Hi, I previously posted about a similar topic and appreciate the input I got. In this post, I’m trying to focus in on wall assembly issues rather than frame vs pole construction. SIPs are not on the table due to cost. I’m NOT trying to innovate a new wall assembly – believe me! Any comments and advice are greatly appreciated - Thanks!

After reading Building Science articles and other posts on this forum, I’m considering one of the two wall assemblies listed below. The building is a 50x30x14 machine shop, 2x6 frame construction, 5” insulated slab with pex. It will have ceiling with blown-in insulation and vented ridge and soffits. We are in a mixed humid climate (MD/PA border) with about 44” of rain/yr. Wind driven rain will be the norm until windbreak trees grow up.

Some of the things that seem different for our building compared to what I read about are:
- use metal siding instead of typical residential siding options
- winter heating to 55 or 60, no air conditioning in summer (So is it really a mixed climate without AC, in reference to the wall assembly?)
- trying for energy efficient so we can attempt solar HW for the in-slab pex (Based on what I’ve read, job 1 in getting solar heat to work is attending to air infiltration and good levels of insulation.)

Option 1: (more insulation value than Opt 2, but moisture problems?)
metal siding
horizontal furring strips (untreated)
1” closed cell rigid foam, no foil
OSB
2x6, cavity filled w/ paper faced fiberglass or possibly blown in
drywall, vapor permeable paint

Option 2: (breaths to both sides, but not enough insulation?)
metal siding
horizontal furring strips (untreated)
some type of wind barrier wrap (?)
OSB
2x6, cavity filled w/ paper faced fiberglass or possibly blown in
drywall, vapor permeable paint

Is this drywall with full ADA gaskets to make it fairly airtight? The foam or housewrap will be taped?

If your goal is for solar to be a significant fraction of the total you're going to need at least R30 clear-wall R-values, R35 would be even better. With 24" 2x6 framing batt insulation and an inch of XPS foam, metal siding you'll be barely at ~ R22. If you go with better grades of blown fiberglass you'd be closer to R25.

In your mixed, but still heating dominated climate you'd want to use vapor retardent paint or vapor retardent facers on the wall insulation with either of your options.

To get it up to ~R30 you'd need to go with the best-grade of blown fiberglass and go 2" on the XPS. If you're really only heating it to ~ 55-60 and it's very tight construction, you'll need an interior side vapor retarder or mechanical dehumidification to avoid condensation within the fiberglass layer. An inch of XPS has a permeabilty of ~ 2.0, so it'll still dry toward the exterior, but at 2" XPS has becomes semi-impermeable at ~1.0perms- still drys toward the exterior, but a half the rate of 1" stuff, so greater care has to be taken in making the interior both air & vapor tight- poly sheeting under the wallboard is in order. Since you're not air-conditioning the place and metal sided, the summertime vapor pressures will be low, and interior temps will track the outdoor average and no condensing hazard within the wall from exterior water vapor exists, so it doesn't NEED to dry toward the interior. (In hotter, more humid climates & masonry exteriors, air-conditioned interiors interior drying capacity is a must, but not here.)

Another interior stackup possiblity would be to add 1-2" of foil faced iso under the wallboard with FSK taped seams & caulked/foamed edges, leaving the exterior side at 1" of XPS. That way the drying capacity is higher (still all toward the exterior), and the R-value can be boosted beyond R30 where it needs to be. It's more labor though, and more expensive than...

Option-3 would be to use fiber-faced iso or iso-panels with nailer-deck on the exterior, mounting the siding on purlins or furring through-screwed into your 2x6 framing. A 4x8 sheet of 3" thick (~R20) fiber-faced roofing insulation runs ~ $55 f.o.b. the distributors' yard (I haven't price the panel-product with the OSB facer recently.) With this amount of exterior foam you won't need interior vapor retarders on a fiberglass insulated studwall, and fiber faced iso is highly permeable (25perms+). OSB nailer decks or sheathing are less permeable than fiber faced iso foam, but variable- over-2-perms when wet, still over 1 when dry at any thickness that you'd actually use under siding. This would be the highest-R solution, with the highest drying capacity,( but 2" thicker then your Option-1.) If you specify R20 minimum on the exterior iso you can safely use foil-faced stuff in your climate and let the studwall dry only toward the interior.

More insulation is more money, but if you run the numbers on how much solar it takes to actually support the load, more insulation is generally more cost-effective than more solar. If as stated you're using the slab as a low-temp radiator you'll need at least R10 of XPS or EPS (and a vapor retarder) under the slab as well. If you have a good exposure on the S side of the building, building-integrated thermal air panels can provide a significant low-cost solar boost to the setup, but you'd have to plan for shading or venting them to the exterior in the summer to keep from overheating.

Posted By BabyBldr on 07 Apr 2010 11:14 AM

Wow Dana1, a great amount of useful and detailed information. I hope you won't mind some follow-up questions, but first I want to take what you've provided and sort through it more carefully, do some additional reading and costing. Thanks again!!

You may want to read up on thermal air panels to work them into the cost analysis. A very-inexpensive version is detailed here.  There are variations on the them, some with blowers, others operating off natural thermosipon convection, but all are much cheaper to implement than hydronic-solar heating. In highly insulated buildings reducing the cross section of the ventilation  area and using active blowers maintains the integrity of the thermal envelope by punching fewer holes in it. there are panelized variations on the theme from small and panelized to not-so-small, and building-integrated.  The size of the array you'd need depends a lot on your R-values, thermal mass (or earth-coupling), and both your overheating & low-temp tolerance for the building.

When you look at the systems-costs for an intermittent use building like this (as opposed to homes, which need to be at comfy temps 24 hrs/day), raising the insulation levels of the wall & roof your overnight heat losses will keep temps closer to the deep-earth temps for location.   Heating it primarily with building-integrated thermal air, (with a small gas space heater) for freeze-protection, if necessary) is going to be a lot less expensive to install & operate than going with an active solar slab.  Subsoil temps in the MD/PA border at elevations below 1000' are in the 58-60F range, so if your heating design goal is 55-60 you'd only need to insulate the slab edge/perimeter when the slab isn't being used as a radiator.  How you insulate the slab-edge will be critical the performance when using the thermal mass & temp of the subsoil to your advantage.  To find out the subsoil temp in your area with more precision, measure the water temp of any well deeper than 20 feet.  (If  you're on a flood plain the water table is less than 5' below your slab you may still have to insulate the center of the slab and you'll only be able to count on the thermal mass of the slab for daily heat storage.)  Insulation money saved on the slab can go directly into beefing up the R of the walls & roof.  Thermal solar panels that use water as a working fluid aren't cheap, nor are the pumps & controls.  Even at wholesale you'd be spending more than $25/ft^2 of glazed area for the system, and it would operate at comparable or lower efficiency than thermal air panels, you'd have to freeze-protect it, etc.

What you give up with going to air as the working fluid is the ability to store substantial amounts of heat, but with a perimeter-insulated slab and 55-60F subsoil and R30 walls the earth is already doing that for you- you're coupled to a large thermal mass (the earth) already at your heating design-temp.  If your air infiltration and glazed area are low, you'll be inherently freeze-protected, and the solar/other will just be "finish heat" to add back what was lost overnight. 

Once you have the building design up to the point where you can calculate the heat loss, you can then size the solar array appropriately for the daily loads. If it's too big you'll overheat even on sunny winter days when it's 5F outside.  Some overheating will be useful for charging the slab & soil temps so that the thermal mass can deliver the heat at night, but you probably don't want to have it be hitting 85F in there any day the sun shines. But 75F might be tolerable in bright winter sun with a lot of snow-reflected insolation, and when the air is 10F+ above the slab temp, you'll be injecting useful amounts of heat into the slab/earth passively.

If you still want to heat it with a radiant slab, burying a large insulated tank (preferably below the slab) as your storage like Reddirt's passive/active solar approach isn't a bad way to go.

Lots of great info in this thread. So to not get it all too mixed up, I’m going to try to tackle one topic at a time. I momentarily set aside the heat source/storage issue to attempt to settle the wall assembly issue.

Dana1 said: “… Another interior stackup possibility would be to add 1-2" of foil faced iso under the wallboard with FSK taped seams & caulked/foamed edges, leaving the exterior side at 1" of XPS. That way the drying capacity is higher (still all toward the exterior), and the R-value can be boosted beyond R30 where it needs to be. It's more labor though, and more expensive …”

This suggestion suits our construction constraints the best, I think. I believe the framing and siding contractors will be more comfortable doing only 1” XPS exterior rather than 2”, in terms of attaching siding and such. For the foil faced iso under the wallboard, my husband and I can DYI install this “at our leisure” without holding up contractors. We plan to do all finish anyway. Also since we’ll have a ceiling inside, we won’t need to do the interior iso on the high gable ends – making it a more doable DIY task for us. And this opens a window of time (and a location to store materials) for us to attempt to scrounge some of the iso from recycle sources or auctions. So, to recap this would yeild a wall assembly of:
- metal siding with horizontal furring strips
- 1” XPS installed per mfg instructions (taped, foam, etc)
- 2x6 24 O.C.
- Cavity? (see question below)
- 1” of foil faced rigid foam (iso type) with foil to the inside, installed per mfg instructions (taped, foam, etc) [2” if the budget allows]
- Wall board, taped and painted

Questions:
1. For the cavity, are you staying to skip the fiberglass altogether because creates a possible is a moisture problem in the wall? Or, are you saying to add the interior iso AND to keep the fiberglass fill in the cavity?
 
2. (Just curious) Does adding the iso move the location of the dew point out of the cavity? Or because the iso is foil faced and taped is moisture (vapor) prevented from even entering the cavity?

3. Does the foil faced iso remove the need for vapor impermeable paint on the drywall?

4. Does this above listed  wall assembly (with or without the fiberglass in cavity as per your pending recommendation) sound reasonable in terms of insulation value for an attempt at solar? Will it avoid moisture/rot issues?

Thank you for your time and help with this – much appreciated!

Ok, in this post I’m trying to make a dent in the heating source/storage, assuming for the purpose of discussion the issue of the wall assembly is settled.

Dana1 said: “…You may want to read up on thermal air panels to work them into the cost analysis.” 
 
I spent last fall reading up on just this topic with tons of time on the BuildItSolar and Home Power sites. But your link on the building integrated system was new to me – great photos BTW. I seriously considered the thermal panel concept for heating the shop.

For site planning, I oriented the long (50’) axis of the building within a few degrees of solar south. The east side of the south wall has a people door and two 3’x5’ windows. The windows allow sunlight onto the always open loading/unloading area of the slab (east wall has 12’x12’ insulated garage door). An awning will shelter the people door and two windows from summer sun. The remainder of the south wall is pretty much available for solar collection. I say pretty much because I do have four 2’x2’ windows at the very top of the 14’ wall for daylighting in the shop, with a 12” roof over hang to provide at least some shielding from the summer sun.

To get a personal feel for thermal air panels I built a prototype (passive). I installed the crudely built panel in a door that we seldom used on the south side of the house. It was a tight fit, with lots of insulation tucked around it and bracing on the outside against the deck. Not very pretty – but then prototypes seldom are. On sunny days between 10:00 and 3:00 I opened the existing door and the panel supplied heat to our dining room. I love the simplicity of the concept. Even if a blower was used on the shop's version it still seemed pretty simple to construct when the ‘fluid’ was good ‘ol air.

But as the winter progressed and many cloudy and overcasts days passed by I began to think storage. Yeah – that’s the ticket! In other research I was reading rave reviews for radiant in slabs. And accounts of how about how plumbers were called out to an older home to service some thing or other and found 20 yr old solar HW systems still working away. So putting that all together it got me to thinking … instead of thermal air panels why not go solar HW with an in-slab radiant. Yeah babe – the idea is evolving…

And now, after all this long windedness, if you’re still reading … I’ve come full circle with Dana1’s post which raises some interesting items. Specifically the idea of using the earth (since it is close to our target indoor temp) as storage instead of water. That had never occurred to me. Wow! Eye opener!! So, to play out the idea, some additional info and questions below. Sorry this is so long….

Cost:
Whether air panels or solar HW-radiant, I was thinking we would design and build the system. Yeah – we may be heading for trouble design wise… but we’d do the best we could. We’d’ mount the panels on the south facing wall rather than the roof for easier install and maintenance. If it was a solar HW system, I was thinking of an insulated underground tank and using a simple drain back system. I wasn’t sure about freeze protection yet, but suspected it would be the safest thing to do for copper pipes in the collectors … but was hoping in drain back configuration it wouldn’t be necessary.

The building:
- isn’t in a flood plan (good)
- will have 22” fall in grade from the north east to the south west + sitting a minimum of 6” above surrounding grade
Question:
For a thermal air panel system that uses the earth as storage, should insulation be laid outside the foundation wall before backfill?

Dana1 stated: “How you insulate the slab-edge will be critical the performance when using the thermal mass & temp of the subsoil to your advantage.”
Question: If I isolate the edge of the slab from the foundation wall and insulate under the perimeter of the slab (X? feet from the edge), is that what you’re referring to? I’ve seen construction drawings of this I can refer to … unless you mean something else entirely.

Question: Under the "have our cake and eat it too" dilema ... Let’s say we still wanted to do pex for in-slab possible future radiant even though we hoped to rely only on the thermal air panels and earth-coupled storage. My husband is having a tough time letting go of the idea of the radiant and doesn’t fully trust the air panel concept. Just a gut feel, no numbers crunched (I know, I know, must do the math). So, is doing pex in an edge/perimeter only insulated slab a waste of time/money?

Question: If we have the space available, can the insulated water tank be next to the building rather than under the slab? What advantage is there in under-slab that off sets the forever disadvantage of not being able to access it?

OK – that’s about it, and it sure was a tome! Thanks for reading.

There's a freebie called BaseCalc out there (google it) that is useful for figuring out how to best insulate the slab edge. At a minimum, using insulated concrete forms (ICFs) when pouring the footings/foundation walls is a start. Then, insulating horizontally either under the slab, or under the dirt outside can make a large difference in seasonal heat loss out of the slab. With ICFs on the walls leave the EPS of the interior side of the form even at the slab and float the slab, thermally isolating it from the foundation & footing, which makes it both a thermal isolator and an expansion joint.

If you put PEX in the slab for potential later use it then becomes a dilemma of whether to insulate under the center-slab or not. If you have 10' of dry soil underneath you can skip it, and still use the thermal mass of the earth to good advantage. The response time for bringing the slab up to temp will be ridiculously slow with solar-only. If you think you'll want something quicker that draws on a big thermal tank, you might consider a baseboard loop around the perimeter as well and run it as a 2-stage system, but that detail is easily done after the fact.

Putting the thermal tank under the slab ensures that most of the heat lost from the tank still ends up inside the building, or at least goes into the thermal mass being used by the building. Putting it outside means it's lost to the outdoors and earth mass NOT used by the building. But if you insulate it well with XPS &/or closed cell spray foam and bury it below the frost line the fraction you lose to the great outdoors or Mother Gaia can be reasonably bounded. (That's how Reddir's sytem is set up.)

Drainback & draindown systems rely on gravity & controls for freeze protection. It requires careful planning and implementation to get the slope right to fully drain the panels. Gary Reysa covers those issues in some detail on his "Solar Shed" solar radiant floor design on the builditsolar site. If you go with his atmospheric pressure system approach, be sure to use only bronze pumps & oxygen-compatible plumbing, and test the PH of the water at least once/year, buffer as-necessary. You need at least 8psi on the system to keep oxygen out of hydronic systems, most are designed to run at 12psi & up.

Don't expect to get days worth of thermal storage out of 1000 gallons of water unless you've insulated the place to over R30 and minimized the glazed area losses. The more you insulate the better earth-coupled your are as well. You might defer the active-solar/storage decision until after you've assessed the performance of superinsulation & integrated thermal air panels. A well-designed earth-coupled building envelope will do far better than the leaky-barns with the oversized thermal-air arrays I've pointed you to on the web, most of which have clear-wall R values barely half of what I'm recommending, and probably ZERO foundation insulation for minimal earth-coupling.

Make sure you don't try to use a concrete floor for both solar storage and radiant heating by conventional means. Every time the latter is used, it wipes out the ability for it to be used for the former. Ie, if you heat the slab with fossil fuels, that leaves no heat storage capacity for solar heat.

On the other hand, interior thermal mass combined with passive solar heating and a tolerance for wide swings in indoor temperature is fine.

Posted By jonr on 08 Apr 2010 01:30 PM
Make sure you don't try to use a concrete floor for both solar storage and radiant heating by conventional means. Every time the latter is used, it wipes out the ability for it to be used for the former. Ie, if you heat the slab with fossil fuels, that leaves no heat storage capacity for solar heat.

On the other hand, interior thermal mass combined with passive solar heating and a tolerance for wide swings in indoor temperature is fine.

That certainly would be an issue at conventional insulation & heating levels.   But at the anticipated BTUs per square foot we're talking here, with R30+ clear-walls & 55-60F heating setpoints, that probably isn't going to be much of a problem. 

The floor is NEVER going to get particularly warm if you're only heating to 55-60F as a design temp.  The slab may only need to be 3-5F above setpoint on the coldest hours of the coldest night of the year, which may only take ~80F heating water to achieve (TBD, since we don't have a Manual-J analysis in front of us).  Overheating the building to 70-75F with solar during sunny days to store more heat into slab & subsoil will probably be tolerable to the users.   Even at on nights where it drops to 0F outside, an R30+  building isn't going to drop from 75F to 55F  overnight if it contains a slab that hit the 70s during the day.

Slabs can only be reasonably be counted on for storing/moderating the daily swings- you can't get a several days or a week's worth of storage in there without really cooking the place during the sunny weeks.  But if you set it up with a mid-70s high limit on the solar, and a 55-60F setpoint on the fossil/other heat source it can probably be tuned to coast along in the 60s & 70s during sunny periods, only dropping into the 50s after a few cold sunless days. It probably won't need much of any input (solar or other) to maintain 55-60F completely passively from mid-March through mid-November, but if you allowed the solar to run it up to 75F starting in mid-October, some amount of seasonal-type storage will be available from higher than heating setpoint soil temps under the slab, which might be sufficient to coast into December-January on solar alone, despite the minimal available insolation.  Hopefully the longer days and snow reflection gains in February will be enough to carry the load without needing to draw further heat from the soil. 

The convenience of having subsoil temps at or near the heating setpoint means that the center-slab soil can always be treated as a longer-period (low BTU/hour rate) buffer, not a net heat loss, so even heat going into the soil from fossil fired appliances would still be largely stored rather than lost.

I don't think the proposed design will run at the implied 6-10 btu/sqf/hr loss on really cold days but it may under more typical conditions. The other thing about a shop is that they are typically not occupied most of the day. This provides opportunities for low mass, temporary heating. Might be more efficient than a high mass design with solar storage.

Posted By jonr on 08 Apr 2010 07:37 PM
I don't think the proposed design will run at the implied 6-10 btu/sqf/hr loss on really cold days but it may under more typical conditions. The other thing about a shop is that they are typically not occupied most of the day. This provides opportunities for low mass, temporary heating. Might be more efficient than a high mass design with solar storage.

It remains to be seen what the design-day heat loss will be per square foot, but bear in mind we're talking about the heat loss at 55F indoor temp (not 70F)  in a location where the outdoor heating design temps are in the mid to high single digits (5-15F above what you need to design for in MI). With a bare-minimum R30 clear-wall values and a 50F delta-T between indoor & outdoor, we're looking at ~1.7BTU/ft^2 of wall & roof area, plus glazing losses.  If the glazed area is well bounded (or has insulated shutters) it could very well come in at ~10 BTU/ft^2 of slab as the peak design-day heat load, maybe less.  Depending on the geometry, walls & roof will worth 6-8BTU/ft of floor area, then it's a matter of the glass, infiltration rates, etc.

Shop buildings go with the low-mass hot-air heaters all the time, and with good reason- it's a cheap & effective method for bringing it up to temp quickly for the few hours you're there, and you can let it coast the rest of the time.   I wouldn't dream of going with fossil-fired radiant heating Maybe a supplemental propane/wood stove or something to cover the shortfall on the coldest-darkest weeks, if it's a place that will be occupied during working hours.

But she seems intent upon a primarily solar solution, and at her 55-60F interior heating temp and similar subsoil temp it's not unreasonble to expect that designing it for a well-insulated (but not super-insulated) R30+ she can make that happen.  If she did the Passivhaus thing and brought it up to a superinsulated R50 she could probably even skip the solar, other than sizing & orienting glazing correctly for a modest amount of E & S side passive-gain and minimal N side loss.  But even at R30 clear the expense of going hydronic solar & radiant floor seems like overkill.  The performance difference between hydronic slab heating/storage vs. simple thermal-air solutions doesn't justify the cost in my mind, but it's her money, eh?  For sure, spending several grand in insulation upgrades makes any solar solution more affordable, and it'll function better with narrower daily temperature swings.

I agree, it remains to be seen and it would be wise to calculate it. When I see "insulated garage door", high ceilings and all the windows, I lean towards high air infiltration and higher loads.

I would differentiate between concrete radiant floors (very high mass) and other radiant heats. Fossil fueled lightweight radiant is common for shops, especially ones that need to open doors. It would be interesting to look at increasing the reflectance of interior walls, ceiling and floors to increase and diffuse mean radiant temperature and mitigate the effect of cold surfaces (like the floor and heavy equipment). Ie, minimize heating of the air and surfaces, allow nearly instant heat up and still create a comfortable environment. Turn it off as the sun and equipment heat the place up.

In a different direction - with the interior and ground temperatures involved, one could get amazing performance from a small geothermal system.

I agree too, it remains to be seen.
...And I hope when I see it I like it. ;-)

Jonr and Dana1 your discussion above was interesting and informative. I gave a late night lack luster attempt at BaseCalc and didn’t even get it to run so that will take some more effort on my part. But I’m not sure how much value it will add …

Just by point of info, the building will be occupied during working hours as we hope to be running a business out of it. But I think the 55 to 60 set point and a pair of thermal undies will be fine for us. Heck, this past winter the thermostat in our house was typically set between 65 (day) and 62 (night). When I see references to 70 degree in-house temps on this website I find it hard to imagine having such warm temps in the middle of winter.

Dana1 thanks for the real talk on the proposed solar in-floor radiant. I confess I was thinking of an atmospheric pressure approach, and hadn’t really thought through the psi issue. No doubt there would be a LOT of details in trying to pull that off and it would not be easy to get right on the first (or fifth) go-round. Honestly it will be a relief to focus on the thermal air panels as a good starting place.

I hope to get the insulation for less then retail by shopping recycled sources, and we’ll do all of the interior work ourselves so hopefully we’ll come out with a quality building without braking the bank. The worst case scenario is that it will be darn cold in the middle of winter, and we need to consider some form backup heat (or more layers). Time will tell.

Thanks again for all of your help. I promise to post back after our first winter in the shop.