How to Insulate a Basement

I am often asked about insulating basements and what the best methods are.  Basements are often the most complex areas of our homes to insulate.  There are a few things to keep in mind before you even start insulating.

First, and for most is moisture control, if there are leaks and cracks in your floor those are the first thing that need to be addressed.  Source control is the only way to stop the moisture from penetrating through the walls’ cracks.  If the wall is in good condition, the next thing to think about with regards to moisture is wicking through the wall.  Since there is no way to ever completely stop this the next best option that I have found is concrete sealer on the wall, by no means is it mandatory but it is a great way to stay a step ahead of the game when it comes to moisture mitigation in your walls.

The next area that I look at is the sill plate and band joist in the basement.  These areas contribute the second largest amount of air infiltration in our homes.  Often times all that is in the way is a fiberglass batt, which as I am sure you have heard me say before is great when used correctly but is little more than a filter when air is allowed to pass through them.  The best way I have found to address this is with foam.  A foam product will both add insulation and air sealing to the space, and at that point you are able to replace the fiberglass batting and get the maximum R-value benefit from the space.

As for the walls, you are truly able to insulate any way that you choose.  Fiberglass batts in an air sealed cavity, full cavity foam, or even wet spray cellulose.  All will not be a problem so long as the moisture source control has been taken care of.

Finally, I am often asked if the walls in a basement should be wrapped in poly to protect from mold.  For the St. Louis area’s vastly changing temperature zone there is no area that is best for applying a poly.   The rule of thumb is that the moisture barrier should always be to the conditioned side of the house.  That means in cold, heating climate zone the moisture barrier should be to the inside of the house and in a hot and humid, cooling climate zone the moisture barrier should be in the outside of the house.  In mixed climate zones like that of the St Louis area the moisture source is dependent on the season and thus there is no recommendation to install a vapor barrier. If the cavity is air sealed I see no reason to add the additional cost and time to a job to add a vapor barrier.

While we are on the subject of wrapping walls with a vapor barrier, I must point out a few things.  Exterior house wrap is semi permeable.  Meaning, it stops bulk wind and moisture.  It is not a full air block which is why it does not air seal a house.  An air sealed house does not need to be wrapped on the interior because by stopping the air completely you are also stopping the moisture.  You have created a healthier environment in your home even within your walls.  With proper moisture mitigation through vent fans and range hoods, moisture in your home should be very unlikely to reach levels where mold is commonly found.

Back to the basements… The common ways of insulating a basement are great when you have furred out walls but, if you do not plan on framing out the walls of your basement you still have the option of applying a foam board to the walls.  One note about doing this is to make sure that you have the proper foam board on you walls.  Common foam board and sprays must be covered with a fifteen minute fire barrier to meet building code.  One of the only products that can do this is Dow Thermax foam board insulation.  We recently had a great deal of success with this and as you can see from the views below.  The look is great for a storage area and the results are great.

 Uninsulated basement

Basement insulted with Dow Thermax Foam Board

We have the experience to help you make the best choice for each building. Give us a call at (314) 428-6095 and we can help you maximize the quality of the space you have in your home and save you money doing it.

1 reply on “How to Insulate a Basement”

Choosing the right basement insulation can be tricky. One local resource may be the local power or natural gas company. In many cases, they can provide general guidelines for insulation types that are best suited for your climate and location.

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