Zone 5 here. The project in question is a very small (53 sq. ft) porch that was at one point converted into a 3 seasonish room. I've already went ahead and removed a low ceiling, removing a ton of celluose. A cathedral type ceiling seems more appealing and just allows more movement in such a small room. Additionally, the wood paneling that made the wall of the room was thrown out and the existing rockwool insulation removed. At this point I'm left with a room down to the studs. Since no sheathing was used on the exterior wood siding I'm not sure exactly what type and how much insulation to use to make this into a comfortable 4 season room. In time I do plan on running a duct to heat and cool the room. I've ruled out fiberglass batts since they would seem to seal the room airleak wise. Spray foam was recommended to me but the price and the fact that if the siding did take in moisture at some point, it would have a hard time leaving ultimately becoming trapped between the foam and paint of the siding. It seems to me that there should be a small gap behind the siding to allow air movement but I'm not sure where I could vent this air space. I've considered foam board between the wall and rafter studs but I am unsure if I should make the joint tight enough to seal out any outside air. Beneath the room, is a crawl space that is vented into the basement so I would imagine that this could vent the entire space if done properly. Any advice on how to proceed at this point would be greatly appreciated.

If you're not going to pull the wood siding and put up sheathing spray-foaming the interior would be a disaster, since the siding needs to dry in both directions.  Does it at least half some 15" felt an window/door flashing to work with?

I'm not sure what the concern about "...since they would seem to seal the room airleak wise." is.  Air tightness is one of your goals here, since that lowers energy use and reduces condensation issues.  Fiberglass batts (even those with facers) are damned-near impossible to make air-tight, but have their own issues. Though it's possible to install them in a way to get the thermal performance out of them, mis-installation is rampant.

Your instinct for leaving a gap in a cut'n'cobbled foam approach is right, and foaming them with  tight seal to the framing (but not the siding) would be correct- you want to create a true air-barrier to keep air from moving into & out of the stud bays, but at least 1/4" of air between the siding and the foam as a drying cavity.  You probably don't need to do ANYTHING to promote venting that cavity to the exterior- most are plenty leaky enough- you just want to be sure that bulk-water that gets by the siding drains rather than wicking into the cavity, and has at least some reasonable drying rate to the exterior.

Ideally both the roof and walls would get exterior rigid foam insulation sufficient for dew-point control at the sheathing using blown cellulose or rock-wool batts snugly fitted to the rafter bays.  To meet code (and avoid problems) without exterior insulation the roof deck needs at least 1.5" of vent space between the insulation and the roof deck, and 2" is even better. That makes getting to a decent R-value a bit difficult.  Otherwise, you can add a couple of inches of closed cell foam directly to the roof deck and safely use cellulose/batts snugged up to the foam, provided the the interior gypsum is air tight (but still somewhat permeable to water vapor.) 

How deep are the rafters?

Is the crawl space air sealed insulated to the exterior?

Is the basement insulated, and air sealed?

There is no flashing to speak of unfortunately but I did go to the trouble of caulking the exterior well last summer when I painted it. Hopefully this did not seal the siding and where it butts against the house too tightly since what you suggest requires the ability for the siding to breath somewhat. Otherwise I'm not sure where the 1/4" gap behind the siding would move moisture to. If I was to use foam board, would it make sense to just use a thin piece covered with a fiberglass or some other air permeable insulation from a cost perspective? The wall cavities are actually 5" deep in most areas due to shimming the walls plumb so they could accept something like I described. The rafters are 2x6 but since they are exposed at this point I could easily make them deeper to allow for air space and more insulation. But as is stands now, there would be no where for that air to go as there is no ventilation for the roof. Though I suppose I could just add a ridge vent easily enough. The basement is air sealed and insulated only at the rim joists and where the top of the block foundation meets the sill plate. However the basement is conditioned to an extent by a a few vents. The crawl space is like the room above. It is only siding nailed to the framing. Is there a product that could seal the foam board in the cavities or do I just rely on a spray foam? Thank you so much for your reply.

Posted By chaz on 18 Jul 2013 08:05 PM
There is no flashing to speak of unfortunately but I did go to the trouble of caulking the exterior well last summer when I painted it. Hopefully this did not seal the siding and where it butts against the house too tightly since what you suggest requires the ability for the siding to breath somewhat. Otherwise I'm not sure where the 1/4" gap behind the siding would move moisture to. If I was to use foam board, would it make sense to just use a thin piece covered with a fiberglass or some other air permeable insulation from a cost perspective? The wall cavities are actually 5" deep in most areas due to shimming the walls plumb so they could accept something like I described. The rafters are 2x6 but since they are exposed at this point I could easily make them deeper to allow for air space and more insulation. But as is stands now, there would be no where for that air to go as there is no ventilation for the roof. Though I suppose I could just add a ridge vent easily enough. The basement is air sealed and insulated only at the rim joists and where the top of the block foundation meets the sill plate. However the basement is conditioned to an extent by a a few vents. The crawl space is like the room above. It is only siding nailed to the framing. Is there a product that could seal the foam board in the cavities or do I just rely on a spray foam? Thank you so much for your reply.

If you have 5" of stud depth to work owith, put 1" of rigid EPS with a ~1/2" nominal gap to the siding, to allow a 3.5" nominal cavity depth, and use unfaced R13 fiberglass (or R15 rock wool) for the rest. Use some cut-down foam as spacer blocks/strips for the offset to the siding, then use can-foam to seal the EPS to the studs.  Expanding foam (eg Dow Great Stuff) or 2-part polyurethane (eg FrothPak) are the only suitable materials for reliaby sealing rigid foam between the studs, and it'll pretty much glue it in place permanently. (It's chemically similar to Gorrilla Glue.)

If you have a source for reclaimed roofing foam (check craigslist) and can find some 4" EPS or iso, or some combination of thicknesses that adds up to 4-4.5" that would be quicker & better (and often cheaper) than a foam + batts solution.

The vent space has as much to do with being a reliable capillary break as it is a true vent, but venting it to the exterior (even if only at one end) will give it much better resilience, particularly on the roof deck.  You can get round retrofit soffit vents as small as 1" , but bigger would be better.  If you're adding 1.5" of depth to the rafters and venting the rafter bays, do it with 2 x 2s perpendicular to the rafters, and put either high-density R15 "cathedral ceiling" fiberglass or rock-wool R15s in the rafter bays, and use split unfaced R19s compressed into the 1.5" micro-cavites.  Better yet, put R15s in the rafter bays and add 1.5" polyiso rigid board on the underside.  Either of those solutions provides significant thermal break over the R1.2/inch rafter timber, delivering measurably better thermal performance.  If taking the rigid-iso road, tape the seams with 2" FSK tape (aluminum duct-tape) for a fairly reliable air seal, and stagger the seams of the gypsum with those of the iso.  But putting 2" of iso above the roof deck and installing rock wool R23s in the 2x6 bays would be higher performance still. (The big box store chains carry Roxul these days.)  With foam above the roof deck you'd need to add a nailer deck for the shingles. Holding the foam in place with 2x4 furring through-screwed to the rafters 24" o.c. with pancake head timber screws and putting a 1/2" or 5/8" OSB nailer would be the preferred method, since you could then vent the channels between the furring with Cor-A-Vent or similar.  But if that stackup is too tall to work, an OSB nailer flat to the foam through screwed 24" o.c. works too (but may have to be replace if the roofing ever leaks.)

The higher the R-value an more air-tight you make the roof assembly the less likely it is you'd end up with ice damming problems.  In zone 5 ideally you'd be able to take it up to R50 or better, but if you deal with the thermal bridging of the rafters, even R35 is pretty good. With R23 batts tight to the underside and 2" of iso above the roof deck you're looking at R35 nominal at center cavity, and would have sufficient exterior R from a dew-point control point of view, but 3" of exterior foam would be better, and it would take 4" to actually meet IRC 2012 compliance.  If you do a combination of thickening up the rafters 1.5" AND exterior foam, 3" of exterior iso (R18-19) would be necessary for dew-point control.

In zone 5 it's long-term cost effective to insulate the basement with 1" of EPS on the exterior and an interior side 2x4 studwall with R13-R15 batts (unfaced batts only), foam-sealing the EPS to your band-joist foam.  Slip an inch of EPS under the bottom plate of the studwall too, as a thermal & capillary break from ground, which has moisture wicking through the slab.

Treat the walls of the crawlspace in the same manner as the room above, but put a 10-mil poly vapor retarder on the ground.  Is there a poured or CMU foundation for the crawlspace, or is the porch supported by piers, cantilevered with the house framing or...???

Thats what I needed to know for the wall cavities. For three of the 4 walls there is a header/beam that that spans the length of the wall. I figure foam board would work in this space as well? For this location I don't think there is much room to work with. Maybe only an 1/2 inch or so of foam board before it would come out past the framing below. I have just looked at the crawlspace and it seems that the sides of the porch are built up from a very small poured concrete foundation of sorts. The sill plate rest just an inch or two from the ground and the floor to the actual porch is supported from from resting on two brick columns that are now enclosed in the crawlspace. It appears as if there is tar or some type of treatment on top of the sill plate. I assume this is for moisture that could come through the siding and drained into the dirt of the crawlspace? I think I may still be concerned about moisture It's very strangely constructed and if I had the time and money I would attempt a real room. Unfortunately the attic spaces are not vented by soffits, only gable vents. This leaves all the soffits of the house enclosed. This is true for the room I am speaking of as well. If was partially insulated before I got to it but there was no venting of any kind to the attic area of the room. Would a ridge vent move enough air? I don't think re-roofing to add more insulation is practical at this point for me due to time and skill restraints so I will insulate what I can in the rafter bays after adding the 2x2 you recommended.

It's probably worth slipping sheet metal or EPDM between the foundation and sill plate as a capillary break against ground moisture wicking into the susceptible wood. (Hopefully the mini-foundation is deep enough to avoid frost-heaving, but it'll be less likely to frost heave AFTER you've insulate the wall than it was before.)

Ridge venting along with 1.5"- 2" air gap between the top of the insulation & roof deck is WAY better than nothing for drying out the roof deck. Air-tight methods on the interior will be more critical than if you had both soffit & ridge venting.

The band joist/beam round the exterior walls of the crawlspace are worth insulating with at least 2" of foam + 3" of fiberglass.