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spray foam with geothermal


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By Chris on 9/12/2010


My wife and I are building a ranch home in Howell, MI and we have been very interested in geothermal systems for years. For this home, we are looking to maximize energy efficiency as best we can afford; so we have been considering using a 1" thick spray-foam insulation in conjunction with cellulose and installing a closed-loop geo system. Our geo contractor is telling us this setup will require an air exchanger (seems reasonable) and a bigger geo system to handle the extra load from the exchanger.

However, I've been checking around and I find online information that says the geo system can be smaller if you are using spray foam. So now I am very confused... I understand the air exchanger, but does the geo system need to be bigger or smaller if using spray foam? Also, one of my o.p. questions was, which system uses less net energy - a geo system w/o spray foam, or a geo system plus air exchanger with spray foam? There must be some formulas out there somewhere that can calculate this type of information.
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By Jere in Ray Twp., MI on 9/12/2010


Chris,
1" thick closed-cell spray with spray-in cellulose for the balance of cavity insulation fill is a cost-effective way to go. The spray foam does a great job of air sealing. I also suggest spray-foaming the rim-joist area... this is where the floor joists sit on the foundation. Also spray-foam 1" or so in the attic on top of the drywall and top plates of the exterior walls, and then blow in cellulose on top of the foam to the desired depth and R-value you want.

Something else to consider is using foam-board insulation on the exterior of the framing of the house. Wood has an R-value of approx. 1.25 per inch, and most homes have approx. 20%-25% of the exterior-surface wood (studs, headers above windows/doors/rim joist, etc.). Adding 1" of XPS or EPS foam board will give you an additional R-5 and will provide a nice thermal break. The foam board would go from top of basement wall to the trusses, so all of the exterior wood framing is covered. Tape and seal all seams, seal/foam around any penetrations to eliminate any air infiltration/air leaks. You could go with thicker foam board if you wanted to... anything over 1-1/2" thick special details must be done for windows, siding, etc..

In the basement, put 2" thick foam board down before the basement floor is poured, this will help keep your heat from escaping through the slab and into the ground.

For your foundation, take a look at Superior Walls precast insulated-concrete basement. This is what I have in my home and I am very happy with it. The basement comes with R-5 insulation built in, and you can add additional insulation between the concrete studs. The concrete studs are 24" o.c. and have wood strips to hang drywall in the event you decide to finish your basement.

Were you looking at using an HRV (Heat Recovery Ventilator) for your air exchanger? I wouldn't think that your geo system would have to be sized larger because of the air exchanger. How did the geo contractor determine the size required for the geo? Homes that are built properly and insulated/air-sealed properly will have a lower heat load; therefore the HVAC is smaller compared to a leaky stick-built house with fiberglass batt insulation. The money saved for the smaller HVAC can offset the cost for the better insulation (spray foam & foam board). Your utility bills will also be much lower with a properly-sized HVAC system too, as a properly-sized system will run longer cycles in the cold winters/hot summers, making it much more efficient. Oversized HVAC will short-cycle because it heats/cools the house too fast. Also in the summer, an oversized system will not remove the humidity as well as a properly-sized system. My parents have geo in their home, Certified Temp Innovations was the geo contractor. You could contact them for another opinion/estimate.

As far as which system uses less energy... I would say without an air exchanger, but it would not be healthy. You must have fresh air coming in and exhaust your stale air... the most efficient and healthiest way is using a HRV. You could have your bathroom fans connected to the HRV, so you are removing any odors and/or moisture from the bathroom. Panansonic makes energy-efficient and quiet bathroom fans.

How far along are you with the house, or are you in the planning stages?

Jere

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By Chris on 9/12/2010


Jere,


Thanks for the advice. We are using Superior Walls and 2" foam under the basement. The 1" foam around the house is out of our budget, unfortunately. We have to draw the line somewhere. :)

I do want to get another geo quote (or two) and after reading more about this, I definitely will. My brother used the same geo guy for a retrofit and was very happy with his work, but something just doesn't add up to me when he says a bigger system is required when you introduce an air exchanger. I will have to go back and check if he's quoting an HRV on something else.

We are still in the planning stage, but very late. Plans are done; we are working on the spec list right now, which is why these questions are coming up. There are some other factors, but we're pretty much ready from a planning perspective. This is why I need to get the geo/insulation thing figured out ASAP.

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By Arne in Houston, TX on 9/13/2010


Chris


Your HVAC contractor should have done a Manual J calculation based on the detail you provided about your plans. I would ask to see the details they used to making the calculation. 

All other things being equal, a tightly-sealed home without a fresh-air heat exchanger would have a reduced load requirement compared to a tightly-sealed home with a fresh-air heat exchanger. Reason for the increase is that air exchangers do create a heat loss of some level, because they are not 100% efficient.   

Depending on the system size increase your GEO contractor wants to do, you could compare that expense (up front and operating) to the expense of increasing your insulation system.

If it were me, I would budget based on a larger system, but before making the system size commitment, have a blower-door test performed once your insulation system is complete and the house is closed in before installing drywall. At that point, you will have a true measurement of air movement plus have the ability to address any leaks that may still exist.   

Arne

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By Jere in Ray Twp., MI on 9/16/2010


I agree with Arne, a Manual J calculation should be done to determine the proper size HVAC. Do you have natural gas or propane where you are building? Take a look at the cost difference between geo and a 90+ or 95+ furnace. I bet you can buy a lot of insulation for a fraction of the cost difference. The additional insulation (foam board on the exterior) along with spray foam in the walls, rim joist, and attic will reduce your heat load requirement substantially. The reduced heat lead with the better thermal envelope will not only require a smaller (but properly-sized) HVAC system, but will run less often and a lot more efficient. If you can reduce the size of the HVAC, your initial upfront costs will be less since the furnace is smaller, and the operating costs, utility bills, etc. will be less too, like what Arne mentioned. Once you have an air sealed, properly insulated home with minimal thermal bridging, the return on investment for geo isn't very good.

Now if you have an old farm house that has air leaks, poor insulation, etc., geo would be a great option because the heat bill with the old heat system would be really high. The heat-load required would still be the same BTU with geo as it is with the old system, just that the geo is more efficient at making the same amount of BTU's.

What you really want to do, is reduce the heat load required to heat your home as much as you can... from there, it doesn't take much to heat/cool the home and a lot less expensive HVAC system could be used. Don't get me wrong, I think geo is great. If money wasn't an object, do both, especially if the government is covering 30% of geo costs. Most people have a certain comfort zone as far as budget goes, and will generally keep the granite countertops and get rid of the better insulation or more efficient HVAC. Countertops are easy to replace/upgrade at any time; adding insulation to improve the thermal envelope is a lot more difficult (attic insulation is easy, but not walls or outside of walls).

BTW, are you planning on building the home yourself?  I owner-built my home over 7 years ago with the help of Pierson-Gibbs Homes. I did my own electrical, painting, installed cabinets and finish plumbing, and my uncle and I custom-built the countertops in the kitchen. I live maybe 45 minutes or so east of where you are.

Jere

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By Arne in Houston, TX on 9/16/2010


Since the topic of Manual J came up, if you use an iPhone or iPad, there was just released an HVAC residential-load calculator app from Carmel Software. It allows you go room by room entering in dimensions and the construction of each opaque (wall, ceiling, floor) surface and determine the heating, cooling and ventilation loads. When you are done, you can email the completed report to yourself and use it in your project.


It's a $24.95 app, but given its capabilities, it is definitely a lot less expensive than buying a full blown Manual J program if you want to run what-if scenarios.  

Again, it's for the iPhone or iPad, so just go to the app store and search for HVAC residential.

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By Kenneth in Lees Summit, MO on 9/17/2010


The blower-door test is a critical part of the equation here.

You want tight construction. There is more than one way to achieve this. Spray foam makes it easier. Poorly-installed spray foam isn't a recipe for tight construction though, just a recipe for spending a boatload of money. My point is that although it is superior material, it isn't foolproof. And if you have superior craftsmen, you can get similar results with more-conventional methods (although perhaps not less cost). The key is performance; materials to achieve this performance should be a secondary part of the equation. And a blower door doesn't measure materials; it measures performance.

When you build tight, you need ventilation. No two ways about it. Again, there is more than one way to do this, but energy recovery (HRV, ERV) is one way to do this. Along with ventilation, I like separate humidity control. The three items you control separately are temperature, fresh air, and humidity. Nice.

Given a complex system (tight construction, ventilation, humidity) that is different from "normal". What you need is an HVAC subcontractor who can handle this. Manual J is just a starting point. For my money, I talk to commercial HVAC subs in this situation. I find that they tend to be more equipped (expertise) for complex situations. I also find that they use smaller equipment (usually a good thing) and better-quality installs (commercial job sites are frequently inspected for quality control by engineers and architects; they don't tolerate sloppy work). Your labor cost will increase, but this may be offset by less equipment cost (due to smaller), and better use of labor such as shop-built steel ductwork. My HVAC guy took measurements, and about a week later my ductwork came on a big truck. Union subs make a good amount of money; the upside is they have a good amount of efficiency so that hourly rate may be offset by less hours.

Once you get your system designed, you can always take some money out of it and your HVAC subcontractor will know how to take it out. For example, I have five zones of ductwork, but my HVAC subcontractor recommended for a cost savings that perhaps we eliminate the zone controller (over $1K). We also eliminated the ERV (another ~$2K). The location for both is perfectly accessible, and five years later I still haven't added them. However five zones of ductwork isn't something I could have retrofitted later. Even the mechanical engineer I hired to do my HVAC calculations didn't believe how small a unit I am using (which is still too big, I really didn't need to spend that money but those are lessons learned only from experience).

In the interest of what do I sacrifice, I would sacrifice the geothermal. HVAC is getting more efficient, the best units from 5-10 years ago can't touch the performance we get today. I will assume that the units 10-15 years into the future will achieve similar levels of improvement. If geothermal fits within my budget, and it offers good return on investment (do you have an HVAC quote for a more traditional system?), then yes I make that investment. If I have limited funds (don't we all), then I take opportunities to maximize the return on my limited investment.


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By Mary in PA on 9/18/2010


Kenneth, you said:

"Once you get your system designed, you can always take some money out of it and your HVAC subcontractor will know how to take it out. For example, I have five zones of ductwork, but my HVAC subcontractor recommended for a cost savings that perhaps we eliminate the zone controller (over $1K). We also eliminated the ERV (another ~$2K). The location for both is perfectly accessible, and five years later I still haven't added them. However five zones of ductwork isn't something I could have retrofitted later. Even the mechanical engineer I hired to do my HVAC calculations didn't believe how small a unit I am using (which is still too big, I really didn't need to spend that money but those are lessons learned only from experience)."

I like the idea of skipping large-ticket items (but planning for them) and then seeing how it goes. So I'm wondering, you skipped the ERV, but I think you would say you have 'tight' construction, right? So how do you manage IAQ? I'm not sure where you are located, maybe even in winter you can still open windows?

Thanks,
Mary


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By Kenneth in Lees Summit, MO on 9/19/2010


I run ventilation through my auxiliary dehumidification system. I run an Aprilaire dehumidifier, and while it is not intuitive they (Aprilaire) have a method to hook these up using two intake sources; interior and exterior. If you look at the Therma-Stor Ultraire-series dehumidifiers (very nice, albeit at a much-higher price point than the Aprilaire), they have two distinct intake sources built into the box itself.

And for a truly-budget install, just put in a skuttle valve for make-up air control - probably ~$15 or so. You need ventilation; you need makeup air. If you want more ventilation, run a bathroom or kitchen vent fan. If you want to be more formal about it, put one of these fans on a timer. It takes a long time to recover enough energy to justify the price of an ERV.

Now then for the truly industrious; I have heard of earth cooling tubes used to supply the make-up air. The proponents like this in that the make-up air temperature is tempered to the ground temperature (think how cold that make-up air is in the winter, what about the heat being introduced in the summer, and now temper it to an average year-round). While skeptics are quick to dismiss this ancient technology, I am not so sure it doesn't have practical use. At the time I built, I considered it but didn't have any design information. I think I could piece together a passable system now.


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