We have built several houses to the NBC which require a blower door test. In Maine the requirement is for a very tight house. We build monolithic slab houses on stem walls. The stem walls tend to run around 3 feet high and usually set directly on ledge. The houses are very tight. So tight that in the summer the AC will hardly run at all. The stem wall and slab are insulated with 3" of foam so what we end up with is a foundation that sits down into the earth which is much cooler than the outside air temperature. So the problem is that in the summer when the outside humidity can be very high - even above 90% - that the air in the home can have very high humidity too, especially if they leave the windows open. If many windows are open the humidity inside the house is the same as outside the house. Now we have very high humidity levels in the house along with a floor that is much cooler than the outside air. This can produce a lot of condensation, especially over the stem walls. Closing the windows and running the AC does not solve the problem because the AC hardly runs. I have also constructed in Florida where high humidity is year around. I have an ICF house here with the same conditions. In this house I installed a whole house dehumidifier in the AC system (separate unit). What i have observed is that when it is below 85 the AC does not run an hour a day but the dehumidifier runs perhaps 3 hours a day (to keep humidity below 55%). So it appears that with the way houses are constructed today that attention needs to be paid to this problem. Can anyone tell me what good practice would be for these kinds of conditions, especially in the north where the problem may only exists for a couple of months for the year.
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Humidity control in tight houses
- Thread starter bcoop
- Start date
Beniah Naylor
SAWHORSE
I assume you have mechanical ventilation coming from the outside, like through a energy recovery ventilation system? If you are bringing in fresh air, I would think the windows would not need to be opened?
Whole house dehumidifier is what I would do, I have one at my house and I love it. I try to keep it down around 40% humidity.
Whole house dehumidifier is what I would do, I have one at my house and I love it. I try to keep it down around 40% humidity.
mtlogcabin
SAWHORSE
Maybe they do not "need" to open. But my wife will open the windows in the dead of winter just to "air" it out and be able to smell the fresh air. If there is a breeze outside she wants it in the house also.
Yes - this is what happens. They want the fresh air from the windows.
Yes - there is an energy recovery system, but i wonder how much humidity it actually takes out. It is also pumping in air that is warm and then needs to be cooled. Not sure that this is all completely thought through.
Genduct
Registered User
Problem is, that you are in an area with the lowest Cooling Degree Days that you have super insulated and sealed.
Basic Residential AC equipment is designed for 75% Sensible Heat and 25% Latent (Humidity). In commercial applications with High latent Loads we use "Deep Coils" and Chilled water to "Wring Out the Moisture"
Your only option might be to control the load with a Humidistat that controls equipment with a Latent Load Discriminator that has an extra coil that will take the Condenser heat to reheat the discharge air you just Cooled off so you could get the humidity out. The job of the unit then is to control Humidity without having Cold and Clammy.
Hope that is clear, You are going to need a well informed HVAC Contractor who understands the problem
Basic Residential AC equipment is designed for 75% Sensible Heat and 25% Latent (Humidity). In commercial applications with High latent Loads we use "Deep Coils" and Chilled water to "Wring Out the Moisture"
Your only option might be to control the load with a Humidistat that controls equipment with a Latent Load Discriminator that has an extra coil that will take the Condenser heat to reheat the discharge air you just Cooled off so you could get the humidity out. The job of the unit then is to control Humidity without having Cold and Clammy.
Hope that is clear, You are going to need a well informed HVAC Contractor who understands the problem
Gregg Harris
Saw Horse
- Joined
- Jan 17, 2012
- Messages
- 934
If your run times are that low the probability that your HVAC contractor has not done there job.Have a true manual J run on the house and determine what your latent and sensible heat loads are. You will probably find your system is over sized causing short run times leading to excess humidity.
Genduct
Registered User
Gregg, your point is generally valid EXCEPT our friends in Maine probably have 600-700 Cooling Degree days' In my Philadelphia area it is probably 1300 In Miami it is 3600
So the challenge is really tough for areas like Maine or Minn. or the Dakota's Well you get the point
I paid for a professional engineer to design the system and did not rely on the contractor or their supplier. The house is about 1700 square foot with large windows. They system in place for the whole house is only 2.5 tons as i recall. The ICF house has a lot of mass (8" concrete walls) and 3"foam both sides, and then cement board siding. It takes all day for the heat to get through the walls. The system runs at the end of the day for a little while when this happens. By that time the sun has set and it is already cooling outside.
Inspector Gadget
Registered User
This sounds to me like your slab is not properly insulated.
Also, is the HRV running? The HRV should remove the humidity from the air in any event.
tmurray
SAWHORSE
Not necessarily. Relative humidity is the percentage of water the air can hold at a given temperature. The warmer the temperature, the more moisture the air can hold.
In summertime, bringing in hot outside air at a high relative humidity would actually increase the relative humidity of the house when it is cooled to room temperature.
This is a major deficiency in the home building industry in that on the mechanical side, there is little knowledge on how to deal with very tight and well insulated buildings.
The slab has 3" foam under it per code. the stem walls have 3" foam each side. I think the stem wall is cooler in the summer because it is so insulated and is down in the earth which is lot cooler than the air, especially in Maine where we are right on ledge (rock) which stays cool under ground.
The purpose of this thread is to discuss these very issues. I do not think that good practice is well understood by many including me. I am describing conditions that are going to be more and more common as people build these houses to the new codes. Some of the replies here are very technical and probably are going to be what is required. I think we have forgotten that the house is for people to live in and the people are not there for the creation of a house that saves all possible energy. It appears that first we spend a lot of money eliminating most of the heat loss and the air flow. Then it is not healthy to live in so we add in replacement air and an HRV. That needs a drain so the plumber got to be paid for that. The drain might dry out so now need a little pump to put water back in the drain. That will break soon and most people do not even know they have one or what an HRV is or even if it is running or not. Now we have this condensation problem so have to have a whole house dehumidifier or other equipment as described above. That will get the moisture out but can't open the windows. That means at times it will be too hot. So now i need a cooling system of some sort as well which most people in Maine do not have because the season is short. Most people means most people in Maine who do not make a lot of money. It appears that on the alter of energy savings we have created homes that are a lot more technical to build and less affordable for most people. Perhaps we went a little to far with energy savings at all costs. The result is that about half the population cannot afford these kinds of houses. The national building code requires this for new construction. In Maine the NBC is mandatory for towns that have more that 5,000 people. Because people cannot afford new construction the working class is getting forced out of towns that have the NBC. They now commute to work from surrounding more rural areas and may not be able to live in the towns they were born in. This needs to be figured out and the industry needs to be trained on good practice. The condensation issue is going to create mold conditions for a couple of months a year. What is going to happen is that people are going to start moving back into trailers again. That does not seem like a good solution. Sorry about the rant. I do not think a can build a new home today to the NBC that someone with a modest income can afford. That is unfortunate.
Genduct
Registered User
A couple of thoughts to share
1 the source of the Humidity is typically cooking, bathing, breathing Outside air (more on that in 2) AND Probably that slab in contact with Moist Earth.
2 A HRV even with a desiccant wheel Is NOT a Dehumidifier that is reacting to Inside temp and dew point. DO consider the OA hass moisture in it How Much, I don't know
3 This is not necessarily going to become a Mold Problem IF BLDG Science principles were followed and the Moisture has a Path to Move through and Out and Not Get captured In the wall or on the surfaces of the drywall or sheathing
PS I very much agree and think it very helpful for you to point out that We Don't Do Best Practices . Tough sometimes to remeber we are the Reviewer NOT the design Professional
IMHO, Mike
Inspector Gadget
Registered User
A lot to unpack here, but let's try.
Before I start, I will point out that I'm building a home intended to be pretty blasted efficient, and I did a crapton of research before building it - and did so before I became a building official.
The situation you describe above is the result of poor planning, nothing else.
Properly ventilated houses will not have condensation problems.
I am still troubled by this declaration you have concrete in contact with earth. When I questioned if it was properly insulated, you said 3" of foam. Either it is insulated, or it isn't. If it isn't, then that's a big part of the problem.
It sounds to me like you might have too many south-facing windows that are exposed to full sun. Again, proper design approaches will eliminate these issues: in my house, I have two massive 4x7 goth-arch windows, but they are under a 3' soffit overhang that screens direct sun from the peak summer months.
Again, don't blame efficiency for what bad design has caused. The general principle for passivhaus construction is to have window area that is no more than 10 per cent of the floor area, largely because windows are woefully inefficient. It's also worth noting that fewer windows = less window condensation.
The above sentence speaks to the shortsightedness that many people have. "I can't afford to build an energy efficient home, but I can't afford to heat and cool a home that isn't energy efficient!"
In beaurocratspeak, many fail to balance capital costs with operating costs. When I volunteered for Habitat for Humanity, which aims to pair modest-income rent-trapped families with affordable homes, energy efficiency was always calculated into the equation. Without going into a bunch of details that I can't remember after 15-plus-years, the proven approach was to spend more up front to reap long-term savings.
I will use an HRV as an example. When I did the research a couple of years ago, the cheapest suitable HRV for my place was $1000. It drew 144 watts per hour. That means its annual cost for electrical service was (3.45 kw/day* 14c/kw=48 cents/day=$176/year. I purchased a unit whose power draw is 18 watts/hour, but it cost $2,000. (.43 kw/day * 14c/kw=6 cents/day=$22/year.) After six and a half years, the "more expensive" HRV has paid for itself.
The best cost/benefit expenditure in home building is insulation. I don't have to explain that to this audience.
Here's the capital/operating cost argument:
Based on conversations, the occupants of a 16x60 trailer go through five to six cords of wood a year. The average price for a cord of hardwood is $325-plus. That means, say, $1,787 in heating costs per winter.
I've spent multiple winters in a well-insulated 500 square-foot cabin (R40 ceilings, R22 walls R26 floors). Wood heat as the only heat source. I've only ever exceeded one cord of wood ONE winter.
The cost of upgrading a trailer's R12 walls to R17 (40 sheets of XPS foam at $30.20) =$1,208 plus tax. Hell, throw in some added vinyl siding and bits for $500. Based on crude calculations/extrapolations, going from R12 to R17 walls will about halve the heating costs in the typical trailer, or in other words (~2000+893*2=$3785; 2x1625=3575) the insulation upgrade will almost pay for itself in two winters, and thereafter be LESS costly.
The same metric applies to all structures. Energy efficiency very often pays for itself in reduced operating costs. Time after time, you will find that in most cases, a cost-benefit analysis will show it is better to have a higher capital cost to invest in energy efficiency, which will recoup that extra investment through the reward of a lower operating cost.
Or, in other words, NOT going to energy efficiency means about "half the population cannot afford" energy inefficient homes.
I am going to go political here: one of the greatest opponents to energy efficiency are the American energy companies who stand to lose a lot of money if people invest - that's a very intentional word - in energy efficiency.
Before I start, I will point out that I'm building a home intended to be pretty blasted efficient, and I did a crapton of research before building it - and did so before I became a building official.
The common approach is to run a small hose to a drain. Proper planning will ensure the p-trap is suitably charged. Often/usually, the HRV hose is looped to create a trap as well. In my house, I ran the HRV hose to the washing machine platform, which is designed to handle failures of both the washing machine and hot water tank. The p-trap for this assembly is charged by the washing machine. Won't dry out unless I don't do laundry for a few months.
The situation you describe above is the result of poor planning, nothing else.
Properly ventilated houses will not have condensation problems.
I am still troubled by this declaration you have concrete in contact with earth. When I questioned if it was properly insulated, you said 3" of foam. Either it is insulated, or it isn't. If it isn't, then that's a big part of the problem.
You have a concrete slab. That should act as a thermal flywheel and keep things cool. I have 5" of concrete on 4" XPS as a slab. When the workers were heating the place last year, the house was comfortable for several days after the fire went out, without extra energy inputs. Likewise, in summer, when it's scorching hot outside, the house is several degrees cooler. That's an established principle in efficient house construction, and manages to keep the house cool in summer without any energy inputs.
It sounds to me like you might have too many south-facing windows that are exposed to full sun. Again, proper design approaches will eliminate these issues: in my house, I have two massive 4x7 goth-arch windows, but they are under a 3' soffit overhang that screens direct sun from the peak summer months.
Again, don't blame efficiency for what bad design has caused. The general principle for passivhaus construction is to have window area that is no more than 10 per cent of the floor area, largely because windows are woefully inefficient. It's also worth noting that fewer windows = less window condensation.
The above sentence speaks to the shortsightedness that many people have. "I can't afford to build an energy efficient home, but I can't afford to heat and cool a home that isn't energy efficient!"
In beaurocratspeak, many fail to balance capital costs with operating costs. When I volunteered for Habitat for Humanity, which aims to pair modest-income rent-trapped families with affordable homes, energy efficiency was always calculated into the equation. Without going into a bunch of details that I can't remember after 15-plus-years, the proven approach was to spend more up front to reap long-term savings.
I will use an HRV as an example. When I did the research a couple of years ago, the cheapest suitable HRV for my place was $1000. It drew 144 watts per hour. That means its annual cost for electrical service was (3.45 kw/day* 14c/kw=48 cents/day=$176/year. I purchased a unit whose power draw is 18 watts/hour, but it cost $2,000. (.43 kw/day * 14c/kw=6 cents/day=$22/year.) After six and a half years, the "more expensive" HRV has paid for itself.
The best cost/benefit expenditure in home building is insulation. I don't have to explain that to this audience.
I live north of Maine. I see homes that are tiny little things, or old-style trailers because people are broke. They're often well less than 1,000 square feet, and often heated with wood. Cords and cords and cords of wood.
Here's the capital/operating cost argument:
Based on conversations, the occupants of a 16x60 trailer go through five to six cords of wood a year. The average price for a cord of hardwood is $325-plus. That means, say, $1,787 in heating costs per winter.
I've spent multiple winters in a well-insulated 500 square-foot cabin (R40 ceilings, R22 walls R26 floors). Wood heat as the only heat source. I've only ever exceeded one cord of wood ONE winter.
The cost of upgrading a trailer's R12 walls to R17 (40 sheets of XPS foam at $30.20) =$1,208 plus tax. Hell, throw in some added vinyl siding and bits for $500. Based on crude calculations/extrapolations, going from R12 to R17 walls will about halve the heating costs in the typical trailer, or in other words (~2000+893*2=$3785; 2x1625=3575) the insulation upgrade will almost pay for itself in two winters, and thereafter be LESS costly.
The same metric applies to all structures. Energy efficiency very often pays for itself in reduced operating costs. Time after time, you will find that in most cases, a cost-benefit analysis will show it is better to have a higher capital cost to invest in energy efficiency, which will recoup that extra investment through the reward of a lower operating cost.
Or, in other words, NOT going to energy efficiency means about "half the population cannot afford" energy inefficient homes.
I am going to go political here: one of the greatest opponents to energy efficiency are the American energy companies who stand to lose a lot of money if people invest - that's a very intentional word - in energy efficiency.
tmurray
SAWHORSE
Largely, the difference in economics between paying more in insulation and paying more in energy is that the added costs for insulation becomes an investment in the equity of the home until the costs are paid back at which time the reduced energy bills become direct savings to the occupant. In contrast paying higher energy bills is simply money that once spent is gone and has no further utility. Furthermore, it places people at greater risk to extreme temperature events and fuel price variability.
You are correct that policy makers must be careful in balancing the approach to low energy use buildings. Too slow and you risk people not being able to afford to live in the houses. Too fast and you risk people not being able to afford to buy/rent the houses.
You are correct that policy makers must be careful in balancing the approach to low energy use buildings. Too slow and you risk people not being able to afford to live in the houses. Too fast and you risk people not being able to afford to buy/rent the houses.
Thank you for taking the time to talk through these issues. It is through this kind of conversation that I learn. My slab and stem walls are not in contact with the earth. the 3" foam underneath and on the sides is the vapor barrier and this is allowed by code as i understand it. The concept that i am not communicating well is that concrete that is in the ground (stem walls) (not touching the ground but in the ground) is not going to warm up quickly. It is probably at the ground temperature and not the air temperature. This is not a design issue it is a reality. This is not about how much glass you have or whether the ERV is working. Warm humid air is going to condensate when it comes in contact with a cooler surface (this is all defined and understood and I know there are variables). The outside walls of the house set on these stem walls. The plate of the wall and the first six inches of the wall from the floor are cooler than the rest of the walls. You can measure it. This is where the condensation is. On ceramic tile floors some times you can wipe the moisture right off of the floor with your hand. The warm moist air comes in the house through normal traffic and air flow. All someone has to do is to open a window and the air changes in the house fairly quickly and then this condition occurs.
The argument whether to insulate or not is not the relevant discussion. The discussion is one about diminishing returns. Good design isn't good because a particular added inch of insulation ALWAYS makes sense. Good design captures most of the bell curve not all of the bell curve. You might spend as much preventing the last 5% of energy lost as you did on the first 75%. A good design is one which is practical and affordable. What good is a design that most people cannot afford? The design that will do the most good for the environment is the one that makes sense for most of the population not just some of the population that can afford to do anything. A good design is one that does not require a lot of maintenance. We had a high efficiency furnace that required a service call every single year. My old one only had a couple over its whole life. Was this a good design? What I am not seeing right now is a "Good design" for average houses that most people live in. Like for example a 1200 sq foot ranch. I do not think that all the accessory equipment required and described above constitutes a "good design". I think current practice was pretty effective until we over tightened up the houses and got into all the air flow issues. I do think that your examples of insulation are good ones but insulation is not really where the problems come in - it is about the air exchange.
Thank you all for weighing in on this. My guess is that we are going to see the building codes changing to address some of these issues over the next few years as this gets sorted out.
The argument whether to insulate or not is not the relevant discussion. The discussion is one about diminishing returns. Good design isn't good because a particular added inch of insulation ALWAYS makes sense. Good design captures most of the bell curve not all of the bell curve. You might spend as much preventing the last 5% of energy lost as you did on the first 75%. A good design is one which is practical and affordable. What good is a design that most people cannot afford? The design that will do the most good for the environment is the one that makes sense for most of the population not just some of the population that can afford to do anything. A good design is one that does not require a lot of maintenance. We had a high efficiency furnace that required a service call every single year. My old one only had a couple over its whole life. Was this a good design? What I am not seeing right now is a "Good design" for average houses that most people live in. Like for example a 1200 sq foot ranch. I do not think that all the accessory equipment required and described above constitutes a "good design". I think current practice was pretty effective until we over tightened up the houses and got into all the air flow issues. I do think that your examples of insulation are good ones but insulation is not really where the problems come in - it is about the air exchange.
Thank you all for weighing in on this. My guess is that we are going to see the building codes changing to address some of these issues over the next few years as this gets sorted out.
tmurray
SAWHORSE
Some of this is also a balance between the costs of maintenance and energy as well. A furnace may well need a service call every year, but in comparison to a lower efficiency furnace, it could still result in cost savings.
One of my primary concerns around the increased requirements around housing is that we are outpacing the occupant's ability to comprehend how the systems need to be operated in their home. Many people I run into don't understand how to operate their HRV, let alone more sophisticated equipment. Maybe we need to move towards BIM in homes.
On of my college instructors had a saying; when it comes to HVAC, the more you can take out of the control of the occupant, the better your system will run.
One of my primary concerns around the increased requirements around housing is that we are outpacing the occupant's ability to comprehend how the systems need to be operated in their home. Many people I run into don't understand how to operate their HRV, let alone more sophisticated equipment. Maybe we need to move towards BIM in homes.
On of my college instructors had a saying; when it comes to HVAC, the more you can take out of the control of the occupant, the better your system will run.
Phil C
Registered User
My wife does the same thing. I come out of my home office after detailing a windows with all of the nuances of todays building science to discover my wife has the window open a bit, freezing outside, heat on.
Phil C
Registered User
Same here, however, not unlike most parts of the country, the air smells so nice here. It would be a shame to not allow that in. I put my foot down if the heat is on, however.
Phil C
Registered User
Houses certainly are getting more complicated, I agree. And the problem above is a tricky one. But I don't know...feels like the cost of lumber that skyrocketed and the insufficient housing inventory pale in comparison to costs associated with more stringent energy code requirements. National association of home builders has the Estimated Costs of the 2012 IRC Code Changes to be about $4,900-$13,800 for more stringent energy efficiency and mechanical requirements. Not to mention you get this money back in energy savings in 8-10 years on average. I know grants exist of compensate for these upfront costs. Although, it would be great if we had more of them.
