New Windows in Old Structures 

 TONER Point of Consideration 

New Windows in Old Structures 

All new windows are made to meet the 2015 IECC Standard. This is governed by the NFRC rating. The biggest change to the NFRC rating is that the air tightness now applies to the frame plus the glass. This makes for a very tight assembly and we lose the minor leakage that always occurred at the frame. 

When you install these tight windows into a loose structure, you will transfer air that came through the window's glass (prior to 2009 Code) and window frame (prior to 2015 Code) to the framing of the window itself. This framing is typically stacked studs and header with no air barrier. 

Once the air has been transferred, it will increase in velocity coming through the small gaps in the framing. The higher the velocity, the higher the moisture content. This will result in the "wetting" of your casings, sills and stools. 

If that trim is made of MDF, then it will swell as the moisture dissipates. If it is painted with an oil based paint, then it will be trapped, turned into water and potentially a mold damaged product. 

My suggestion is that the following should be added to your standards: 

•When installing new windows into old structures, ensure you also seal the framing from the outside with a full air barrier. 

• If the outside framing can’t be adjusted, then apply the same principle to the interior framing. 

• Install all window trim in a historical manner with a 1/8" shim to allow for moisture dissipation to come from behind the assembly. 

• Only use water based paints. 

• Do not use MDF trim on exterior walls or T-Walls. 

• If the windows are good original windows, consider keeping and restoring them. Windows only account for 4-5% of energy loss in our climate. 

Thanks –Toner 

TONER Home Matters 


Coryne Rich