Mudroom – inside or outside conditioned space?
Hi folks,
I’m looking for perspective on potentially including an unheated room as part of the “conditioned space” of our home.
We have a 1950’s single-story house in northern New Hampshire (climate zone 6). It’s a rectangle of just under 1000 square feet with a full, unfinished basement. There is a mudroom (120sqft) which was built as an addition, with a concrete slab and concrete steps into the basement, closed off from the main house with an exterior door. The mudroom has no heat, but has insulated walls and ceiling. A blower door using the exterior door of the mudroom achieved 1 ACH.
The energy efficiency contractor we had here was pushing us to treat the mudroom as outside the conditioned space, but we’re interested in bringing it fully inside (opening the door to the main house) to bring it above freezing in winter.
I’d appreciate any perspectives on including or excluding this kind of space from the conditioned envelope of the house.
Thanks,
Doug
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Replies
Doug, what reasons did the contractor give you for treating it as outside the thermal envelope? Probably severe thermal bridging through the concrete?
I've done unheated mudrooms for clients at their request, and in my 1830 Maine farmhouse we have a mudroom that is unheated. I think a heated mudroom would be much nicer, especially if it's the main entry to the house.
Michael,
The contractor seemed to think that because it currently is unheated it should be treated as "outside." He did not reference any particular reasons.
I'm leaning toward since it's currently insulated, it can be "inside."
Any building/performance concerns with the middle ground of leaving the door to the living space open, but not adding a heater? (we're comfortable with that space just being a little cooler if that's the biggest repercussion)
If the exterior walls & ceiling of the mudroom are insulated, it's technically conditioned space, even if the partition walls/floor to the rest of the house are insulated, and even if that room isn't actively heated.
The difference in thermal performance of the house by leaving the partition wall door open will be pretty small, but would increase slightly. There is probably more surface area to partition wall & floor assemblies than the surface area of the exterior walls. Heat loss is a function of area x U-factor x temperature difference. With the partition door closed the difference in temperature between the mudroom and outdoors might be 30F (10F outside, 40F inside), compared to 50F (10F outside, 60F inside) when the door is open. So the heat loss of that section of wall will go up by about 50/30 = 1.67x, but only for that section of wall, not the whole house. If you want to calculate it, figure out the U-factors and surface areas of the relevant exterior surfaces , then take some temperature measurements.
See:
https://www.greenbuildingadvisor.com/article/the-fundamentals-of-series-and-parallel-heat-flow
https://www.greenbuildingadvisor.com/article/how-to-perform-a-heat-loss-calculation-part-1
I would want my mudroom to be conditioned space. In the summer it’s not a big deal, but in the winter that “mudroom” is going to be a “snow and ice” room. If you take off your boots in there and they stay frozen with ice clods on the bottom, that’s not going to enhance the comfort of your home.
Even if conditioning the mudroom means a little more energy use, I think it would be worth it for the more comfortable living conditions it would offer. I suppose you could do something of a hybrid with the mudroom on a separate heating zone, maybe with a small wall-mounted heater, but that would probably be too small of a space for a mini split or other more efficient heating option.
Bill
You are correct the portion of the room not occupied by the stairs is an unfinished, uninsulated slab.
Why can't you just add a section of baseboard off your heating system? Or some type of hxchg off your wood stove to supplement the entire house.
Thanks for the replies folks. I'm still trying to determine how to address the thermal bridging through the concrete that Michael identified. We've been leaving the door to the mudroom open and it seems noticeably colder in the house, and the surface of the concrete is quite cold.
So, how do we mitigate heat loss from the uninsulated and exposed concrete foundation of the mudroom and steps to the basement?
I’m concerned that insulating the exterior wouldn’t be effective, unless we did the entire house, because the main house foundation is going to be insulated on the inside, and the adjoining concrete would create a thermal bridge. But let me know if you think otherwise.
I’m tempted to go with insulating the interior with EPS under plywood as per https://www.greenbuildingadvisor.com/article/installing-rigid-foam-concrete-slab. But if we do that, how do we handle the steps? Is it possible/practical to run foam and plywood over each step? I can’t find guidance/details on this, to create both insulated and solid steps. We would have to raise the doors – but that’s fine, we’re planning to replace them and have the headroom.
Would appreciate any insights.
Thanks,
Doug
>"I’m concerned that insulating the exterior wouldn’t be effective, unless we did the entire house, because the main house foundation is going to be insulated on the inside, and the adjoining concrete would create a thermal bridge. But let me know if you think otherwise."
I think otherwise.
Heat loss is a function of temperature difference, surface area, and R-value.
The surface area/width of the thermal bridge is quite wide if the foundation is uninsulated, and the R-value is quite low. Insulating even part of the foundation wall for several feet either side of the mudroom (overlapping where it's also insulated on the interior) means the heat has to flow through a much smaller cross-section area, and for longer distance. The overall heat flow will be MUCH reduced, despite the thermal bridge.
Concrete is not a refractory metal- it has both thermal mass and R-value. Most concrete used in foundations runs R1.25-R1.5 for an 8" thick foundation wall or stem wall. While modeling 3D heat flows in concrete structures gets complicated- doubling the length of the thermal bridge doesn't automatically double the effective R value, but does make an easy to measure (if not easy to model) difference. Even an R5-ish thickness of concrete (R5 using a dumb R per foot type of model) over a much reduced cross sectional area compared to the outward facing side of the foundation makes a large difference.
IRC code min slab-edge insulation in your area (climate zone 6) is R10 down to 4 feet below grade or the stem wall footing, whichever is shallower. That would can be done with 3.5" of Type II EPS or 4" of Type VIII (<< often available for dirt- cheap as reclaimed roofing foam culled from commercial/industrial building re-roofing or demolition) EPS. The exterior EPS would have to be protected (particularly above grade) and flashing would need to be installed where the concrete transitions to framed wall, but overall it has be easier/cheaper/better than trying to insulate under stair treads etc for a much lousier net improvement.
Thank you for the detailed reply Dana! That gives me more confidence to try the exterior insulation once things thaw out this spring.
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