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What type of building envelope should I use on my resided house in Seattle?

Dale Luhman | Posted in Green Building Techniques on

We have a two story house in Seattle that was built in 1976. It has 2×4 exterior walls with R-11 rolled fiberglass insulation between the studs – has brown kraft paper vapor barrier on the inside of the house walls, has foil vapor barrier on the inside of the unheated garage; then on the studs is foil backed kraft paper, ¼” of white foam insulation board, then aluminum siding. It looks like they use slanted 1×4 bracing in the corners for shear bracing. There doesn’t appear to be any plywood on the exterior of the walls. Looking at current earthquake building standards in Seattle, we are nervous about the lack of shear wall support. So we had an engineer do an earthquake retrofit design that removes all siding down to the studs. We anchor the sills to the concrete walls, brace the walls with plywood, and connect the two floors together for the earthquake retrofit. Now we need a new house exterior. And since we are adding plywood and other items with depth to the studs, we thought it would be a great time to replace the half of the windows in the house that are single pane.

I thought this would be fairly easy, but with lots of considerations there are lots of questions about how to reside the house. We plan to leave the R-11 insulation in between the studs, and then nail the 1/2” plywood to the studs on the exterior for shear strength. We did not want to lose the current R value from the ¼” foam that is removed if possible. Looking at various Green Building Advisor topics I can get various pieces of information, but can’t complete a final wall profile.

Seattle is in climate zone 4C marine, moderate temperatures year round, 30 to 80 mostly, but can get down to 10 and up to 90 occasionally, but moderate humidity much of the year. British Columbia to the north has required a 10 mm or 3/8” wide rain screen gap since 2006, and Oregon to the south has required a 3 mm or 1/8” rain screen gap since 2010. Seattle is at least as wet as they are, so it seems like we should be including a rain screen in our residing plan. Reading many articles, including Joseph W. Lsitburek’s articles, “The Perfect Wall”, and “Mind the Gap, Eh!”, gives me insight, but not total clarity for my total house reside. I have read Martin Holladay’s numerous articles, including “Calculating the Minimum thickness of Rigid Foam Insulation.” As far as we know, our current siding has not had water issues for 38 years, we would like this residing to last that long in our damp climate with no or low maintenance, and to not break the bank.

So calculating the minimum thickness of rigid foam insulation for Seattle is ≥ R 2.5. Using that with the Perfect Wall – the Residential Wall, it seems like I could construct my new wall, inside to out as –

1. Keep this existing –
a. gypsum board interior walls,
b. 2×4 studs,
c. R-11 rolled fiberglass insulation that has brown kraft paper, between the studs;
2. then add new –
a. ½” plywood for structural sheathing attached to studs,
b. Hardie Wrap weather barrier – the items above would dry inward, the items below would dry to the outside
c. ½” thick R 2.5 XPS solid foam board,
d. 6 mm (1/4”) thick homeslicker rainscreen – provides gap for drainage
e. Hardie Plank lap fiber cement siding

Questions –
1. Have I interpreted the readings of the above mentioned articles correctly so my house will be able to breathe and dry and not mold? If not, what changes are needed?
2. Will nailing the hardie plank through the ¼” thick rain screen and though the ½” thick XPS hold the hardie plank securely to the studs? So the Hardie plank won’t sag over time?
3. Will the above nailing compress the homeslicker rainscreen so much that it will not be effective? Would using ¼” thick lathe instead of ¼” thick rainscreen provide a longer lasting and cheaper separation barrier gap?
4. Lstiburek says a 3/8” gap for a rainscreen is conservative and it is required in British Columbia. Oregon only calls for 1/8”. Is ¼” enough, or since we are going to all of this trouble, we might as well go for a 10 mm or ¼” gap? We want a reasonable long lasting good, dry house that won’t have to cost a whole lot.
5. In Seattle, 15 pound felt and just a water resistive barrier like tyvek is all that is required to meet code I believe. So most contractors are not familiar with installing rainscreens. But it seems like with our damp climate a rainscreen system makes sense. I hate to hire someone to learn how to do this system on my newly resided house. The basics seem to be fairly straight forward, but the details around the windows and drainage seem to be where we could get into trouble.
6. Other suggestions on materials that might work as well as or better than above? I really don’t have a big preference in any particular product; the above just seemed like options that might work.

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  1. Saul Kerner | | #1

    I work for a GC in the Seattle area, and I can tell you that, while it's not a code requirement, most general and siding contractors in the area are familiar with installing a rain screen system. I know you can install the Hardiplank over the rainslicker mat without compressing it, but you'll have to talk with your contractor/architect about the 1/2 insulation underneath.

    The standard rain screen assembly for most contractors in the area is going to be a batten style, typically 1/2" P.T. plywood strips, or a pre-made batten from cor-a-vent or vaproshield. If you went to a batten system, you could also look into using a thicker foam on the exterior.

  2. David Meiland | | #2

    The 1/2" thick foam you are proposing is inadequate. I would install the shear plywood and tape the seams, install 2" polyiso rigid insulation over the plywood including solid wood furring around the openings, wrap with Typar or similar over the polyiso with new windows installed at the same time, vertical 1x3 or 1x4 furring strips over the wrap, then finally nail the Hardie to the furring.

    Read up on air sealing and window flashing / sill pan details and take care to execute them correctly.

    You could go thicker on the foam, but with 2" and R12 your total is around R23.

    Does the house have an attic? What is the heating equipment?

  3. Dale Luhman | | #3

    the house attic has 12-15 inches of loose cellulose insulation that at 3.8/inch I figure provides about r-40 to 50. we have natural gas heat, and since our 1900 sq ft house only uses about $400/year in heating, I don't see the point in adding a whole lot more insulation. 1/2" of xps insulation should give me r-2.5, which is more than I have now to be above the dew point, and its double the thickness of the insulation that I have now.

    and you would put the tyvek house wrap on after the plywood and the foam board insulation are installed? that should not affect the dew point condensation?

    and you would use furring strips rather than some of that thicker homeslicker rainscreen material?

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