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strategery 11-27-2011 08:55 AM

Any reason not to put on an inch of foamboard insulation under new siding?
I'm looking at having new siding put on and I currently have only blown-in insulation for my exterior walls. I got a bid from a siding contractor for hardie siding and I asked if I could put an inch of foamboard under it to beef up my insulation with house wrap. He said that it isn't a good idea because my house would "rot from the inside out from moisture and condensation."

I live in Iowa. I know that walls need to be able to dry if they get wet, but the house wrap and the foamboard are both vapor permeable, correct? Won't that be enough to let it dry out should it get wet?

Windows on Wash 11-27-2011 09:38 AM

Paging GBR.

In a word, yes.

What is the interior wall detail like? Is there a vapor barrier to the inside of the home (poly plastic).

Hardie = rainscreen wall.

strategery 11-27-2011 02:06 PM

No. No house wrap on currently either.

Windows on Wash 11-27-2011 08:27 PM

House wrap is not a vapor barrier. It is not even a class 3 vapor retarder.

It is design to be a bulk moisture rain screen and air barrier (when installed properly.

Non coated foam boards are vapor permeable and could be used and would allow drying through them.

If you do not have an interior vapor barrier, it is possible you could install a vapor barrier foam (foil faced iso) and still be fine.

GBR knows this stuff backwards and forwards.

Gary in WA 12-05-2011 11:52 PM

WW covered most of it in good answers.

Read the sidebar #2 (keep in mind you are 6400 Heating Degree Days (HDD)) here; Now go back and read the rest, Lol. Pages 15, 16 for a cold climate with and without foam- notice the temperature of dew-point on your sheathing (ply/OSB);

Problem with Hardie on foam is it can trap water at the top edge of each course;

Where does the housewrap go?

Properly install the H.W.:

Optimum window/door head flashing detail;
A bene.;

I suggest two layers of housewrap, Tyvek (58perms) rather than Typar (-12perms) when the optimum low perm rating for walls is 18perms.

More points on outside foam;

Worried about the foamboard permeability……..

In your cold climate, you want the wintertime heat from the sun;

Some savings w. foam in my area:

1. Describe the wall. e.g. OSB,f.g. batt, faced?, drywall, plaster, etc… trying to determine rainscreen or not…

2. What is the closest big city?


strategery 12-06-2011 02:14 AM

Wow that is a lot of information. THANKS!

1 the siding now is cedar and the insulation is blown in cellulose. I believe it's drywall over plaster. Is there a way for me to check that? It's pretty old so I seriously doubt that there's ANY housewrap. Others have told me the same.

2 I'm in Des Moines. More wind here than in Chicago. Temperatures are extreme in the winter and summer. Bitter cold windy winters and hot humid summers.

So if I'm understanding you correctly, foamboard is NOT a good idea and two layers of tyvek is a better way of dealing with the weather here?

Windows on Wash 12-06-2011 06:02 AM

Not at all. Exterior foam is a great option provided you don't create a double vapor barrier.

If you are going to run Hardie, you should have a rainscreen wall. If you installer is capable, there is no reason you couldn't put 6" of EPS (if you wanted to) to the outside wall and do a rainscreen effect. Whenever you use foam, you need to stagger your joints and seal your seams.

You could also do 2" of fiberfaced iso to the outside and still keep your perm rating as a class II.

With any rigid foam layer, you will also need to build out the windows.

Are you replacing the windows?

strategery 12-06-2011 01:01 PM

I really hadn't planned on replacing the windows but I suppose I could. I currently have Anderson double pane windows vinyl installed in 96. They're great windows. Can the siding be done with the foam boards in a way that accommodates the existing window?

Windows on Wash 12-06-2011 09:18 PM


You will just need to furr/trim out around the windows to accommodate the sunken look.

WRB goes over the sheathing now and ties into the windows. Foam goes over top that.

Gary in WA 12-06-2011 09:53 PM

Exterior foam is good. Two layers of housewrap is good. Now for the cedar siding....
With the Hardie, you could get by with just a special drain-able housewrap with a high perm rating like Tyvek. With the cedar, because it wicks water, I suggest a ventilated (that’s vents bottom and top, not a vented- just bottom vent) rainscreen with strapping.

Note your locations 33” annual rainfall as per article sited a rainscreen is needed.

Cedar siding wicks water, back-prime the cedar siding, even with a rainscreen;

Cedar on a housewrap can’t dry out from the backside;

Cedar siding is similar to hardboard siding when on foam, pp.65:

Go with a rainscreen.


strategery 12-07-2011 03:04 AM

You guys are awesome. Thanks for the information and the links I really appreciate it.

Windows on Wash 12-07-2011 08:01 AM

+1 on Rainscreening (see my first post). More expensive and a bit more difficult but immeasurably better.

strategery 12-15-2011 05:19 PM

One more question relating to this.

If I have foamboard insulation installed underneath the Hardie siding, it's going to leave a big noticeable difference with the siding sticking way out there between the wood-framed first level and the concrete foundation that's above grade. What's the answer to that? Should I have the peremeter of the foundation excavated a few feet so the foamboard can be installed all the way down below frost level?

Windows on Wash 12-15-2011 08:27 PM

Excavating to foam on the concrete is overkill.

The step out from the siding and the foundation wall is not that much of and issue and can be trimmed out for a nice look.

strategery 12-17-2011 01:48 AM

Do you have any pictures I could see where there's 4 or 6 inch of foamboard insulation underneath siding trimmed out like you say?

Nevermind, I found a few searching on google.

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