Hello. I am a residential builder, and this will be the first time that I have been asked to bid a green project. The owner is looking to build to "Near passive standards". The house will be a single plane shed roof design, with the high side of the roof facing south. We will be building in Zone 5a. Southern Michigan, only 20 miles north of Ohio.  I think that the architect has done a good job in the details, but I am curious if there is a more cost effective way to achieve the same R-value. I should mention that the owner is not only looking for energy savings, but is also sensitive to the impact of materials on the environment. I would assume that XPS insulation would not be acceptable.

Wall details:
T&G siding 0.75"
Air space with pressure treated battens 1.5"
Siga Majvest membrane
Studs @ 24" o/c with cellulose dense pack 9.25"
Osb .5"
Service space framing with Roxul insulation 3.5"
Gypsum board - painted .5"

Total wall thickness 16"

Roof Details:
Standing seam metal roofing
Roof underlayment
Plywood sheathing .625"
1x2 battens .5"
Siga Majcoat membrane
MDF - Agepan DWD diffusion board 2"
16" TJI floor joists @ 24" o/c with cellulose dense pack 16"
OSB .75"
Service space framing with Roxul insulation 3.5"
Gypsum board - painted .5"

Total roof thickness 23 7/7"

Floor Details:
Floor finish .75"
EPS insulation / subfloor 1.125"
Concrete slab 4"
Polyethylene
EPS ridgid insulation 6"

In zone 5 frost protection for footers is 42". Would it make sense to use ICFs for the foundation with slab on grade? Or would pouring a frost footing with 2" foam on exterior side be sufficient? I am always inclined  to put radiant tubing in a concrete floor, if for no other reason than to future-proof the design. In this instance it could be used to move heat in the slab from solar gain to other areas of the house. Would that be worth the cost? I'm sure that I will have more questions as we go along. Thanks in advance for your responses.

HI MattM
There are a variety of different ways to achieve that R value. One nice feature of Passive House is that builder's and architects have the freedom to choose design, insulation methods and insulation types. Having said that, though, and since you are new to Passive House, I'd suggest going with the Architect's plans. Roux and cellulose are becoming the preferred insulation due to their low global warming potential (GWP) and the fact that they are both post-industrial recycled materials, so you'd be hard pressed to achieve the same thing without increasing the GWP.

The use of ICF's would be up to you, the architect and the homeowner. The slab, however it's built must be free of thermal bridges, meaning that the slab cannot contact the ground, or contact other concrete which is in contact with the ground or ambient (outside) air. Radiant heat tubing would be an unnecessary expense. The floor will be no cooler than room temperatures if you are close to PH levels.

There are many issues other than insulation levels that make PH unique and unusual, but the fact is, it works. And it will become, as it has in other parts of the world, the standard way to build, so read up on it and take care in building the house so you can look forward to building many many more like it.

Radiant slab might be justified if the homeowner's lifestyle demands a floor warmer than the inside air temperature. For example, bare feet on a slab at ambient temperature will still feel cold since the slab will conduct heat away quite fast. In other words, the slab can be the same temperature as everything else in the house and still feel cold to the touch. Floor coverings with some R-value would be a less costly way to accomplish comfort.

Thanks for the responses. I will press on with the current design.

Alton - in a Passive House, if the floor temperature is higher than the air temperature, it will warm the air to the same temperature. If you want to heat the house that way, that's fine except that the air and floors will be the same temperature, not warmer. Another issue is that radiant heat is slow to change temperature, so if the floor is 70o by 8 AM, and you have southern glass, by 10 Am the house could be overheating. These are amazingly efficient homes that are simple in many respects but built differently and have different requirements.

I read that radiant in-floor heating works decent in homes that are NOT energy efficient but not so well in energy efficient homes, especially a passive house. Like Bob mentioned, overheating the home with radiant flooring is really common in such homes. Especially homes that don't have any slab foam will see vast temperature swings. The lag in thermal recovery time causes great temperature swings throughout the day. There are better and more energy efficient ways to heat a home.

Just to be clear on the radiant, I wasn't planning on actually heating the water with a secondary source. I was thinking that the heat from the area of the slab that is heated from the sun could be distributed to other areas of the house. Wouldn't that help balance the temperatures through out the house, and help with the issue of overheating in the rooms with southern exposure? It might even be possible to heat the garage. I really don't know, I'm certainly new to this type of building.

Yes, you can certainly move heat from passive solar heated areas of a building to other areas of the building needing more heat using a hydronic radiant system. You can even move/reject excess passive solar heat outside the building using a hydronic radiant system. If you do not know what you are doing when designing a passive solar heated building, you are at great risk of over-heating the building and even a hydronic radiant system may not save you.

It is true that the slab will only be a couple degrees higher than the desired indoor air temperature in a well-insulated and well-sealed energy efficient building. So the slab won’t feel nearly as comfortable as a slab in a building that is not well-insulated or not well-sealed where the slab temperature may be as much as 20 degrees higher in temperature than the desired indoor air temperature. However, the slab will certainly feel more comfortable than it would if it was NOT heated! Furthermore, being able to adequately heat the building with this lower temperature slab also results in lower slab heat loss, lower required hydronic radiant supply temperatures and lower associated operational cost than would otherwise be the case.

Integrating passive solar heating and hydronic radiant system heating in energy efficient buildings is starting to become more common now that there is a growing community of designers/builders that actually know how to successfully do this. If you live in passive solar friendly location, like sunny areas in buildings, and are pouring a slab anyway, this integrated system approach will by far provide the lowest acquisition cost and lowest operational cost heating system solution. Perhaps check out the design information and associated software on our website.

Slabs are slow to respond - but they won't cause much over/under shoot in a low load house. For example, a slab that never needs to exceed 75F will never heat a room above 75F (but other things might).

...and still feel cold to the touch. Floor coverings with some R-value would be a less costly way to accomplish comfort.

It would be interesting to quantify this effect. For example, what temperature does a radiant tile floor need to be to feel as comfortable to bare feet as a wood or carpeted floor.

MattM: consider a frost protected shallow foundation instead of a conventional footing.

I don’t think a tile floor or hardwood floor can ever feel as comfortable as a carpet or rug. Both tile and hardwood floors are quite hard and not very comfortable on bare feet even when warm. This is why people normally put area carpets/rugs in the sitting areas of hydronic radiant heated floors.

Also please keep in mind that the designer needs to factor in the lost hydronic radiant emitter area that is blocked by area carpets/rugs in these zones when designing the system. The designer also needs to factor in the increased R-value of hardwood and the associated reduced heat transfer capability of the hardwood-covered hydronic radiant emitter when designing the system too.

As the building becomes increasingly more well-insulated and more well-sealed, the required cooling/heating system authority and response time becomes increasingly less important. If taken to an extreme, the large internal thermal mass can in fact maintain a constant temperature without any cooling/heating system whatsoever…think about a cave!

Posted By sailawayrb on 15 May 2015 09:59 AM
Yes, you can certainly move heat from passive solar heated areas of a building to other areas of the building needing more heat using a hydronic radiant system. You can even move/reject excess passive solar heat outside the building using a hydronic radiant system. If you do not know what you are doing when designing a passive solar heated building, you are at great risk of over-heating the building and even a hydronic radiant system may not save you.

It is true that the slab will only be a couple degrees higher than the desired indoor air temperature in a well-insulated and well-sealed energy efficient building. So the slab won’t feel nearly as comfortable as a slab in a building that is not well-insulated or not well-sealed where the slab temperature may be as much as 20 degrees higher in temperature than the desired indoor air temperature. However, the slab will certainly feel more comfortable than it would if it was NOT heated! Furthermore, being able to adequately heat the building with this lower temperature slab also results in lower slab heat loss, lower required hydronic radiant supply temperatures and lower associated operational cost than would otherwise be the case.

Integrating passive solar heating and hydronic radiant system heating in energy efficient buildings is starting to become more common now that there is a growing community of designers/builders that actually know how to successfully do this. If you live in passive solar friendly location, like sunny areas in buildings, and are pouring a slab anyway, this integrated system approach will by far provide the lowest acquisition cost and lowest operational cost heating system solution. Perhaps check out the design information and associated software on our website.

Thanks, that confirms what I was thinking.

Posted By jonr on 15 May 2015 12:21 PM

MattM: consider a frost protected shallow foundation instead of a conventional footing.

 Thanks for that advice. I will definitely look into it.

What's the slope on that roof that faces North? And what color will the roof be?

Passive houses require very small inputs to maintain temperature when heating or cooling. Typical house max loads are usually described as in the ball park of 1500 watts or the amount a hair dryer produces. The internal temperatures even out naturally. Remember that forced controlled air exchange is required in PH homes because they are required to be below .6 ACH 50 for air tightness . This will continuously move the air through out the home balancing out the temperatures. When the Air handling systems are off during certain construction periods the heat from appliances and solar create noticeable temperature difference in areas. These houses have to have the ventilation running all the time when occupied. It typically uses very little energy. The lungs of the home should be well invested if you want the building to operate properly.

In floor radiant tubing in such homes can be used for temperature homogenization if specific areas have higher thermal loading do to solar exposure. It also facilitates Solar thermal collection and distribution of low grade heat sources. Such systems are particularly useful in areas that have any severe climate time frames. Having large internal thermal masses in PH homes is always a benefit because it reduces temperature fluctuations and improves temperature stabilization between solar absorption time frames.

Cellulose in a properly built open diffusion wall system is a great alternative. You need to be careful in humidity management of the walls. The cellulose area in the wall should not have moisture content exceeding 20%. at any time to avoid any chance of mold and or mildew. Great care should be taken to have a pressure boundary that is a completely sealed layer continuous without penetrations. Making the central cavity wider and only insulating it is less expensive than filling multiple cavities. It also facilitates easier production and substantial labor savings. It does lead to a slightly thicker wall.

In high humidity areas it is problematic to keep the humidity of the heavily insulated cellulose walls in the safe moisture content realm without forced drying.

ICFs are faster, lower labor content, provide easier sealing, are structurally capable of handling any earthquake, tornado or hurricane hazards, bullet proof, wrought, mildew and mold proof. They present very low long term risk for the healthy building and have extremely long usable building life cycle. Using super insulated ICFs give you a 20 inch thick wall that is R-50 actual R- value with a captive thermal buffer zone in the contained concrete. Standard ICFs thermal average climate extreme over several days to a weak. Super insulated ICFs thermal average more than a month of the local weather affects. Obviously these numbers are very climate specific. The point being there are many advantages in designing high performance buildings with ICFs.

We moved our passive net zero homes to building with Super Insulated ICF walls with cellulose filled trusses and a foam cap eliminating the internal condensation issues. We also monitor the moisture of the cellulose and have the facility to ventilate and dry if ever necessary with additional HRV or ERV capacity designed in. This approach in our opinion is far less sensitive to potential failures. The down side of the ICF approach is that it is higher in embodied energy than the cellulose approach. We have made this our standard base offering.

@ gokite - The entire roof is a 1:12 shed roof (one single plane) sloping upward from the north. I am not sure of the color, but I will make suggestions to the owner if lighter/darker colors are preferable. We are considering changing the roof to a gable design so as to be able to install PV shingles on the south side.

Have you found any PV shingles that are not cost prohibitive? I have always liked the idea, but everytime I have looked they have been priced out of economical reason making them very boutique. PV installers as a norm charge higher cost than reasonable. PV installations are not difficult. We have always ended up installing with our own labor at a substantial advantage in cost. I have had an installer actually tell me they charge high rates because the consumer is seldom well informed.

Posted By zehboss on 25 May 2015 01:56 PM
Have you found any PV shingles that are not cost prohibitive? I have always liked the idea, but everytime I have looked they have been priced out of economical reason making them very boutique. PV installers as a norm charge higher cost than reasonable. PV installations are not difficult. We have always ended up installing with our own labor at a substantial advantage in cost. I have had an installer actually tell me they charge high rates because the consumer is seldom well informed.

A person with moderate to advanced building skills be able to install a PV solar ground mounted system?

The wiring up portion is something that I would leave to the electrician but the is the rest something a DIYer can do?