background:

My new ICF home will be going up this week (footers are in). however, between the main ICF portion of the house and the garage, there is a mud room, which is almost like a 'connecting hallway' between the house/garage. It's only 1 story and angled so the garage has an angle to the house (see attachment). The mud room and the garage will be stick-built to save some money. They will both be 2x6 framed (will be confirming this with my builder this week, but that's what the plans specified).

I'm planning for blown cellulose in between the studs, but I'm trying to figure out what my most cost-effective setup will be, such that I'm not canceling out the benefit of the rest of the house.

A few questions:

1) should I bother with exterior insulation over the sheathing?
1a) if so, how much is necessary and what type? (I've read the building science article about dew points on the sheathing when using exterior insulation)
1b) would the air barrier/house wrap go over or under said insulation?
2) does blown cellulose require a vapor barrier on the interior? I'm assuming it does.
2a) if yes, what type of vapor barrier is typical and cost-effective?
3) is typical tyvek house wrap 'good enough' for such a small area? or should I use something else?

Thanks!

It depends. Where are you located?

Is the mud room part of the conditioned envelope? How many square feet is the mud room? Main house?

South-Central PA

Yes, it's part of the conditioned envelope. mud room is approx 125 sq ft; entire house is approx 5946 including basement.

South central PA is US climate zone 5, unless you're in the yellow zone (4):



In zone 4 you don't need an interior side vapor retarder.

In zone 5 you can skip the vapor retarder as long as you have "vented cladding" such as vinyl, or build with a air space/gap between the sheathing & siding, and assembly type referred to as "rainscreen".

In either zone 2x6 w/cellulose meets the IRC code minimum for thermal performance without exterior insulation.

In zone 5 if you install exterior insulation  it needs to be a minimum of R7.5 if 2x6 or R5 if 2x4 for dew point control at the sheathing.  If you use less exterior-R than that, a Class-II or variable permeance vapor retarder is required on the interior. Do NOT install polyethylene or foil (or even vinyl or foil wallpaper), since that will only impede drying capacity (severely!).   The sheathing can't dry toward the exterior at a reasonable rate through an inch of foam (unless it's unfaced EPS, AND rainscreened).  A variable "smart" vapor retarder such as 2-mil nylon (Certainteed MemBrain) is far more forgiving and robust than half-perm paint or asphalted kraft, etc.

A 2x4 cellulose + R5 foam wall meets code min as well, but to approach ICF performance you really want to go with 2x6 cellulose + R8, which comes in at about R23 "whole wall" after factoring in the thermal bridging, and has adequate dew point control even in zone 5 with some margin.

The greenest foam sheathing solution is EPS, which has a stable R value over many decades, and is blown with a relatively benign blowing agent (pentane), which is often recaptured at the factory and burned for process heat.  XPS is the same polymer and has a ~15% higher labeled R value, but that higher R is temporary, and dependent upon the amount of retained HFC blowing agent.  The most common HFC used for blown XPS in the US is HFC134a (automotive AC refrigerant) which packs a very powerful greenhouse punch, ~1400x CO2.  As the blowing agent leaks out over a few decades the R-value sinks to that of EPS of similar density (about R4.2/inch.)   From a design robustness point of view it's best to assume the fully depleted performance, not the labeled performance.  From a $ per labeled-R point of view EPS is cheaper anyway.  You'd need at least 1.5" of XPS to meet the R7.5 labeled IRC prescriptive for 2x6 framing, but 2" of EPS (R8.4) is usually cheaper, and will still be performing at R8.4 in 50 years.

The placement of the housewrap depends on how you are mounting & flashing the windows.  If the glass is going to be co-planar with the structural sheathing ("innie" mounted) the housewrap should be a crinkle type  (eg Tyvek DrainWrap) and goes between the foam and the sheathing, properly lapped with the window flashing.  If the glass is roughly co-planar with the siding ("outie" mounted), you can use flat housewrap, and it goes between the foam  and the siding.

Tape the seams of the structural sheathing and caulk the sheathing to the framing inside each stud bay as the primary air barrier.  While it's sometimes possible to detail the housewrap as an adequate air barrier, it's really nowhere near as robust for air tightness. Don't forget to caulk under the bottom plate of the studwall, and between doubled up top plates too- those long skinny seams are much bigger holes than they appear to the naked eye.  Tape the seams of any exterior foam too- it will help performance if it ever shrinks over time (foam shrinkage issues have improved a lot since the 1970s, but are you really going to count on it to be a perfect air barrier for a century?)

awesome info. Thank you!

So I would be in the green area just a tiny bit north of that small yellow patch, which is York County (I'll be in Cumberland County).

So if I'm reading your post correctly, My setup should be as follows from the interior to the exterior:

Paint : ship lap (this is what the wife wants in the mud room instead of drywall) : blown cellulose w/2x6 studs : OSB sheathing : Air Barrier : EPS : vertical furring strips (rainscreen) : vinyl siding

There won't be any windows in the mud room, so it sounds like I could technically choose air barrier in either location although i would think inside would be easier to attach it.

And the EPS on the outside I was just planning to do an R10 since I'm already ordering R10 for underslab, i was just going to order the extra for those walls in the same thickness (i think it comes to like 2-3/8")

does that sound like a good plan?


As a follow-on question, since the mud room is so small, do you think it would be easier to put the ceiling insulation in the roofing system instead of blowing it in like a normal attic? I've read about this before, but I have no experience with it and wanted to see if anyone has a good reference design for that as well? I'm drawing a blank on the right term to search, but basically it would be a 'conditioned attic' space for just that small section.

Thanks!

All fiber insulation needs both an interior and exterior side air barrier. While dense packing the cellulose would be sufficient for maintaining it's thermal performance in your stackup,  ship-lap ,t & g or any other planking it isn't even close to being an adequate air barrier, and air transported moisture will find it's way from the interior to the cooler sheathing. 

With R10 EPS on the exterior of R20-ish cavity fill you have adequate dew point control at the sheathing for 5 perm paint on theinterior, but not enough for an air-permeable interior side sheathing.  You have a few options for air barriers here:

The most robust would be thin gypsum board between the ship-lap &  insulation, detailed as an air barrier, and painted with single coat of standard latex primer. 

Non-perforated 1/4" fan-fold XPS siding underlayment (seams taped) would also work, but would also limit the drying rate a bit (most of those products have thin polyolefin facers bringing it's vapor permeance down to under 2 perms.) 

You could also use 2 mil nylon Certainteed MemBrain, which is a variable permeance "smart" vapor retarder, but be sure to tape any nicks or tears that it might pick up when installing the ship-lap.  This would offer the most optimal drying, but is more susceptible to damage.  (If you can't find it through distributors, Lowes & HD both are offering smaller sheets on their websites now, at somewhat inflated pricing over typical.  An 8' x 100' sheet costs about $100 at the midwestern box store Menards (alas, none in PA)  which is cheaper than Lowes/HD and it's usually cheaper still at distributors catering to the trades. I don't imagine you have enough wall area in a mud room to make searching for better than box-store pricing worthwhile.

Under no circumstances should you use polyethylene sheeting in your stackup.

In a zone 5 location with a cathedralized ceiling at least 40% of the total R needs to be above the roof deck, or as closed cell foam at the roof deck if it's all under the roof deck.  (The IRC prescriptive is R20 minimum above the roof deck at a code-min R49.)  If it's a single flat pitch putting 5-6" of EPS above the roof deck (R21-R25), and dense packed 2 x 8 rafters ( 7.25" of cellulose, for ~R27) works fine.  If you use rigid polyiso above the roof deck you can get away with as little as 4".  The labeled R of 4" polyiso is likely to be R24-R25, but when the average temp through the foam is under 40F it takes a significant performance hit (unlike EPS, which sees a performance gain), and for design purposes needs to be derated to about R5/inch in your climate.

The Mass Save Deep Energy Retrofit Builder Guide written specifically for Massachusetts retrofitters a useful reference for building better than code new construction in your area, since many of the details are zone-5 specific (such as the 40% total R above the roof deck minimum, etc.).  There are good detail drawings on how to build with exterior foam on walls, variations on how to safely do high-R unvented roof systems, etc.   Another set of design and implementation tips/resources can be found on this site.

I cannot express my appreciation for your wealth of knowledge!

I think I will convince the wife to put 1/4" drywall up under the ship-lap with the primer.

I'll keep doing more research on the unvented roof concept. Thank you for the links and other info.

Standard spec 1/2" drywall at the builders supply house probably costs less than 1/4" drywall because 1/2" is used so much while 1/4" not so much (~10% lower cost for 1/2"). Plus, you can get it in 12' long sheets which reduces the amount of joints to mud and the new lightweight 1/2" board would probably be easier to handle than full sheets of 1/4" (less stiff, prone to breaking). Plus, if you are using 1/2" gypsum ceiling board for the ceiling you would probably end up with less waste if you used 1/2" everywhere.  I can't think of any advantage 1/4" has over 1/2" in this application.

Thanks Arkie! I was just assuming 1/4" would be cheaper. I'll definitely look into the 'lightweight' board.

If you are going to add rigid foam anyway, you can move it to the interior, tape (SIGA?) it as the interior side air barrier and save the cost of interior gypsum.

Posted By jonr on 22 Jul 2016 02:00 PM
If you are going to add rigid foam anyway, you can move it to the interior, tape (SIGA?) it as the interior side air barrier and save the cost of interior gypsum.

Interior foam has more penetrations, more thermal bridging, and is more difficult to install than putting it on the exterior. If it were easier or more effective/cost-effective we would see that done more often. Interior side foam usually done a last-resort move for improving thermal performance in retrofits where they don't want to (or can't) touch the exterior.



To date I can't recall ever seeing that done on new construction, and it's for a reason (or multiple reasons.)

Plus, we wouldn't want to do that anyway since we don't want to lose that space on the interior.