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How to deal with an interior vapor barrier on a rigid foam retrofit.

I would like to add foam board to the exterior of my house to beef up the R value. The walls currently are 2x6 with fiberglass batts and plastic sheeting as a vapor barrier between the studs and drywall. Most of the stuff I've seen says you don't want a vapor barrier on the inside so your sheeting can dry to the inside but I haven't found much information on what to do if the vapor barrier is already in place. Thanks!

Asked by Jacob Roark
Posted Aug 28, 2014 2:16 PM ET


3 Answers

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To give the assembly some drying capacity you have to be very careful in your selection of exterior insulation, and how the siding is applied.

The most vapor permeable rigid insulation currently on the market is high-density rigid rock wool (eg: http://www.roxul.com/products/residential/roxul+comfortboard+is ) which is a bit north of 30 perms, which more vapor permeable than some housewraps, and has an R-value of about R4/inch. It also has the advantage of being completely fireproof. Roxul Comfortboard IS comes in thicknesses from 1.25" (R5) to 5" (R15). Installation is pretty straightforward, but read the fastener guides for tips:


Next in line line is unfaced EPS, which an order of magnitude drop in vapor permeance, but still good enough as long as you don't go ridiculously thick. Type-I EPS (1lb per cubic foot nominal density) is about R3.9 inch and about 5 perms @ 1", which is about the same permeance as #15 felt, not a big deal, but at 5"/R19 it's about 1 perm, which starting to be a bit vapor-tight, but still OK.

Type-I EPS is a bit easy to ding up or break during installation, and Type-II EPS (1.5lbs nominal density) is usually preferable, but it's a bit tighter, 1-perm @ 1", but it's also 7-8% higher R at R4.2/inch. It takes 3"/R12.6 to hit the 1-perm threshold, but that's still pretty good- it would literally DOUBLE the performance of a 2x6/R19 wall with studs 16" o.c..

When applying rigid foam it's useful to use 2 layers and overlap the seams, and tape the seams on each layer, since there will be both seasonal expansion/contraction causing air leakage and thermal striping if left unchecked. Rigid rock wool has a coefficient of expansion closer to that of wood, and is more flexible, making that less of an issue if you go that route.

Whatever you put up there, draw the line at 0.75 perms minimum, and create a "rainscreen" gap between the siding and insulation. The most common way to do this is by through-screwing furring to the studs with pancake head timber screws 24" o.c., and mounting the siding to the furring.




With the poly vapor barrier in place DO NOT USE FOAM WITH FOIL OR PLASTIC FACERS or you'll be risking creating a moisture trap. Unless you're re-mounting the windows, the housewrap goes between the rigid insulation and the siding, taking extreme care to lap the window & door flashing correctly. If going with rock wool any standard housewrap will do, but if going with foam it's better to use a crinkle-type housewrap,(eg Tyvek DrainWrap) which provides a bit of capillary break & drying channel for any bulk water intrusions that make it to that layer.

Answered by Dana Dorsett
Posted Aug 28, 2014 3:00 PM ET


This question crosses my desk every two or three months, so I have developed a standard answer. Here it is:

Many energy experts worry that it may not be a good idea to install exterior foam on a house with interior polyethylene. Although it would be better if the poly weren't there, the fact is that tens of thousands of Canadian homes with interior poly have been retrofitted with exterior rigid foam, and there haven't been any reports of widespread problems. According to building scientist John Straube, all indications show that these retrofits are "not so risky as most people think. These homes will probably be fine."

That said, the installation of exterior foam is not advised on any home that has suffered wet-wall problems like leaking windows, condensation in stud cavities, or mold. If you plan to install exterior foam during a siding replacement job, keep an eye out for any signs of moisture problems when stripping the old siding from the walls. Investigate any water stains on housewrap or sheathing to determine whether the existing flashing was adequate.

If there is any sheathing rot, determine the cause -- the most common cause is a flashing problem, but condensation of interior moisture is not impossible -- and correct the problem if possible. If you are unsure of the source of the moisture, hire a home performance contractor to help you solve the mystery.

If your sheathing is dry and sound, I don't think you need to worry about adding exterior foam. Adding a rainscreen gap will certainly go a long way toward avoiding future moisture problems. Of course, it's important to be meticulous with your details when you are installing your new WRB and window flashing. It's also important to keep your interior relative humidity within reasonable levels during the winter. Never use a humidifier.

To summarize, here are four caveats:

1. Be sure that your foam is thick enough to keep the wall sheathing above the dew point in winter. Read more on this topic here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

2. When the siding is being removed, inspect the existing sheathing carefully for any signs of water intrusion, and correct any flashing or housewrap problems.

3. Install rainscreen strapping so that there is a ventilated gap between the new exterior foam and the siding.

4. Keep your interior humidity under control during the winter; if the interior humidity gets too high, operate your ventilation fan more frequently.

Answered by Martin Holladay
Posted Aug 28, 2014 3:02 PM ET


Thanks so much for information on the insulation options, Dana. Very useful!

Martin, I appreciate the response. This was my gut feeling on the subject, but I like to get a proffessional opinion before I push forward with something. Thanks again!

Answered by Jacob Roark
Posted Aug 28, 2014 8:34 PM ET

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