3 Reasons to Rethink Crawlspaces in Passive House Design

On NESEA’S BuildingEnergy course Passive House Design course discussion board…

Takeaways:

Typically, Passive Houses do not feature crawlspaces (a foundation that is five feet or less, and fully enclosed — or, not vented to outside) because:

• If the thermal boundary is at the home’s foundation walls, there is the added cost of the foundation wall and slab insulation.
• At a minimum TFA (Treated Floor Area) is habitable space, with daylight, finished surfaces, electricity, and sufficient height clearance. Passive Houses use the Annual Space Heating/Cooling and Primary energy per square foot or meter of TFA as the key metrics for comfort and environmental impact. Basically, the more TFA you have for the cost of the building, the better. Crawlspaces do not provide acceptable TFA.
• Instead, walkout basements can be included in a thermal enclosure. Though the above grade portion requires insulation for PH (Passive House), it can also have windows for natural light, and even solar gains, so the basement can be quality living space. This is a TFA (Treated Floor Area) return on the investment.

DISCUSSION BETWEEN STUDENT 1 AND MIKE DUCLOS

STUDENT 1:

What are your thoughts on crawlspaces (for a Passive House)?

MIKE DUCLOS:

Your climate zone is much different than up here, so you will need to interpret this answer. Also in my area there is a strong bias to having full basements to store ‘stuff.’ We tend to use full basements in renovations to expand living space without the expense of expanding the footprint. In the European homes I’ve been in, they tend to have much less stuff, what they have tends to be of higher quality and durability.

We often have walkout basements here, they are included in the thermal enclosure, the above grade portion requires much insulation for PH (Passive House), but the above grade portion can have windows for natural light, and even solar gains, so the basement can be quality living space, so there is a TFA (Treated Floor Area) return on the investment. For the sake of this reply, I’d like to define a crawlspace as a foundation that is five feet or less, and is fully enclosed – unvented to outside. I’ve never heard of a PH with a crawlspace.

If the thermal boundary is at the foundation walls there is the cost of the foundation wall and slab insulation and probably no TFA to compensate for that expense. TFA is ‘quality living space’ that’s why it excludes stairs, interior walls, etc. PH sees TFA as the quality for which we construct buildings. In the few retrofits I have seen with crawlspaces, the approach has been to bring them into the thermal enclosure. So a considerable expense for insulation and no TFA reward. If you need space for stuff, can a garage with an attic suffice?

PH is a large investment for TFA, so we need to be careful with defining the building enclosure to maximize the TFA and minimize the cost to construct. I think it would be possible to exclude the crawlspace from the thermal enclosure, mechanicals in a PH take up little space, perhaps that would be a solution in your area?

Why do people have crawlspaces, and what do they do with the space?

STUDENT 1:

When we moved here from Colorado, where we had a full basement, I asked that very question “where do we store our stuff?”

Crawlspaces are considered an upgrade in quality over a plain slab and therefore used in most upscale homes. People associate walking over a “soft” wood-framed floor with quality instead over a “hard” concrete slab included in most low-priced production homes. Basements are never used except when a walkout condition exists on a sloping site. So when I had a sloping lot, I would always include a walkout basement with plenty of windows in the wood-framed walls for daylight to reduce the dark, dank feeling of a basement.

The two LEED projects I did had enclosed, conditioned crawls with the mechanicals located in that space. I often suggest this strategy to clients, but they question the extra expense.

I try to explain that they should think of a crawlspace as a “short man’s basement”. So I have a dilemma down here with the use of crawls and PH.

I am curious with your suggestion of not including the crawlspace in the thermal enclosure. What do you think of the idea of fully enclosing and insulating the crawl (but not conditioning), and pouring an insulated 2″ concrete “rat” slab? The average crawlspace height is around 36″-42″. I have not progressed through the course enough to understand the tradeoff of PH extra expense and TFA. Could the crawl be considered storage or the same as a mechanical room and thereby not be included in the calculations for TFA, etc? This concept would satisfy the quality concerns of the upper market client and their storage needs.

MIKE DUCLOS: 

I can certainly emphasize, our attic, basement and house are full of ‘stuff’ we plead an exemption because we have ‘stuff’ from both our parent’s houses, but we’re as guilty as many.

One of the best lectures I’ve ever attended was by Bill Rose at UIL Champagne/Urbana, one of the many things he showed us was a map of all 50 states, he said ‘ in these states a slab on grade is the mark of a quality builder, and in these other states a basement is the mark of a quality builder.’ Local customs vary, I’ve not heard of crawlspaces being favored in our area. Up here we’re basements with a few slab on grade, with crawls typically only where granite ledge is near the surface, I’m in one of those on Cape Ann right now.

I don’t relate to the ‘soft’ wood floor argument. I’ve spent over two months in European homes with concrete floors, I’ve no issue with them, I appreciate the temperature stability and ‘feel of quality.’

I like your solution to sloped land: ‘quality living space’ or at least the potential for it – in basements – if you’re to have a basement, make it useful, ideally enjoyable, habitable space, so maybe the whole house can have a smaller footprint, perhaps be lower cost due to the greater utility of the footprint, etc. IMHO, customs can mean we continue thinking in one way, if you can show someone a different way that works better, gives us more for less – more durability, living space, quality of life for less money, environmental impact, etc. – let’s consider that carefully and not dismiss it out of hand because it is local custom.

Treated Floor Area – TFA – I’ve interpreted this as the PH way of saying the reason we construct buildings is for quality living space. Let’s distill that into space with the highest utility and quality (e.g. You can’t live inside thick walls, you don’t enjoy being in a dark basement without natural light, etc.) so they set a high bar for quality living space, then say ‘look this is really important, we are going to use a lot of resources to make this building, so let’s really do it right, let’s make a building for 200, 300, 500 years, let’s take the long view – as opposed to what I think of as Walmart houses – designed and constructed to yield as many square feet of space for the lowest cost as the top priority, with operating expenses & environmental impact, comfort and indoor air quality a lower priority, and which can be considered ‘disposable’ e.g. we’ll only think about a 3-5 year ROI for energy related improvements, and we’ll tear it down after 50-100 years. So this means that a house that would last for 100 or years will spend how many years of its life on the wrong side of the annual energy cost versus investment curve?

So PH uses the Annual Space Heating/Cooling and Primary energy per square foot or meter of TFA as the key metrics for comfort (heating / cooling) and operating energy (Primary energy) related environmental impact.

The more TFA you have for the cost of the building, the better, since TFA is in the denominator of the Annual Primary energy use / TFA equation = KWHR/SM/Yr, KBTU/SF/Yr

So concrete that goes into a space (say a 42″ high crawlspace) yields no quality living space, and so no TFA in the denominator, so the Primary Energy per TFA number is not reduced by incurring the cost of that crawlspace. It’s the viewpoint I understand from working with the PH standard, it is certainly not the only way to view / value buildings, but I see it has merit to consider and weigh, considering the alternatives.

Make sense?

You could do what you suggest: insulate under the rat slab, air seal the crawlspace so it is part of the air pressure and thermal boundaries, and indirectly condition the crawlspace (heating it via the uninsulated floor above) and that’s often what happens up here when we find the rare crawlspace to fix. I didn’t intend to indicate a crawl should not be included in the thermal enclosure, but rather why would you invest the resources in a crawlspace? Maybe you have reasons I don’t see?

STUDENT 1:

The best way to explain the use of a crawlspace is that people perceive that it elevates them up off of damp dirt. I am trying to come up with an efficient concept that will satisfy a client’s quality expectations. I understand the tradeoffs of TFA. I am trying to integrate the PH concept to the mainstream in this area.

I guess one way would be to show an effective “break” between the ground and the bottom of slab and thereby providing a “warm” floor. I am trying to come up with a concept to use in my capstone project.

MIKE DUCLOS:

Steve, you might try telling them there would be more insulation under the slab than in the walls of the home they live in now, and the earth is warmer than the air on the other side of their walls.

STUDENT 1:

I’ll look at replacing crawlspace storage with an attic area.

I have been using SPFcc sprayed under the roof sheathing to locate the thermal layer and thereby semi-condition the attic area. This will locate the insulation out of the way to use structurally designed “floor” area for storage. I assume this space would not be designated as TFA in the PH calculations?

MIKE DUCLOS:

I think the space you describe is definitely not ‘quality living space’ so no TFA there. At a minimum TFA is habitable space, with daylight, finished surfaces, electricity, and sufficient height clearance. Consider TFA a desirable place to live and you’ll be in the right ballpark.

 

About Mike Duclos – Principal, DEAP Energy Group, LLC

Mike Duclos is a principal and founder of The DEAP Energy Group, LLC, a consultancy providing a wide variety of Deep Energy Retrofit, Zero Net Energy and Passive House related consulting services. Mike was an energy consultant on the Transformations, Inc. Zero Energy Challenge entry, and has worked on a variety of Zero Net Energy, DER and Passive House projects, including two National Grid DER projects which qualified for the ACI Thousand Homes Challenge, Option B, and a feasibility study of a retrofit to the Passive House new home performance standard. Mike is a HERS Rater with Mass. New Homes With ENERGY STAR program, a Building Science Certified Infrared Thermographer, a Certified Passive House Consultant who certified the second Passive House in Massachusetts, holds a BS in Electrical Engineering from UMass Lowell, and two patents.

Additional Resources: 

• FREE LECTURE: Passive vs. Conventional Floor Planning
• FREE LECTURE: All About Windows
• EXPERT COURSE: Passive House Design
• FREE TOOL: Passive House Sample Design