Should I build a basement or slab on grade?
Our buider tells us that building a full basement is much cheaper that a "slab on grade" with a four foot frost barrier at the perimeter. We don't understand how this cold be true. It would also mean that it would not make sense to use infloor heating ane geothermal heating as we had planned. Your toughts?
Robert Baker
A lot of builders recommend building a basement instead of a slab on grade mainly because of familiarity. Most don’t know how to build a slab on grade so they tend to steer you way from them simply for lack of knowledge and experience.
We have many pages on how to build a slab on grade, but this page addresses your dilemma specifically-
Slab on Grade or Foundation and Basement; Which is Best?
Part of the problem with calculating the cost difference between a basement and slab on grade is comparing them accurately. The numbers get skewed when you factor finished or unfinished basements, and what you calculate as the entire square footage of livable area. And more importantly, the sad reality is that very few basements are actually built properly and most end up moldy. You won’t get that with a slab.
The price to build a proper wall below grade is always more expensive than the cost to build above grade. It will be cheaper to build an energy efficient and healthy two story house above grade than the same quality house with one level below grade.
Hi Robert. We are in the same exact situation. How are you folks proceeding. Everything I am reading indicates a slab on grade is a cheaper option. With the environmental impacts being less severe, I really want to go this route. As with your project we are banking on the slab being cheaper so that we can install radiant floor heating as well. The Geothermal company I spoke to practically laughed at the idea we are building on a slab.
Hey Matthew, Where are you building, and how big? Geothermal is a costly but noble undertaking as it extracts heat from the ground, and unless you are building a 7-10 thousand square foot house or you plan to build an extremely inefficient one, it has a terrible return on investment. It will cost you anywhere from 25-40K to cut your bills in half (this is what installers have always told me) so you're bills need to be extremely high for it to make sense. For any reasonably-sized code-built house, you could invest half that amount in extra insulation and sealing the envelope properly and probably cut your bills by 75% or more. And about slabs - they are often rejected by those that don't understand the benefits, don't know how they work, or by sales reps and contractors that can profit more by talking you out of them.
As an example - our LEED Platinum Edelweiss House is a 1450 square foot house in Quebec, Canada and is heated for under $300 a year. Yoda wouldn't live long enough to get his money out of a geothermal install on that house, we put the money into insulating instead.
Different guy but appreciate the answer. I have no plans on geothermal but maybe radiant heat and wondering if your answer would be the same. I know the key seems to be insuring the envelope is insulated right.
Thank you for the detailed response! I agree the the cost in undertaking geothermal may have a very poor ROI. With that said there are significant Grant's and tax savings that can cut the cost of geothermal by nearly 50%. We would also use the geothermal for a forced air heating and cooling system. What is the heating and cooling system installed in the Edelweiss house?
50% off for sure makes it more appealing! Energy modelling showed that about half of the heat load of the Edelweiss House is met with passive solar heating, so that's a huge part of it. We also have a radiant floor system, but that is usually kept very low simply to provide comfort. The house is extremely airtight, and with it being very open in concept and with good circulation, most of the remaining heat demand comes from the mini-split heat pump. The house is only powered with electricity and no gas at all (including heating, water heating, cooking, drying etc) So the whole house electrical load (heating, water heating, cooking, clothes drying - everything ) comes to under $600 a year. This is why we advocate for designing the home to use minimal power rather than look at supply options. So things like solar panels or geothermal aren't necessary as the demand is so low to begin with. Here is a more detailed page about the Edelweiss House heating systems.
And out of curiosity, I'd love to hear why the geothermal company laughed at a slab, those reasonings are always fascinating to me :) And where are you buildling? Climate zones are important for best design.
All this information is amazing. Thank you! I've heard a couple folks mention energy modeling. I'll definitely look into some consultants to see if we should have some energy modeling done.
So, it just seems like folks are not very familiar with building on slabs in New Enlgand and its associated benefits, namely the cost savings and environmental benefits. I am afraid that this lack in knowledge will result in higher costs negating one of the large reasons why we want to go with a slab. For example, the geothermal company quoted us 10k more to route the HVAC ductwork for the slab than if we were to go with the basement. If that's the true cost, then we might as well put the 10k into a basement. Our biggest hurdle right now is figuring out how to route the ductwork for a slab at a comparable cost to a basement build.
We are in climate zone 5.
How big is the house you plan to build Matthew? And will it be one story?
Hi Mike, our house is going to be roughly 1900 square feet with two floors. The entire first floor is open concept. After a month of working with our builder we are leaning towards building our home on a slab. Our plan is to polish and seal the slab to finish it as our primary flooring for the first floor for additional cost savings. A couple of questions for ylu Mike if you have time.
1) Do you need radiant floor heating for concrete slab floors in the north east? What ive read is that due to concrete thermal conductivity concrete will always feel cool to the touch if set at a comfortable 70 degrees. For concrete to actually feel warm you would have to set your radiant floor heating much warmer to the point that the room temp would be far to hot.
2) Basements are the normal in New England and other cold climates. Do you find that this decreases the home value and makes it harder to sell down the road?
Thanks for all your great thoughts!
Hey Matthew –
1 yes, the conductivity of concrete will always have it feeling cooler than other flooring materials like wood, marmoleum, cork, or carpet. You don’t *need* radiant heat, but personally I wouldn’t build a slab without it. Radiant floors are an efficient way to heat a home, and even healthier as you aren’t blowing dust particles around as with forced air. Here is our page on designing a home for thermal comfort, you can read a lot more there.
2 I can’t speak to the real estate market in a given location, but what I have heard from agents at least in Ontario where we are, is that you may be at a slight disadvantage as many buyers are expecting a basement, but slab on grade fans are often quicker to buy it as they know the advantages of slabs compared to basements. So that’s a tough call, a house can sit on the market for a year or sell in the first hour of an open house if you get the right buyer. I would personally only build a slab home regardless if it was for myself or if I was building on spec to sell.
And here is our page about the best non-toxic way to seal a concrete slab on grade floor in case you go that route.
Here is another page that may be of value - Legalett slab on grade kits with air heated radiant floors - They distribute across north America, we've built several of them, its the only way I'd do a slab now.
Here is a time lapse Legalett installation video from one we built recently.