Insulating attic

Speakerboxx

Hi there,

I have an airtight house but would like the insulate the attic myself especially the areas between the rafters. There are noticeable wind drafts coming down into the attic from the roof when I go up there.

I have rockwool on attic joists already and osb floor boards over the joists to walk around the attic. I was going to put another layer of rockwool on top of the rockwool thats already there. This would only be put on the layers and areas where there are no floorboards attached next to the eves. I would like to insulate the roof rafters myself. I am not interested in spray foam but rather a cheaper alternative that would be effective.

I was thinking of getting the thick foamed insulation sheets and cut and put them into the rafters keeping them 50mm out from the roof itself. I was then going to apply a layer of airtightness tape all around over the insulation sheets in the rafters. If I do it this way, will it be any way effective? All I want to do is stop any Cold drafts entering the house.

Thanks.

Biker1

Speakerboxx wrote: »

Hi there,

I have an airtight house but would like the insulate the attic myself especially the areas between the rafters. There are noticeable wind drafts coming down into the attic from the roof when I go up there.

I have rockwool on attic joists already and osb floor boards over the joists to walk around the attic. I was going to put another layer of rockwool on top of the rockwool thats already there. This would only be put on the layers and areas where there are no floorboards attached next to the eves. I would like to insulate the roof rafters myself. I am not interested in spray foam but rather a cheaper alternative that would be effective.

I was thinking of getting the thick foamed insulation sheets and cut and put them into the rafters keeping them 50mm out from the roof itself. I was then going to apply a layer of airtightness tape all around over the insulation sheets in the rafters. If I do it this way, will it be any way effective? All I want to do is stop any Cold drafts entering the house.

Thanks.

Assuming you have an airtightness test result to show that you have an effective airtight barrier then that is all that matters. The drafts in the attic would mean there is adequate ventilation in the roof space and putting insulation and an airtight barrier is a pointless and costly exercise. If increasing the insulation at ceiling level then you must raise the flooring to prevent compression of the insulation.

Markcheese

Be careful not to block air coming into the attic , your roof timbers could rot over time ,
Are you getting cold air coming down into the house ?

Speakerboxx

Markcheese wrote: »

Be careful not to block air coming into the attic , your roof timbers could rot over time ,
Are you getting cold air coming down into the house ?

Yes I am getting cold drafts coming down from the roof into the attic space. Would insulation of the roof rafters with the foam sheets of insulation be OK to do provided I give a 50mm gap between the sheets and the roof itself? Or would rockwool be any better? I just want to stop cold drafts there without effecting the ventilation.

Speakerboxx

Biker1 wrote: »

Assuming you have an airtightness test result to show that you have an effective airtight barrier then that is all that matters. The drafts in the attic would mean there is adequate ventilation in the roof space and putting insulation and an airtight barrier is a pointless and costly exercise. If increasing the insulation at ceiling level then you must raise the flooring to prevent compression of the insulation.

I got an airtight result of 1.9 for the entire house so not 100% effective I reckon.

Penn

Speakerboxx wrote: »

I got an airtight result of 1.9 for the entire house so not 100% effective I reckon.

No house is going to be completely airtight. Even Passive Houses can be up to 0.6.

1.9 is a great result for airtightness, and I'm assuming you have a heat recovery system too. Unless you're storing something in particular in the attic space that could be badly affected by the cold, I'd have no concerns about the attic space being cold and think insulating it wouldn't be of any benefit. So long as the cold draughts aren't getting into the main house (which with an airtightness of 1.9 I strongly doubt they are), I don't see what the issue is.

Calahonda52

I agree with Penn, can't see the issue here, 1.9 is a grate result
its cold roof, well ventilated, which it needs to be, and I an assuming that you have at least 300 mm insulation in the ceiling and that the ventilated eaves detail is done properly.
Assuming the ceiling insulation id done properly, m then adding a layer to the rafters is a waster of time, counter productive and may inhibit properly ventilation of the roof timbers


have a read of this
https://www.nsai.ie/about/news/publication-of-sr-542014-code-of-practice

the_pen_turner

What do you mean by coming from the roof to the attic space.
Are there rooms in this attic space like a dormer


If the attic is outside the livable parts of the house then it sounds like everything is working fine. You need ventilation in an attic

Speakerboxx

the_pen_turner wrote: »

What do you mean by coming from the roof to the attic space.
Are there rooms in this attic space like a dormer


If the attic is outside the livable parts of the house then it sounds like everything is working fine. You need ventilation in an attic

I still see people insulating attics and was wondering if people were doing it in new builds as well afterwards. People always say you can't have enough insulation. I thought everything was fine with ventilation when you leave 50mm space between the insulation board and roof timbers (rafters)?? Correct me if I'm wrong.

mickdw

Your airtightness is good.
If you want more insulation, put a further layer on the ceiling, not on the slope.
Insulating between the rafters will be wasting your money unless you go to town on it and ensure all ventilation stays out of the attic space itself and just rushes through the gap behind the attic boards.
If you were to do this, you would need to introduce ridge ventilation also.

MicktheMan

Speakerboxx wrote: »

I still see people insulating attics and was wondering if people were doing it in new builds as well afterwards. People always say you can't have enough insulation. I thought everything was fine with ventilation when you leave 50mm space between the insulation board and roof timbers (rafters)?? Correct me if I'm wrong.

The point previous posters have been trying to make is it would appear your attic space is OUTSIDE your thermal envelope, i.e. insulating / airtightening your rafters has the same effect of insulating/airtightening your back garden wall .... none.

Because the space is outside the thermal envelope it needs to be properly ventilated. The situation where 50mm ventilation is required at the back of an insulated rafter roof is for cases where the space immediately under said roof is within the thermal envelope.

Adding insulation to the floor of the attic MAY be worthwhile depending on what is there already, how well it is installed etc but the law of diminishing returns also kicks in at a certain point.

Speakerboxx

MicktheMan wrote: »

The point previous posters have been trying to make is it would appear your attic space is OUTSIDE your thermal envelope, i.e. insulating / airtightening your rafters has the same effect of insulating/airtightening your back garden wall .... none.

Because the space is outside the thermal envelope it needs to be properly ventilated. The situation where 50mm ventilation is required at the back of an insulated rafter roof is for cases where the space immediately under said roof is within the thermal envelope.

Adding insulation to the floor of the attic MAY be worthwhile depending on what is there already, how well it is installed etc but the law of diminishing returns also kicks in at a certain point.

OK so adding insulation to floor only of attic would be the only alternative option possibly?

I have loads of insulation boards on my shed which I have no use for and have cost me nothing to have. I don't trust airtightness. Who's to say that the electrician or plumber didnt punch numerous holes on the air tightness membrane and I will never know about it, making it useless then. Some of the rooms of our house in the thermal bridge feel colder than others and that's why I was looking into these options. But an airtightness of 1.9 feels suggests I have nothing to worry about maybe.

MicktheMan

Speakerboxx wrote: »

OK so adding insulation to floor only of attic would be the only alternative option possibly?

I have loads of insulation boards on my shed which I have no use for and have cost me nothing to have. I don't trust airtightness. Who's to say that the electrician or plumber didnt punch numerous holes on the air tightness membrane and I will never know about it, making it useless then. Some of the rooms of our house in the thermal bridge feel colder than others and that's why I was looking into these options. But an airtightness of 1.9 feels suggests I have nothing to worry about maybe.

Just a couple of points to keep in mind;
1. If you have an air tightness issue, adding insulation won't generally help.
2. Air tightness is one of very few aspects of a build that can be actually tested objectively so if your final att result is 1.9 m3/hr/m2@50Pa then why not trust this result? It is a real, repeatable result.
3. Certain rooms not feeling the same temperature could well be down to a (combination of) heat distribution issue, orientation (north or east versus south or west) or aspect (higher external heat loss area to floor area ratio, for example).