Home Energy 101 Part 2: Rim Joists


When I go into homes and do an energy assessment, I normally go straight to the basement.  Once I’m there, one of the main things I look at are the rim joists, which are located essentially where the foundation wall meets the first floor. It usually looks like an empty rectangular box. For whatever reason, most rim joists are completely exposed - no insulation and no air sealing. Occasionally I will see some fiberglass batts that have seen better days but unless the home is recently built, most of them are empty. These are significant entry points for air, moisture, and pests. Cobwebs are definite signs of air infiltration. There are some relatively simple solutions that are low-cost and medium-effort that can greatly improve this notorious draft culprit.

Solutions Big and Small


The first solution, and in my opinion the biggest bang for your buck is air sealing.  The second option is insulation.


1. Air Sealing (also known as “spray foam sealant”): Most common types of air sealing are the one step or two step expansion foam that you can find at any hardware store and some larger retailers. For rim joists, I recommend the red can of GREAT STUFF - the cost for a two pack is about $8. You can also use caulking which costs about $3 a tube.


a. Application: Simply clean the area first and spray/caulk in a picture frame design. Make sure you are using precautionary measures such as a face mask, gloves, long sleeve shirt and safety goggles. Spray foam sealant is a (insert prefered expletive here) to get off skin.


2. Insulation: There are some variations on the recommended R-value (measure of thermal resistance) for rim joists - some say 7 others 14, while I say whichever one is on sale. The higher the R-value the better, but once the rim joists are air sealed, the insulation is an added bonus. Don’t get me wrong, insulation does slow the penetration of heat and cold and creates a more comfortable home (especially if you have wood or tile floors above the basement), but in terms of the cost-energy assessment after R-10 it doesn’t make a substantive difference.


The two main types of insulation for rim joists are fiberglass batts and polyisocyanurate board, (say that five times fast) also known as foam board. I personally prefer the foam board for a couple of reasons - one, it doesn’t deteriorate as easily as fiberglass and two, if installed properly, foam board can be cleaner looking and have a tighter seal. The benefit of fiberglass batts is are their prices, as they tend to be very economical. Fiberglass batts are also more malleable, as you can cut them to size if you have pipes of wires, while foam board may take a bit more effort.


Fiberglass batts             vs           Foam Board

Dimensions:  ½” x 15” x 39’       Dimensions 2” x 4’ x 8’

Cost $12                                   Cost $30

R-13 valueR-13 value


All in all, the do it yourself or with a friend you plan to repay with beer and pizza shouldn’t take longer than a long weekend afternoon.


If you happen to have a finished basement, these areas may not necessarily be insulated, but it may not be economically sensible to tear down the walls just to address this problem. However, if you ever plan to do renovations make sure to take care of it then. Also, some people have partially finished basements that have some exposed joists. Even if it’s only a handful exposed joists, I would still recommend taking the time of sealing those up - every bit helps.


rim joist.jpgBEFORErim joists sealed.jpgAFTER

Programs of Interest


If you decide to take a crack at this DIY project, keep in mind there are tax credits that can help reimburse you for the supplies. One example is Montgomery County Maryland tax credit. There is a $250 reimbursement available for installing energy conservation devices. Check to see if your county has something similar.


Montgomery County



If you decide that the job is too big for you or that you’d rather have a professional do it, there are programs that can help pay for these types of renovations. The one I highly recommend for large projects is the Home Performance with Energy StarTM offered by PEPCO. It offers 50% off all air sealing and insulations upgrades to attics, basements, and crawlspaces up to $2,000. There are a couple of catches, the first, you have to get an home assessment (audit) done. It determines the safety of the appliances (primarily gas appliances) and estimates how leaky the home is. Once you have the audit completed, any upgrades you do through a pre-approved PEPCO contractor (second catch) is eligible for the rebate.The other benefit is what is called a “test out” - another audit is done after the renovations to make sure the improvements where done correctly, safely and determines how “tighter” the house has gotten.This can give you a better idea of the rate of return on your investment and how much energy you will be saving due to the upgrade. The audit normally costs $400-500 but PEPCO subsidizes the fee so that homeowners only pay $100. Special note: once you have the home assessment done, you have up to a year to take advantage of the rebate so there is no time pressure.


Here is a link to all the PEPCO energy efficiency programs currently going on...

http://www.pepco.com/energy/conservation/meiin/ All programs


http://homeenergysavings.pepco.com/home-performance-with-energy-star-program Home Performance


Video Tutorials


First video explains how the rim joists affect the overall performance of the home and some professional options for remedying the problem.



Second video has more of a DIY feel giving some options for a weekend project.




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