40 AM ET


The stated goal is comfort. With only single pane windows (and no storm windows), the existing comfort shortcomings are likely to be PRIMARILY all about windows!

The U-factor of wood sash single-panes is usually about U-1.0, aluminum sash single panes run about U1.2. That is about 10x the heat loss of a cedar clad 2x4/R11 wall per square foot, about 4x the heat loss of UNINSULATED 2x4 wall, and the windows likely to dominate the current heat load numbers of that addition, while dragging down the mean radiant temperatures.

With windows that lossy the temperature of the inner surface of the glass at 40F, is lower than a clear-glass double-pane at +10F, or a code-minimum low-E double pane at -20F. (See: http://s3.amazonaws.com/finehomebuilding.s3.tauntoncloud.com/app/uploads... ) The effect that has on the mean radiant temperature of the room (the most important factor for comfort) can be felt 10 feet away. Having glass that cold also generates convective drafts that can be felt when near the window.

In Seattle's climate triple panes would be overkill (unless going for PassiveHouse levels of energy use). Even double-low-E #2 & #4 surface double panes might be overkill, but they're guaranteed to be higher comfort than a code-min window. A pretty good U0.28-U0.30 argon filled double pane with low-E only on surface #2 would be a huge improvement over what you currently have, if only slightly better than current U0.32 IRC code minimum for zone 4C.

Answered by Dana Dorsett
Posted Oct 13, 2017 1:31 PM ET


Martin, if I run into board sheathing I'll add a barrier. The first thing I did when I moved into the house was seal the inside wall penetrations. Sealing around the sockets, lights, switches was both relatively inexpensive and noticeably effective.

Dana, thank you. My goal is comfort. I didn't realize the role windows played in that regard. I thought in terms of R value rather than mean radiant temperature. And it's true, they do feel cold from 10 feet away. I have cheap blinds up now that provide very little help, if any at all. I'll pay attention to your window recommendations.

Answered by Brad Steeg
Posted Oct 13, 2017 3:48 PM ET

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