Best Ventilation Bang For The Buck In My Climate
I am in the process of a planning a new house. Approximately 2,000 – 2,500 square feet. ICF construction with sealed attic (spray foam under roof deck). I expect the house to be tight and want to make sure I provide for proper ventilation. I’ve read about the various options and I am now trying to determine what would be the best bang for the buck in my climate. The house will be built in the Lubbock Texas area (360 miles west and slightly north of Dallas). Climate is fairly dry. Not as dry as Arizona, New Mexico or the El Paso region of Texas but much drier than the eastern half of the state. We’re practically a desert compared to Houston.
Lubbock Climate Data:
Annual HDD = 3431
Annual CDD = 1689
Annual Days w/max temp over 90 = 80
Annual Days w/min temp below 32 = 93
Average annual rain = 18.6″
Average annual snow = 9.9″
Average wind speed = 12.4 mph
I plan to install some type of energy efficient fireplace with a source of outside air for combustion. I have not yet decided on whether or not to vent the range hood outside.
So, what do you think? Exhaust ventilation, supply ventilation, HRV, ERV, or something else?
Also, if anyone knows of a good builder with ICF experience in the Lubbock area, please let me know.
DC
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Replies
Cheapest option is central-fan integrated supply ventilation with a motorized damper on the passive air intake and a FanCycler control.
Dan,
Martin's suggestion is what we currently use in Dallas
You might look to some of the New Mexico Builders and Designers for ideas.
(Armando Cabo maybe)
I think Lubbock would be good for thermal mass.
Have you considered Adobe?..how about an Earth Ship?
Kiva Fireplace etc....
I wonder what the summer night time temperature is like in Lubbock?
Martin,
Cheapest as in up-front costs or life cycle costs? Is the way you describe the best bang for the buck? I don't want to compromise proper ventilation for cost but I can have more house by limiting both the up-front costs and life cycle costs. Energy costs and proper ventilation are more important than up-front costs.
DC
Sorry Doug,
your post first showed up as being posted by Daniel Morrison.
oh darn,darn,doh
your name is Don
John,
I'm pretty much sold on ICF. We do have the potential for tornadoes here so I want a strong home. Summer nights here are perfect. Summer night time lows average in the upper 60's to lower 70's.
DC
Dan posted the first post for me because the GBA system rejected my email address as being spam. For some reason I can now post using that same address. Name is Donald or Don but I also go by DC hence the signature.
DC
A 10-year simulation of various asphalt-shingled insulated roofs in both cold and hot climates, with a one-time rain leak introduced in the third year of the study, concluded among other things:
The least tolerant roof assembly in either climate is the unvented closed-cell polyurethane insulation roof assembly with SRAM (self-repairing adhesive membrane) applied over the sheathing. This roof assembly creates a vapor trap and is slow to dry although the SRAM is supposed to prevent leakage from wetting the sheathing. Additionally, the closed-cell polyurethane foam will not allow leakage water to filter through and can promote deterioration of the wood roof structure with no visible indication of a roof leak.
If you really must used a sealed roof, then make provisions for venting it under the sheathing. There are vent baffles made for sprayed foam applications.
In your climate, either exterior insulated concrete or Thermomass with insulation between two layers of concrete will give you much better dynamic mass benefit than ICFs.
For ventilation, you don't want exhaust-only, since you should have either neutral or slightly positive pressure during the cooling season. Supply-only, as Martin suggests, or a balanced ERV would be the best options.
Thanks Robert, You've given me a lot of think about and I appreciate the insight. As I explained in my response your answer on the out-gassing Q&A, one of the reasons I am considering sealed attic construction is that I've come across some research which suggests sealed attics are stronger in tornado and hurricane situations than vented attics. I also want a space for my duct work that doesn't get to 120+ degrees in the summer and in west Texas it's always nice to have insulated storage space that is relatively dust free.
I've got a few more questions (not necessarily for Robert only, others feel free to chime in):
1) Would open cell foam change anything in the simulation scenario you described?
2) Is SRAM necessary in this climate?
3) What do you think of a radiant barrier that is designed to be installed on top of the roof deck beneath the shingles? Here is a link to the product I found: http://www.truprotect.com/ Supposedly, this extends the shingle life and helps protect the shingles from hail damage (another west Texas weather consideration).
4) Sounds like the supply ventilation option you guys have described is the best bang for the buck but I am curious about your suggestions of ERV instead of HRV. I've only talked to one local person about this and I was told I did not need ERV because I would not need the moisture control. Any thoughts about that?
FYI, If I go with concrete construction I'm afraid my only local option is ICF.
Great site. Glad I found it.
DC
DC,
You are in climate zone 4 similar to Albuquerque but a bit more moisture & wind. ICF is a great wall assembly for your climate, but it usually costs 15%-20% more not the typical 5% advertised, and use an experienced crew to install it.
If you want a non-ventilated attic I would use open cell, just make sure you do a good job installing your roof. I don’t know about TrueProtect but I know you can use TechShield AND spray under with open cell (per Peter Pfeifer) or if you use metal roofing (better for hail storms) you can install it with vented with nailers. Some of the metal roofs come with radiant barrier coatings. Also, I would design the roof with at least 24” overhangs, 36" would be better.
On ventilation I would use a balanced system with 95% furnace & 16SEER AC unit, both with variable speed capabilities and fresh air makeup through a mechanized damper ran by an IAQ thermostat. I would spend NO money on HRV or ERVs, you DO NOT NEED IT IN YOUR CLIMATE, specially with a balanced hvac system.
On the fireplace, I would recommend a sealed combustion unit. It has it own fresh air make up.
Be careful on the stove you choose, you must know the required hood size and provide makeup air equal to the hood and wired simultaneously. Do not use the hvac system for this makeup air.
Lastly, 90% of Green Building is in the design!!! Make sure is well done upfront so you don’t spend unnecessary money later on.
Armando,
Thanks much for your response. 15 to 20% more for ICF would be disappointing but I would probably find a way to do it anyway.
What is the thought on plywood for roof decking as oppose to OSB? I am told plywood breathes better.
If I go with a sealed attic and foam insulation under the deck, would I need to do anything special to prevent moisture in the house from migrating to the attic?
Between you, Martin and others who have responded, I am comfortable with no longer considering HRV or ERV. That saves some cash I can use for other things.
Thanks for the recommendation on the fireplace.
My range and oven with both be electric. My current home has a recirculating vent hood above the range and it's okay most of the time but there are some times I would prefer an external vent. My new home will be sealed much tighter so, as you suggest, I need to make sure I do this right.
Here is a good article on Plwd vs OSB, http://www.pathnet.org/sp.asp?id=17336 plus there many more on the web. Plwd wears better with moisture, but you don't have that much moisture in LBB. Normally, most builders use OSB.
To prevent moisture to enter the attic, make sure you have good & proper installation of your roofing and roofing paper, install all kinds of flashing and roof edge, seal all penetrations and seal, seal, seal. You can Google for roofing details on the web, and make sure they are on the drawings!!!
With an electric vent you don't need a big hood, but it needs to be vented to the outside and accounted for on the air makeup of the house if less than 400 cfm. Bigger than 400 cfm you need a separate system as I said before.
If you really want a good guide, you can go to the book store at http://www.eeba.org and buy a Builders Guide for Hot-Dry/Mixed-Dry Climate ($45.00) that would give all the information and details you would need for your home plus more.
I should have said: With an electric stove....
DC,
You asked about open-cell foam. Here's the rest of the conclusions of the ASTM study. Vented roofs always outperform unvented roofs (as other studies have also demonstrated). However, roofing material and color and orientation have a larger effect on preventing attic overheating.
Sandwich radiant "insulation" is snake-oil. A radiant barrier under the roof sheathing or roof framing is helpful in reducing attic temperatures, though it will increase roofing temperatures. But its value will be negated if spray foam is applied directly to it without a vent channel or air space.
SRAM is not necessary in any climate except perhaps a hurricane zone where a non-detachable secondary roof weather barrier may occasionally be necessary.
The ThermoMass system is available by truck shipment anywhere, will outperform ICFs, is less expensive, requires no additional finish, and can be installed by any competent concrete foundation contractor. http://www.thermomass.com/company/directory.htm (no, I'm not connected with them, but very impressed with what they offer and will likely use it on my current project).
Vented roof systems with permeable insulation in cold climates are durable because they include redundancies that can tolerate incidental moisture and provide visual indicators of roof leakage.
In cold heating climates, vented roof assemblies clearly outperform unvented assemblies with respect to drying potential. Multiple types of vented assemblies in this climate are considered durable, including permeable insulations such as glass-fiber batt and open-cell polyurethane insulation.
In hot humid climates, the most durable roof assembly is the vented open-cell polyurethane with either felt or SRAM applied over the sheathing due to decreased drying time of the interior gypsum wallboard when compared to the unvented roof assembly. However the addition of venting in a hot, humid climate further complicates the location and construction of the air barriers and vapor retarders. Functionally, the foam needs to be applied to a second layer of sheathing, which also serves as a redundant air barrier. A layer of blocking would form a vented cavity between two layers of sheathing. The first layer of sheathing could also be changed to glass-fiber faced gypsum sheathing, to improve the fire resistance of the assembly.
Unvented permeable shingled roofs are also a viable consideration in hot humid climates, although it would be slightly less durable. The unvented roofs would be less expensive and simpler to construct that the vented roofs. These unvented roofs result in increased drying time of the inboard gypsum wallboard. The insulation must remain permeable to avoid a vapor trap.
Armando - Thanks for the input. I will definitely purchase the book you recommended.
Robert - Thanks to you as well. I will look into the Thermomass system but I believe mine would be the first such house constructed in this area and I'm not sure I want to be first. I've learned there are several builders in the area who build with ICF. However, I am intrigued that Thermomass claims their system is tornado resistant which is one of my primary criteria. Do you know of any sources of info that compares the tornado resistance of ICFs with the Thermomass system? Any idea how the cost compares?
Also, I appreciate that you are not in favor of sealed attic construction but the study you cited compared hot/humid and cold climates, neither of which is my climate. What do you think about this article by Joseph Lstiburek - http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation?full_view=1
His conclusion is: "Both vented and unvented attic/roof designs can be used in all hygro-thermal regions. However, the designs need to be climate sensitive."
DC
DC,
The study I quoted limited itself to two US climate extremes, but the findings apply as well to any climate in between. That was one of only two studies I'm aware of (the other is the MEWS wall study) that incorporated the kind of leak that every house experiences at some time. Both studies demonstrated that a thermal envelope that limits drying potential by use use of low-permeance foams can result in long-term conditions conducive to structural rot.
I'm aware of everything that Lstiburek publishes, and I disagree with his insistence that a wall or roof assembly can be made "perfect" by the addition of exterior foam layers. Such an envelope remains perfect only as long as there are never any imperfections, which are an inevitable part of the life history of every building.
I haven't done a face-to-face cost comparison, but ThermoMass should be less expensive than ICF, though they both require a concrete pumper, and any foundation contractor can handle it. There is more concrete (a full 8") and the inner and outer sections are bound by fiberglass connectors that have the same tensile strength as ½" rebar but without the thermal conductivity. The system has been in use for 30 years in large-scale commercial and industrial facilities and is moving now more into the residential market. It's far superior to ICF in terms of thermal mass advantage, considerably stronger and does not require a fire barrier.
Here's what the building science experts have to say about roof venting.
Bill Rose, ASHRAE, Illinois Building Research Council:
Airtight ceilings are a more reliable way to ensure a dry attic than venting, but in practice most houses fall into a middle ground where venting balances moisture input.
Don Fugler, research director, Canada Mortgage and Housing Corporation
Houses with ceilings tight enough to meet Canada’s strict R-2000 standard “probably could get by without roof venting.” But most Canadian houses, even new ones, don’t have such perfect ceilings.
Ned Nisson, author, The Superinsulated Home Book, editor, Energy Design Update:
"I hesitate recommending [unvented roofs] to clients unless I’m absolutely assured of impeccable quality control. In my opinion, roof ventilation is cheap insurance against expensive callback problems. Why gamble?"
Wayne Tobiasson, research engineer U.S. Army Corps of Engineers, Cold Regions Research Center:
“Because of the monumental problem of ice damming, there is no question in my mind that the ventilated roof is an order of magnitude better in cold regions.”
Anton TenWolde & William B. Rose, members, ASHRAE:
We recommend venting of attics in cold and mixed climates. However, if there are strong reasons why effective attic vents are undesirable, unvented attics can perform well in cold and mixed climates if measures are taken to control indoor humidity, to minimize heat sources in the attic, and to minimize air leakage into the attic from below, or vice versa. The necessity and effectiveness of vents in cathedral ceilings in cold and mixed climates is still a contested issue. Unvented cathedral ceilings can perform satisfactorily in cold and mixed climates if the cavity is properly insulated, measures are taken to control indoor humidity and minimize air leakage into the roof cavity, and a vapor retarder is installed in the ceiling.
Paul Fisette, director of Building Materials Technology and Management at the University of Massachusetts, Amherst:
"There are many ways to treat the symptoms [of ice damming], but proper air sealing, insulation, and attic venting are the best ways to eliminate the problem."
Joe Lstiburek, Building Science Corporation:
"Vented attic/roof designs have the advantage of a long, proven historical track-record. However, they work best with airtight ceiling/attic interfaces and where ductwork and air handlers are not located within attic spaces. The increase in the use of complex roof shapes and cathedral ceilings has resulted in problems with vented roofs."
"In extreme snow regions it is necessary to add a vented air space between the roof cladding (shingles) and the rigid insulation to avoid ice damming. The vented air space is needed to flush heat away trapped by the insulating value of relatively thick snow."
Armando,
I am familiar with the components of the central-fan-integrated supply ventilation system....
In fact your description sounds almost like my supply system.
What is the exhaust component of the balanced ventilation system that you are talking about?
Is it merely bathroom exhaust fans controlled by the IAQ thermostat "controller"?
Can you give a product example of the IAQ thermostat?
Robert,
For reference on the Thermomass foundation system:
Prices just came in for 232 linear feet of 8' high foundation, one being the Thermomass (but 2" rigid) system;
10" Thermomass (2" rigid) - $41,400
10" ICF (6" concrete) - $43,600
8" Foundation w/ 2" rigid and 2 x 6" Denspack cellulose interior wall - $42,272
8" Foundation w/ 4" rigid and furring interior wall - $39,470
All concrete prices include Polywall waterproofing, while the ICF wall includes rubber membrane and stucco system above grade. Thermomass has no interior finish, ICF and remaining walls have 1/2" drywall finish. I'm working on estimate on the Thermomass with 4" rigid, but imagine it wall surpass the cost of ICFs.
Jon,
That's useful information. What size foundation is that and where are you building? Depending on climate, you will get significantly more dynamic mass benefit from either the externally insulated wall or the Thermomass than with either ICF or interior insulation.
ICFs generally offer no more than 1.5 DBMS, while exterior or CIC walls can offer 2-3 DBMS, in which case more than 2" of internal or exterior foam may not be necessary.
Nevermind my question on size of foundation. I missed that you included linear feet.
Robert,
The foundation is in Colrain MA and is not rectangular - thats why I gave linear feet, and does include a 26' x 32' attached garage. I understand about the mass, but am hesitant on insulating outside of the foundation in this very wooded site with high watertable. That's why I'm getting local concrete contractors to investigate the Thermomass system. I am also doing a framed double wall system and would have to cantilever the floor deck over the foundation for exterior wall insulation. If the building were a more regular shape, I'd consider the Larson Truss system.
A good part of the basement will be finished, which makes me lean toward finishing the interior as per the Cold Climate Concrete Foundation Strategies and Details.
We're getting tight on budget so the 4" rigid options not going to work.
Jon - thanks for posting the price comparison. Based on that, my finish preferences and the fact that I don't think anyone in my area is currently using Thermomass, I'd be surprised if I could beat the price of ICF. Deed restrictions will require that I use brick and/or stucco for the exterior finish of either choice and my personal preference is to have drywall finish on the interior. I don't like the idea of unfinished concrete for an interior wall so I need to consider that when pricing Thermomass.
You can control balanced ventilation measuring exhaust fans (hallway, bath, kitchen, dryer) but always maintaining positive pressure. I've starting to specify makeup air in kitchens where hoods are over 400 cfm, that has become an interesting deal since is in the 2009 IRC and the kitchen stove & hood manufacturers don't have solutions for residential makeup air. Honeywell Visionpro IAQ Thermostat (±$150) does a grit job of a programmable TStat + Air Cycler all in one. It controls temperature, humidification, dehumidification, and ventilation all in one place.
Depending upon how tightly sealed a home or commercial building is, and the presence of odor and contaminant producing components such as some types of paneling, carpets, and kitchen construction; an ERV (humid climates) or an HRV (dry climates) allows for fresh air to be introduced without the related energy losses of unbalanced HVAC supply/exhaust systems.
Especially, in commercial applications such as conference rooms, casinos, or nightclubs, an ERV or HRV can be critical components for energy savings and customer comfort.
Anonymous (I don't understand why people won't put their name to their statements),
Let's be accurate. HRVs and ERVs reduce, but do not eliminate ventilation heat losses (or gains) and they have associated energy costs. Their annual efficiencies are often considerably less than rated.
I have to weigh in on the ventilation component. To date, many H/ERVs have been less than efficient. That has changed. The second generation of H/ERVs have efficiencies of 90% recovery, and can provide a COP of better than 4.0. The use of an H/ERV in this project, with 170 days per year of extreme heating or cooling, would provide a significant reduction in energy use. You can also eliminate the bath fans, the exhaust hood in the kitchen (use a re-circ hood for grease/oil and exhaust the kitchen with return ducts to the H/ERV), and significantly improve indoor air quality. This has been done in Europe for years, and this technology is available here.
It makes little sense to go to the trouble and expense of building a really tight envelope, only to use a fan to bring in hot or cold air much of the year. You might as well just punch holes in the walls. Not to mention that with a really tight house, air does not move in or out of the space easily.
Barry,
I doubt that any ERV/HRV will offer a net annual efficiency close to 90%. That may be the optimum operating efficiency, but actual annual efficiencies are typically quite different.
And I have to disagree that "It makes little sense to go to the trouble and expense of building a really tight envelope, only to use a fan to bring in hot or cold air much of the year. You might as well just punch holes in the walls."
Holes in the walls is exactly what every tight building must have, whether they're ducted through a recovery ventilator, the HVAC system or simple make-up air inlets. What I think makes no sense is building a hermetically-sealed house and then relying on artificial respiration to keep the indoor environment livable. That's like deliberately putting yourself on a heart/lung machine instead of breathing on your own.
Houses should be tight enough to eliminate most of the stack effect air exchange and prevent infiltration/exfiltration moisture damage. But they should be designed to maintain a living environment regardless of whether the grid is up or down.
I would never cook on an electric range (no good chef would think of it and it takes the "chi" or life force out of food) and I would never use anything but an externally-vented hood with a gas range. No recirculating hood is going to eliminate all the air-borne grease and some of it will find its way into a centrally-ducted HRV if there is a kitchen intake. And no HRV, even with fan boost, as powerful enough to move the moisture from a hot shower out of a bathroom quickly enough to avoid damage.
Also, in a heating-dominated climate, maintaining negative indoor pressure prevents the warm air exfiltration that causes condensation in the thermal envelope and consequent moisture damage. So a simple, exhaust-only ventilation system based on efficient bath fans (with a kitchen hood for source removal) and passive make-up air inlets is often the best approach to a durable and livable house.
Barry - thanks for the advice. I'd be interested to learn what "significant reduction" would be. Electricity costs are low here and I need to consider the payback as well. So far I've not been able to find a local HVAC person who can spell HRV let alone design and install a system. Right now, I'm favoring Armando's approach.
I am not expert, but have been looking at cost effective options for roofing/attic in hot climate. I would consider using above sheathing ventilation and a metal roof. The vented airspace between the metal roof and sheathing provides significant reduction in heat transfer to the attic space. You could still foam insulate under the sheathing, but may not need to. Unfortunately, wind loading may be an issue.
Taking the topic back to ventilation -
I think that a home in Lubbock could benefit from "night purging". HVAC engineers also call it a type of "Free Cooling". It can prevent the need for AC, thereby saving a lot of kwhs and money.
In Denver it's usually done with a whole-house attic fan. I prefer using a swamp cooler. Both methods need a good way to prevent air leakage during the heating season.