Upgrading From Uninsulated Compacted Aggregate Slab
Hello,
I live in the foothills of Colorado outside of Boulder. It is a concrete block house with brick flooring. Under the floor is sand and then compacted decomposed granite, which is basically the dirt around here. The main body of the house is built into a hill with a large bank of south facing windows. This portion of the house also has a wood stove, and I am able to heat 1000+ square feet with passive solar (the bricks soak up a lot of sun) and a wood stove. However, the master bedroom is not so lucky. It stick out from the main body of the house, is a half step down into the earth, and does not get a lot of sun. The people that built this house had great spirit and some solid ideas, but the bedroom is “warm” to us in the winter if it’s 60 degrees.
I’ve insulated the ceiling better, which bumped up our average temp in the winter by about 5 degrees, and I’m going to insulate the roof exterior when I replace the roofing system down the road. However, I’d like to upgrade the floor in the master for heating and radon reasons. I was planning on removing the brick, digging down, recompacting the earth if I need to, laying a vapor barrier, two 2 inch layers of rigid foam (crossways), and then two perpendicular layers of subfloor OSB (screwed and glued). I could then lay vinyl or hardwood on top of this system.
My questions would then be:
a) Does this sound like an OK plan? Vague, I know, but I appreciate feedback. I’m looking to avoid pouring a slab, for what I think are some legitimate reasons, but if avoiding that is silly, let me know!
b) Is radiant flooring a good option here, and if so, where in this layer cake of flooring would I put it? I assume in between the foam and OSB, but I could be missing something crucial there.
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Replies
Marco,
You would want 2x4 flat 'sleepers' at 16" beneath a single 3/4" subfloor if you need space for radiant tubing. That, or an 1-1/2" self-leveling gypsum underlayment.
Foam needs to be underneath the vapor barrier.
Jason,
Isn't it only important to have the foam under the VB when you are pouring a slab above?
If the foam is installed airtight and never sees a plumbing leak, sure, it would probably stay dry enough to hold its R value. But why chance it with a wrong-side vapor barrier, given the opportunity otherwise? We know the foam can stay drained and dry underneath.
I guess that I should add some specifics to the first question. If I should be doing an analysis before I decide on a heating method, that would be good to know. I don't know the quality of the envelope. I assume that it's poor, but, for example, last night it must have been near zero degrees. We went to bed with the room being 60 and woke up with it being 54. That doesn't strike me as being a terribly leaky envelope. However, it could just be the thermal mass of the brick and CMUs. Any insight is appreciated. I'm just a homeowner, but I'm also an engineer, so don't be afraid to throw anything complicated my way
You should absolutely do an analysis before deciding on a heating method. It's a code requirement now. Even if you don't you're shooting in the dark if you don't. If you Google "Manual J" (or search this site) you can learn about it and get software to help with the calculations.
The methodology of the Manual J is to assume that the heat flow through a wall is directly proportional to the temperature difference between the two sides, and inversely proportion to the insulation level. This is high school physics. In the Manual J process you assume an indoor temperature, and an outdoor temperature, estimate the area and insulation level of the walls, and from those calculate how much heat will be required to maintain the indoor temperature at that outdoor temperature.
You're going to run into a few hitches with that method. First, the Manual J assumes that the interior temperature is constant for the entire building. so there is no flow through the interior walls and they can be ignored. In your situation that's not the case.
The other issue is the floor. Earth that is covered by a house will stay close to the year-round average temperature year-round. In most of the US that's in the 50's. When the room is above ground temperature heat will flow into the ground, and when it is below ground temperature heat will flow out of the ground. To heat the room you have to account for that flow.
Once you've done the Manual J you can see where the heat flow are, which tells you the most effective spots to add insulation, and gives you an idea of what the impact will be. The general rule is add insulation to the most effective places first, and then add until you hit the point of diminishing returns.
It seems like the main goal is to increase comfort versus increasing energy savings, correct? Making the master suite its own zone with R10 under a heated floor will get you comfort. Just insulating and changing the floor finish from brick to wood will increase mean radiant temperature and improve comfort.
Analysis of existing homes is a rabbit hole. An energy audit can show you how deep it goes. I tend to focus on the weakest links. Air leaks, rim joists, ceiling air barrier, and old crappy mechanicals.
MarcoB,
You mentioned radon. Have you tested to measure the indoor levels? If radon is a known issue in your area, I would check that first. You may have to seal the floor and mitigate to mitigate your health risk.
A blower door test also would tell you how leaky the home is and give you a baseline for making decisions on where to improve. It may be you also need mechanical ventilation.