I've got access to some discounted (factory seconds) of rigid foam board and looking to cut it up and place it against the exterior walls on the inside of an old (1894) converted house that is now four 1br apartments. Thinking of going with 1" of rigid foam board and then adding a fiberglass batt on the inside of that. With the old house, the 2×4 studs are actually at least 4", not 3.5.
It's in southern Illinois and I'm trying to get some good advice on whether I should be concerned about vapor barrier and if the fiberglass batt makes this better or worse?
Rigid foam board in walls between studs, plus fiberglass batt
insulation
Related Solutions
Yes, Rigid foam (XPS and ISO) boards are frequently used in exterior applications, and can be a great option for improving efficiency. There are a couple of issues with your proposed approach, though:
- Vapor barrier location. The vapor barrier should be on the warm side (probably inside, unless you live in a very hot & humid climate). Ideally, this is immediately behind your interior drywall. You also do not usually need an extra vapor barrier layer adjacent to rigid foam sheets, they are typically highly resistant to moisture.
- Attachment. You cannot rely on glue to support foam and siding panels. These need to be mechanically fastened.
On a wood frame house, you'd want to use this order:
- interior drywall
- possibly a vapor barrier (e.g. poly sheeting), though the wisdom of this is debatable (see the comment from iLikeDirt below)
- wall studs (optionally with batt or cellulose insulation in between)
- exterior sheathing
- rigid foam
- wooden strapping for attaching siding, screwed to studs through foam. This provides support for the siding and also allows air circulation behind the siding, so any moisture that gets behind the siding can dry out.
- siding panels, nailed to strapping
Here's a visual from Fine Homebuilding, which has a couple of good articles on this:
Note that adding foam on the exterior of your house complicates some things. You will likely need to redo the trim on your windows and doors, which will become more in-set. These inset areas will need to be flashed properly or you will have leaks, which have more potential to do damage when you've got multi-layered, thick walls that could trap moisture. If done well, you can get very good insulation. But make sure to work with someone who knows what they're doing!
It depends on where you live and how you plan on attaching everything. For interior insulation you want the vapor barrier on the warm side so the colder regions would always do the interior facing and some warmer climates would do the exterior side of the insulation. The underlying rule of where would the moisture most likely be trapped still applies but it typically biased because of installation, how you plan on attaching everything. For interior walls, no vapor barrier is recommended if you use a full fill spray foam because it is a close cell foam (over 2lb density like most spray foam) and is meant to be a tight squeeze unlike most insulation and stops airflow instead of slowing it down. The same can be true for rigid foam especially for outside insulation. If you plan to use contact adhesive, liquid nails, etc glue to hold the rigid board to the house then you'll be essentially stopping all the air flow and that side would not be a candidate for a vapor barrier, additionally if you are gluing the siding on, the same rule would apply.
To sum up: If you are in a cold climate, even seasonally, it is a good idea to put a vapor barrier on the outside of the outdoor insulation as long as you are not gluing the next layer onto the insulation. This will help the trapped moisture wick out better.
Related Topic
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Best Answer
I am doing exactly this, but the reason for the rigid foam is to create an air channel along the underside of the roof to allow airflow up to the ridge vent. The layering is, from the outside inwards, roof, 1 in. airspace, rigid foam, batts, vapor barrier (plastic), drywall.
Understand that the function of the vapor barrier is to prevent moisture from migrating from the warm side into the total thickness of insulation towards the cold side, where at some point the temperature gradient will drop enough to cause condensation. You do NOT want this to occur inside the insulation. The fact that the rigid foam is also a vapor barrier is not a problem as long as you have a correct barrier immediately behind the drywall.
If by "adding batt inside" you meant "inside the wall - towards the roof", then I would not rely on the rigid foam acting as a sufficient vapor barrier unless you made a point of carefully sealing all the edges with Acousiseal. Preventing air migration past the insulation is critical to its effectiveness, and you must also provided for ventilation of the roof material itself.