Attic Ventilation and Insulation (part 2)

I con­sider the insu­la­tion in the attic to be the most impor­tant of the insu­la­tion in the house. Heat rises, there­fore the attic is where most of the heat in your house is going to try to escape.

I have built or added insu­la­tion to build­ings where the only thing that we started out with was insu­lat­ing the ceil­ing of the room. The dif­fer­ence in the tem­per­a­ture of the room or area that has the ceil­ing insu­lated is amaz­ing. Even with no insu­la­tion on the walls, the room holds two to three times more heat.

In order to max­i­mize the heat­ing effi­ciency the walls must be insu­lated as well but the ceiling/ attic insu­la­tion is prob­a­bly the most dramatic.

In part 1 of the attic ven­ti­la­tion and insu­la­tion post I talked about the ven­ti­la­tion and what it means to your house (or other build­ing). Here I will fin­ish up by fur­ther explain­ing the proper air seal­ing and insu­la­tion of the attic space and how it ben­e­fits the home.

Air seal­ing means keep­ing the air from flow­ing through the insu­lated areas of the house. On most con­ven­tion­ally built houses in this region (wood framed houses with insu­la­tion installed inside the cav­i­ties of the studs) the air seal­ing comes from installing mois­ture bar­ri­ers of plas­tic or house wrap under the fin­ished sur­faces of the walls and over top of the insu­la­tion in the attic. Other types of con­struc­tion will have dif­fer­ent require­ment for the walls, but most of them still have the same needs in the attic.

Insu­la­tion works by trap­ping air and cre­at­ing a bar­rier between the heated areas and the cold exte­rior. With­out insu­la­tion there is no bar­rier to keep the heat or cold from being trans­ferred from on area to the other. Heat trans­fer can also travel through the studs and fram­ing of a house. So when insu­lat­ing an attic it is a good prac­tice to cover all of the exposed fram­ing mem­bers attached to the ceil­ing in order to elim­i­nate this transfer.

Insu­la­tion and air seal­ing go hand in hand; one with­out the other is point­less. Have you ever worn a thick coat in win­ter but you were still cold because the wind cut right through the fab­ric of the coat? Or, have you ever worn a thin leather jacket that no wind can pass through but you still froze from the tem­per­a­tures? Now, have you ever worn the wind breaker jacket over top of the thick coat so that now no wind can get through and you have insu­la­tion against the cold temperatures?

This exam­ple of the coats that you wear is really no dif­fer­ent from the insu­la­tion in your house and how it works. Air pass­ing through insu­la­tion (drafts) ren­ders insu­la­tion almost use­less, where air seal­ing with­out insu­la­tion will stop the air flow but there is noth­ing to hold in the heat.

A lot of the attics that I have been in there is only insu­la­tion installed in between the ceil­ing joist and the tops of the joist are exposed, allow­ing the heat to be trans­ferred into the attic through the wood of the joist. A very select few I have seen have had an addi­tional layer of insu­la­tion installed over top of the joist to elim­i­nate the heat transfer.

In addi­tion to this extra layer of insu­la­tion that should be in place, there should also be an air seal­ing layer over top of the insu­la­tion to keep the air from flow­ing through the insu­la­tion. This is espe­cially impor­tant in attics that are vented prop­erly. Most attics do not have a fin­ished sur­face to pro­tect the top side of the insulation.

One thing to keep in mind about insu­la­tion is that it should never be crushed or packed down; this causes the insu­la­tion to lose its insu­lat­ing value. Attics are more prone to this prob­lem than walls because of the insu­la­tion being exposed. Peo­ple walk­ing in the attic or stor­ing stuff in the attic spaces may do a lot of dam­age to the insu­lat­ing value.

I really don’t rec­om­mend using an attic for a stor­age space because of the loss of the insu­la­tion value. If you install a floor in the attic space in order to be able to store items or walk around with­out hav­ing to make sure you step on a joist, you will not be able to have the extra layer of insu­la­tion, (unless of course you build the floor of the attic up above the sec­ond layer of insu­la­tion, but that would be very expen­sive. For the cost of this you could prob­a­bly build a small shed that offer you more stor­age space with­out hav­ing to lift things into the attic.