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</o:shapelayout></xml><![endif]--></head><body lang=EN-CA link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>I have looked at a lot of attics (all vented) and one of the things I have noticed in pretty well all of the attics is some blackening of the sheathing. I have also noticed that the moisture evidence is more predominant on the shaded side of the roof. This makes sense if you consider solar exposure heating the roof and generating convective airflow at the sheathing line drying the sheathing. I don’t think roofs always vent at the ridge terribly well (lot of turbulence and conductive loss before the airflow even hits the ridge and backdrafting from the exterior airflow) which may be why there is such evidence of condensation on the shaded sides. Just guessing.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>The other problem though with a roof that is ridge or top vented <u>only</u> is that it can function as a solar chimney – depressurizing the attic somewhat and pulling air from the interior – which is why you would need an intake at the eaves (somewhere) to balance it. The problem with an older building is that you also need a slightly higher ratio of insulation at the eave/wall juncture than is in the wall – so really the only solution is to install vents (plastic type) in the actual roof a foot or so up from the wall.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>The other option is to close off all the vents and do a decent job of sealing the interior at the drywall level. You still have air movement due to heating of the roof but the energy is dissipated by conductive loss to the sheathing and framing and with the exception of metal roofing drying will happen to the exterior. Some people might not like this option and while the jury is out on that one the codes still require ventilation and require a ratio of both eave and ridge.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>That said - I think the obvious place for roof ventilation is between the sheathing and the roofing to provide both drying and cooling. If you look historically at tile roofing (rounded shapes) or even old shake roofs they worked wonderfully to allow for continuous air flows keeping things dry and roof temperatures down.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0cm 0cm 0cm'><p class=MsoNormal><b><span lang=EN-US style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span lang=EN-US style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> greenbuilding-bounces@lists.bioenergylists.org [mailto:greenbuilding-bounces@lists.bioenergylists.org] <b>On Behalf Of </b>Sacie Lambertson<br><b>Sent:</b> November-14-11 6:56 AM<br><b>To:</b> Green Building<br><b>Subject:</b> Re: [Greenbuilding] attic insulation in Seattle house<o:p></o:p></span></p></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><span style='font-size:13.5pt'>Back to the question of insulating an attic in the PNW (Seattle) I have found nothing from Lstiburek specifically addressing this issue in this area. This 1906 house has those small mushroom vents high up the roof but nothing down low. No soffit vents. The attic is currently nicely dry--it does have some old loose material between some of the 2x4 joists, material that was probably across the attic floor at that little depth before the house was updated. Those folks never got around to insulating the attic floor except where a new ceiling was put in below, where they put in fiberglass batts above. (We will take these out, but leave in the loose stuff). <br><br>Without a clear context, below is what L. does say about ventilating. I'm inclined to seal the perimeter to the degree I can and then simply blow in a foot of cellulose over everything. Several of you have suggested this, however I would love to hear from John Straube on this issue.<br><br>thanks, Sacie<br><br><i>“Is it better not to vent a roof?” </i></span><o:p></o:p></p><p><i><span style='font-size:13.5pt'>Lstiburek’s response: “I like to vent whenever I can. But when roof venting cannot be done so that it will be effective or when it is difficult (e.g. with hipped roofs or multiple dormers) it is better to have an unvented assembly.”</span></i><o:p></o:p></p><p><span style='font-size:13.5pt'>And from <span style='color:#1F497D'>cmhc</span>:</span><o:p></o:p></p><p><i><span style='font-size:13.5pt'>Attic computer models show that attics in damp coastal climates may actually be drier with less ventilation.</span></i><o:p></o:p></p><p class=MsoNormal style='margin-bottom:12.0pt'><br><br><o:p></o:p></p></div></body></html>