Last week we talked about Minnesota code requirements for insulating foundations in new construction. This week we’ll hit existing homes.
There are a lot of potential issues with insulating existing foundations. What kind of foundation? Block, poured, stone or even brick? Do they have waterproofing on the exterior? Are they structurally sound? There is not a one insulating technique that fits all when dealing with existing foundations and it’s important the right choices are made.
Heat loss through foundation walls will the keep soils along the foundation from freezing in a cold or very cold climate. Frozen soils can push against a foundation, causing structural damage. I have seen several bank owned homes that were winterized and allowed to freeze during the past financial crisis. Some of these homes had foundations that had been frost damaged. Bulges and cracks in the concrete that were not present prior to freezing. All the damage I have seen has been in concrete block or CMU constructed foundations, most were not constructed correctly. Insulating existing foundations can have similar effects, reducing the heat loss to the soils.
What are the options? I’ll start with the most expensive, but depending on how the foundation is constructed, may not be the best.
Insulating from the footing to the mudsill on the exterior is the most expensive (and most work) option. This will require excavating the foundation to the footing. Be sure water proofing is present on the foundation, if it’s not, add it. I would then follow the Minnesota (or your local) codes for new construction, at least R-15 total interior and exterior with no less than R-10 on the exterior in Minnesota. If there is no footing drainage, this is a good time to add it. The foundation needs to be sound, the soil will probably freeze along the foundation, problems worst than elevated heating costs and cool basements can result.
The next option would require some excavation, but only a foot or two below existing grade. Adding insulation four feet horizontally away from the foundation and then insulating the foundation wall above. This technique is similar to a frost protected shallow foundation, slightly raising the soil temperatures below the insulation to prevent the soils from freezing. (Frost protected shallow foundations will be the topic of a future blog posting.) The horizontal insulation should have a slight pitch to allow any moisture to move away from the foundation. In my opinion, this is the best option if you are worried about foundation damage due to freezing soil movement. One additional benefit is the soils below the horizontal “wing” insulation should stay dryer, which may help improve issues with a damp basement.
The last exterior option, and the least expensive, is to cover the exposed above grade foundation to around a foot below grade. The exposed above grade portion of a foundation has the highest heat loss due to the greatest temperature differential. Expanded Polystyrene (XPS), Extruded Polystyrene (EPS), and rigid Rockwool all work well. I would try to achieve at least R-10. It would be best to flash the insulation to the water resistive barrier of the above grade framing. The insulation needs to be covered to protect from degradation due to UV and atmospheric pollutants. (and the weed whip). This above grade insulation protection is a requirement of all three exterior insulation options.
One additional option which I have seen used once is below grade exterior spray foam. The foundation was entirely exposed and sprayed on the exterior with closed cell spray foam. An additional coating material was spray applied to protect the foam. I do not know a lot about this process, may be a topic for a future blog posting.
If your trying to insulate the foundation of an existing home, interior insulation is also an option, but to tell the truth, it scares me. There are a few potential problems. The first, freezing moisture that is present within the concrete. Concrete could care less if it’s wet or dry, as long as wet concrete doesn’t freeze. Lowering the temperature of the foundation by insulating the interior can cause the moisture trapped within the concrete to freeze and damage the foundation walls. For this reason, exterior insulation is always the first choice.
The second I already discussed, lowering the heat loss through a foundation wall which reduces the temperature of the soil to the point where it freezes. Soil movement in cold climates can damage foundations.
Lastly, when you bury one side of concrete with soil that is wet, (and there is no or a poor water proofing strategy on this exterior foundation wall), the moisture will move through the concrete to a dryer area, which will be the basement. The second law of thermodynamics wet to dry! Capillary and hydraulic pressures may also move moisture through concrete. Minnesota requires extensive air sealing measures to be completed when insulating foundation walls. Are we trapping that moisture behind a product that may have a low perm rating? Codes in my area require the use of low perm vapor retarders when insulating with either fiberglass or open cell spray foam. The retarders often end up being polyethylene sheeting. Not a good idea. Never insulate a basement on the interior that has moisture issues.
For these reasons, I will only use two methods to insulate the interior of a basement.
Rigid sheet insulation, either EPS or XPS, sealed on all four sides, and less than R-11 is the first method. Neither of these insulations are supposed to promote mold growth, but they do create a toxin when burned. Almost all rigid foams will need to be covered with a fire resistant covering, such as drywall. Install the foam against the concrete foundation wall and build a wall against the foam.
The second option is closed cell spray foam, which I try to keep under two inches. Again, I don’t want to reduce heat loss to the soil to the point of freeze concerns, a couple inches is plenty. The foam needs to be covered. Build a wall one inch from the concrete, then add the spray foam and cover with an approved material, such as drywall. Neither closed cell spray foam or rigid insulation are supposed to absorb moisture, though I have heard instances where XPS has.
This covers the basic strategies for masonry foundation insulation. Two foundations that would be very difficult to insulate are a rubble or stone foundation or a structural brick (which I have never seen). There are other foundations that are required to be insulated. Pressure treated wood and frost protected shallow foundations are a couple that come to mind. As I said earlier, there is not one strategy that will apply to all foundation insulation. Be sure to follow any of your local codes.
Question – building kitchen addition with poured foundation 42″ deep with 24″ crawl space. I plan on heating crawl space. 1) Where should insulation be placed – on the outside of foundstion wall or inside wall? 2) should outside of wall be sealed with waterproof sealer befor placing insulation? 3) Should floor of crawl space be -dirt – crushed limestone gravel or 2-3″ of concrete? 4) Should I lay (how many inches thick) ridgid foam insulation under crawl space floor before putting down crawl space floor? 5) Should I put plastic sheet vapor barior on insides of foundation wall? And under or on top of crawl space floor? 6) Should I insulate in-between the crawl spaces overhead floor joists? With what type of insulation? 7) Should I put wing board rigid foam insulation out from outside of foundation wall? How far? How thick? How deep? 8) Any other considerations you think I should know?
Hi Russ,
I can answer a few of your questions, but others I will need a little more information, mainly where you are building the project. Based on the depth of your foundation, I would guess you are in climate zone 6. Here’s an article on climate zones so you know which one you are located. https://www.northernbuilt.pro/why-knowing-your-climate-zone-is-important/ If you are in a northern climate, with little risk of termites, then the insulation can be installed outside the foundation. If you are in an area with termite risk, then the insulation should go interior. Here’s a map showing areas of risk. https://basc.pnnl.gov/images/termite-infestation-probability-map-adapted-2021-international-residential-code-irc-figure
The outside of the foundation wall will need to be sealed. What you choose for a sealant (it could be a roll-on product, spray-on product or self-adhered membrane) will need to be installed before any exterior insulation is installed. The only foundation that would change this order would be an insulated concrete form (ICF), in which case the foundation sealing product will almost always be a membrane installed over the foam insulation. I recommend extending whatever sealant used to the connection of the foundation of the existing structure.
The floor of the crawlspace can be finished with any number of products, but it needs to have some sort of vapor retarder placed either on the dirt floor, or over any stone. If you are using insulation over the crawlspace floor, the vapor retarder typically goes on top of the insulation. A 6-mil polyethylene sheet product will work well if you are not using the crawlspace for storage or have to enter often to service any heating/cooling equipment. If the space is often visited, I would install a 10-mil or heavier polyethylene sheet or pour a concrete floor, that product is much more durable and won’t be easily damage by foot traffic.
As far as how thick any insulation installed on the concrete floor, this will depend on your climate. If you are in a heating dominated climate, R-10 is enough. A mixed heating and cooling climate could use R-5.
The interior walls of the crawlspace do not need a vapor retarder covering them, but you should seal the floor vapor retarder to the walls, up a few inches. Hopefully you’ve done a good job a waterproofing the exterior of the foundation and the risk of an interior water leak is low. There is a risk of moisture moving into a concrete wall by way of capillary action. This is the result of the concrete footing of the foundation being in contact with the damp ground. Moisture is able to move through the concrete footing, into the foundation wall and then dry to the interior of the crawlspace. This risk can be eliminated by painting the top of the footing with a heavy layer of any type of paint before the foundation walls are set. This is called a capillary break.
If you are insulating the foundation walls, you don’t insulate the addition framed floor. This is called a conditioned crawlspace. If you do not insulate the foundation walls, then you will want to insulate the framed floor and vent the crawlspace to the outside. This is called a vented crawlspace. I don’t recommend choosing the vented crawlspace option, go with the conditioned crawlspace.
The only time you are required to place “wing” or horizontal insulation around a foundation is if you are choosing to use a frost protected shallow foundation system. If you are pouring a foundation 42″ deep, you are constructing a traditional foundation below frost depth. No wing insulation is needed.
A couple other considerations, when building a conditioned crawlspace, the space needs to be conditioned. You will need to have some sort of heat supplied to the space. You will want to maintain at least 60°F or the floors in the addition will be cold to walk on. Another consideration is dehumidification. You do not want the space to become too humid. I would install a dehumidifier than does not need to be emptied, one than can drain to the home’s plumbing system, or one with a pump that pumps the water to the plumbing system.
Hope this information helps.
Randy