sprayfoam and a HRV system

Topper74

Hi,

Can someone explain to me what happens in the attic of house, if you are putting in sprayfoam and a HRV system in the house.
I was told that in this case you don't have any ventilation area in attic wallplate level then.

Is that correct?

muffler

Above post moved to new thread.

sas

Topper74 wrote: »

Hi,

Can someone explain to me what happens in the attic of house, if you are putting in sprayfoam and a HRV system in the house.
I was told that in this case you don't have any ventilation area in attic wallplate level then.

Is that correct?

Are you planning on insulation on the slope or on the flat? Sounds like on the flat.

If it's a cold roof and you are insulating on the flat, you do require the ventilation area at the wall plate as you suggested.

I've never seen spray foam used on the flat. It would be a strange insulation option for anywhere you may want access to again.

Topper74

Hi Sas,
No I think I'd be foam insulating the slope(in betweent joists) down to the floor of attic. My question I guess is, Does the foam seal up at the point of contact with wall/attic floor.

Also do you need a membrane for airtightness in this case (if you're going for that) if you have the foam sprayed on. btw its going to be a block house.

sas

Topper74 wrote: »

Hi Sas,
No I think I'd be foam insulating the slope(in betweent joists) down to the floor of attic. My question I guess is, Does the foam seal up at the point of contact with wall/attic floor.

Also do you need a membrane for airtightness in this case (if you're going for that) if you have the foam sprayed on. btw its going to be a block house.

As I understand it, you need to maintain a ventilation gap on the cold side of the spray foam insulation (or any insulation) at this location.

The gap at the eaves as you are referring to, would then allow the air to pass between the insulation and your felt.

If a breathable felt is used this may not be necessary but I'm not an expert on this so I'll leave it to someone else to clarify.

You really should use a membrane for airtightness. The NSAI certs for these system appear to be vague and recommend that you only omit the membrane if calculations are done to prove it's safe to do so. However, you'll be hard pressed to find someone that will give you a legally binding commitment that this is the case and so I'd assume you have to use it.

There's a new fella (macspower) doing the rounds here that is an installer for one of the spray foam products so hopefully he'll correct anything I've gotten wrong.

Topper74

Thx,
How exactly is that attic membrane put in to form airtightness, And at what point does the membrane go in, staight after roofing or after sprayfoam goes in.
Be great to clear this up, along with the ventilation query that you mentioned in relation to the Sprayfoam.

sas

Topper74 wrote: »

Thx,
How exactly is that attic membrane put in to form airtightness, And at what point does the membrane go in, staight after roofing or after sprayfoam goes in.
Be great to clear this up, along with the ventilation query that you mentioned in relation to the Sprayfoam.

Membrane goes in after insulation. It's done later in the build basically. The membrane is stapled to the rafters and the joints are taped. Airtightness material installation isn't rocket science but it needs to be planned. You need to have a plan for how you will connect your membrane to any block work for example. There are a couple of cheap courses on how to install the materials run by the suppliers. It's incredibly time consuming too so don't take it on yourself unless you have the time.